8. Control-Flow and Arrays

What We Will Cover

Elucidations

Homework Questions?

Quiz Questions?

Questions from last class?

  • Comments on using the suggested organization for your project?

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8.1: Relationships, Truth and Conditions

Learner Outcomes

At the end of the lesson the student will be able to:

  • Use conditional statements to compare numerical and string data values

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8.1.1: Flow of Control

  • As in most programming language, PHP handles flow of control with branching and looping statements
  • PHP branching and looping statements are the same as C, C++ and Java and very similar to other languages
  • Most branching and looping statements are controlled by a test condition
  • The test condition evaluates to either true or false

Relational Expressions

  • The simplest test conditions are comparison of two values such as: 
    $count <= 5
  • These comparisons always have one relational operator comparing two values

    relational expression

  • The operators are known as comparison operators or relational operators
  • The following tables shows the PHP relational operators you can use to compare two expressions

Table of Relational Operators

Math Name PHP Example Result
= Equal to == or
===		 5 == 10
2 == 2		 false
true
≠ Not equal to != or
!==		 5 != 10
2 != 2		 true
false
< Less than < 5 < 10
5 < 5
5 < 2		 true
false
false
≤ Less than or equal to <= 5 <= 10
5 <= 5
5 <= 2		 true
true
false
> Greater than > 5 > 10
5 > 5
5 > 2		 false
false
true
≥ Greater than or equal to >= 5 >= 10
5 >= 5
5 >= 2		 false
true
true

Comparing Strings

  • Note that comparison operators work with strings as well as numbers
  • With strings, comparison is performed in alphabetical order using ASCII:
    1. Digits before letters
    2. Uppercase letters come before lowercase letters

What is Truth?

  • "What is truth?" may seem more appropriate for philosophy than programming
  • However, in programming we have to know how the computer interprets truth
  • PHP interprets the following as false and every other value as true:
    • boolean false
    • integer 0 (zero)
    • float 0.0 (zero)
    • empty strings "", and the string "0"
    • arrays with zero elements (not covered yet)
    • objects with zero elements (not covered yet)
    • special type NULL (such as variables not assigned a value)
  • Note that the boolean value false is displayed by PHP as the NULL character ('\0')
  • However, the NULL character ('\0') does not display anything in a browser
  • To view information about boolean values, you can use var_dump()
  • Thus, to see the truth of a relational expression, you can use something like: 
    var_dump(5 == 10);
  • The following example shows several relational operators in action
  • To view the example, click here

Example Displaying Several Comparisons

<html>
<head><title>PHP Script</title></head>
<body>
<pre>
<?php
    $count = 5;
    echo "$count == 5: ";
    var_dump($count == 5);
    echo "$count != 5: ";
    var_dump($count != 5);
    echo "$count &lt; 10: ";
    var_dump($count < 10);
    echo "$count &lt;= 10: ";
    var_dump($count <= 10);
    echo "$count &gt; 10: ";
    var_dump($count > 10);
    echo "$count &gt;= 10: ";
    var_dump($count >= 10);
?>
</pre>
</body>
</html>

More Information

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8.1.2: Using the if-else Statement

  • An if-else statement chooses between two alternatives based on a single test condition
  • For example, if we are writing a simple guessing game and want to comment on a users guess: 
    if ($guess == 7) {
    echo "<p>*** Correct! ***</p>\n";
} else {
    echo "<p>Sorry, that is not correct.</p>\n";
}
  • General syntax: 
    if (test) {
    // execute statements only if true
} else {
    // execute statements only if false
}
  • The test condition is in the parenthesis and evaluates to a Boolean value of true or false
  • The statements to execute are usually enclosed in curly braces
  • If the curly braces are left off then the if and else only applies to the next statement
  • Notice that there is no test condition for the else clause
  • For clarity, the if and else are written on different lines than the nested statements
  • Also, the statements are indented

Conditional Example

  • In this example we create a simple guessing game application
  • We use a simple form to collect a guess from the user
  • Save the following as guessform.html in the htdocs/phptest directory of your localhost Web server 
    <html>
<head>
<title>Guessing Game!</title>
</head>

<body>
<form method="get" action="guess.php">
<p>I'm thinking of a number between 1 and 10.<p>
<p>Can you guess it?</p>
<input type="text" name="guess" size="20"></p>
<input type="submit" name="submit" value="Submit">
</form>

</body>
</html>
  • We report the success on the following page, where the form sends its data
  • Save the following as guess.php in the same directory as guessform.html

    <html>
    <head>
    <title>Guessing Game Response</title>
    </head>
    <body>
    <?php
    $guess     = $_REQUEST["guess"];
    if (    $guess == 7) {
        echo     "<p>*** Correct! ***</p>\n";
    } else {
        echo     "<p>Sorry, that is not correct.</p>\n";
    }
    ?>
    </body>
    </html>

Omitting the else

  • Sometimes you want one alternative to do nothing at all
  • In PHP, you can accomplish this by omitting the else clause

More Information

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8.1.3: Nested Conditional Statements

  • We can have several statements in either the if or else clause
  • One of these nested statements can be another if or if-else statement

Multiple Alternatives

  • By using collections of if-else statements, a program can distinguish between multiple alternatives
  • For example, we can rewrite our guessing game to provide hints on the guess: 
    $guess = $_REQUEST["guess"];
if ($guess == 7) {
    echo "<p>*** Correct! ***</p>\n";
} else if ($guess < 7) {
    echo "<p>Sorry, that is too low.</p>\n";
} else {
    echo "<p>Sorry, that is too high.</p>\n";
}
  • By nesting an if statement in an else clause we are able to choose from multiple alternatives
  • The test conditions are evaluated in order until one of the tests evaluates to true
  • At that point the statements in that part of the structure are executed and then the rest of the if-else-if is skipped
  • The final else clause is optional and serves as a default in case all the other conditions are not met

Using elseif

  • Since an if-else-if structure is so commonly used, PHP has a shortcut version for the else-if part: elseif
  • As the name suggests, elseif is a combination of an else followed by an if
  • This variation saves a whopping one space in our source code for every elseif we use
  • For example, rewriting our previous example to use elseif: 
    $guess = $_REQUEST["guess"];
if ($guess == 7) {
    echo "<p>*** Correct! ***</p>\n";
} elseif ($guess < 7) {
    echo "<p>Sorry, that is too low.</p>\n";
} else {
    echo "<p>Sorry, that is too high.</p>\n";
}

More Information

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8.1.4: Using Logical Operators

  • Sometimes you want to consider several test conditions at once
  • For this we can use logical operators
  • Use ! (NOT) operator to reverse value of expression
  • Use and to AND two or more conditions
    • Also, you can spell AND as: &&
  • Use or to OR two or more conditions
    • Also, you can spell OR as: ||
  • and operator returns true only if expressions on both sides are true
  • or operator returns true if either expression is true

and (&&) Operator

If expr1 is... And expr2 is... Then expr1 and expr2 is... Example Result
true true true 5 < 10 and 5 > 2 true
true false false 5 < 10 and 5 < 2 false
false true false 5 > 10 and 5 > 2 false
false false false 5 > 10 and 5 < 2 false

or (||) Operator

If expr1 is... || expr2 is... Then expr1 or expr2 is... Example Result
true true true 5 < 10 or 5 > 2 true
true false true 5 < 10 or 5 < 2 true
false true true 5 > 10 or 5 > 2 true
false false false 5 > 10 or 5 < 2 false

! Operator

If expr is... Then ! expr is... Example Result
true false !true false
false true !(5 < 2) true

Logical Example

  • Suppose we want to verify that users enter correct values such as a number between 1 and 10
  • We can use a logical or to check the value of two fields at once 
    if ($num < 1 or $num > 10) {
  print "Error: enter a number between 1 and 10";
}
  • An alternate way of checking uses the NOT operator with the AND operator 
    if (!($num >= 1 and $num <= 10)) {
  print "Error: enter a number between 1 and 10";
}
  • We can convert between these two ways of expressing the same condition using De Morgan's laws
  • Some people like to construct a truth table to verify the logic of a condition

Check Yourself

  • What is displayed by running the following code? 
    $bb = true;
if ($bb or !$bb) {
    echo "That is the question!";
}

More Information

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Exercise 8.1

In this exercise we explore using relational and logical operators with if-else statements. As an example, we will calculate a student's letter grade according to the following table:

Numerical Grade Letter Grade
greater than or equal to 90 A
less than 90 but greater than or equal to 80 B
less than 80 but greater than or equal to 70 C
less than 70 but greater than or equal to 60 D
less than 60 F

Specifications

  1. Open the XAMPP Control Panel by double-clicking the icon on the desktop or by using the Apache Friends entry in the start menu.

    XAMPP icon

  2. Start the Apache and MySQL modules, if they are not already running.

    XAMPP control panel

  3. Start TextPad, or another text editor, and enter this starter code:

    <html>
    <head>
    <title>Score Entry Form</title>
    </head>

    <body>
    <h1>Find Your Grade</h1>
    <form method="get" action="grade.php">
    <p>What was your score?
    <input type="text" name="score" size="10"></p>
    <input type=submit name="submit">
    </form>
    </body>
    </html>

  4. Save this code in a file named grade.html to the htdocs directory and phptest subdirectory of your Apache (XAMPP) installation (htdocs/phptest).

    If you installed XAMPP on Windows using the default settings, the htdocs directory is C:\xampp\htdocs. Also, the Start menu has an entry that opens the htdocs directory by following Start -> Apache Friends -> XAMPP -> XAMPP httpdoc folder. On other systems you will need to search for the htdocs directory. If the subdirectory phptest is missing, then you will need to create a new one.

  5. Open a new Web browser tab or window and enter localhost/phptest/grade.html in the address field.

    You should see a page like the following in the browser:

  6. Copy the following PHP script into a text editor and save the file in your phptest directory as grade.php: 
    <html>
<head><title>Grade Report</title></head>
<body>
<?php
$score = trim($_REQUEST["score"]);
print "The score was $score, which is an ";
// Enter code here

?>
</body>
</html>

    This is the file in which you will be making changes.

  7. Add conditional statements to the grade.php file that uses the $score variable to determine the grade based on the score. For example, your first statement might be: 
    if ($score >= 90) {
    echo "A";
}
  8. Once you have completed writing the statements, test your code with several scores to make sure it works correctly.
  9. Submit your final grade.php file (the one you made the changes to) to Blackboard as part of assignment 8.

As time permits, be prepared to answer the Check Yourself questions in the section: 8.1.5: Summary.

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8.1.5: Summary

  • As in most programming language, PHP handles flow of control with branching and looping statements
  • PHP branching and looping statements are the same as C, C++ and Java and very similar to other languages
  • Most branching and looping statements are controlled by a test condition
  • The test condition evaluates to either true or false
  • he simplest test conditions use relational operators
  • Relational operators include: 
    ==  !=  <   <=  >   >= === <== >==
  • Similarly, you can compare letters since letters are stored as numbers
  • Relational expressions always evaluate to either true or false
  • To make choices in your scripts, use one of the following: 
    if
if else
if elseif elseif ... else
  • We can have several statements in either the if or else clause
  • One of these nested statements can be another if or if-else statement
  • By using collections of if-else statements, a program can distinguish between multiple alternatives, like: 
    $guess = $_REQUEST["guess"];
if ($guess == 7) {
    echo "<p>*** Correct! ***</p>\n";
} else if ($guess < 7) {
    echo "<p>Sorry, that is too low.</p>\n";
} else {
    echo "<p>Sorry, that is too high.</p>\n";
}
  • As a shortcut for the else-if part, PHP provides elseif
  • For example, rewriting the previous example to use elseif: 
    $guess = $_REQUEST["guess"];
if ($guess == 7) {
    echo "<p>*** Correct! ***</p>\n";
} elseif ($guess < 7) {
    echo "<p>Sorry, that is too low.</p>\n";
} else {
    echo "<p>Sorry, that is too high.</p>\n";
}
  • When you need to compare several conditions, you can use logical operators 
    !   and   or
  • For example: 
    if (($num < 1) or ($num > 10)) {
  print "Error: enter a number between 1 and 10";
}

Quick Quiz

  1. If a = 4 and b = 5, the expression a > b evaluates to .
  2. You may nest ________ else-if statement(s) in a PHP program:


  3. The else statement is the default conditional selected when all possible conditions are not met.


Check Yourself

  1. What is meant by the term "flow of control"? (8.1.1)
  2. What is a relational expression and why are they used? (8.1.1)
  3. What are the two possible values to which a relational expression evaluates? (8.1.1)
  4. In PHP, what values evaluate to true? (8.1.1)
  5. True of false? PHP can compare both numbers and strings with relational expressions. (8.1.3)
  6. What is an if-else statement used for? (8.1.2)
  7. Does the syntax for an else clause include a test condition? (8.1.2)
  8. True of false? The order of if and if-else statements never matters. (8.1.2)
  9. True of false? An elegant way to choose among multiple alternatives is to nest if statements in an else clause. (8.1.3)
  10. When does an AND (&&) of two or more conditions evaluate to true? (8.1.4)
  11. When does an OR (||) of two or more conditions evaluate to false? (8.1.4)
  12. What is the effect of the NOT (!) operator? (8.1.4)

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8.2: Using Loops to Repeat statements

Learner Outcomes

At the end of the lesson the student will be able to:

  • Use while and for statements to repeat sections of code
  • Apply commonly-used looping patterns
  • Avoid common looping problems

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8.2.1: Introduction to Loops

  • Looping statements in PHP are the same as C, C++ and Java and very similar to other languages
  • PHP has three general-purpose looping statements: 
    while
do-while
for
  • In addition, PHP has a special purpose foreach looping statement we will cover later
  • The code that repeats in a loop is called the body of the loop
  • Each repetition of a loop is called an iteration

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8.2.2: Using the for Loop

  • Whenever we work with a loop, we need to consider these things:
    1. Loop body -- which statements to repeat
    2. Test condition -- when to loop and when to stop
    3. Test update -- what must change to exit the loop
    4. Initialization code -- how to start the loop correctly
  • The for loop provides a syntax for these things in a compact form: 
    for (initialization; test; update) {
    // statements to repeat
}
  • Where:
    • initialization: statement to execute before the loop starts
    • test: condition to evaluate at the start of each iteration
    • update: statement to execute at the end of the loop
  • The statements to repeat are enclosed in curly brackets
  • For example: 
    <?php
    $max = 5;
    for ($i = 0; $i < $max; $i = $i + 1) {
        echo "i = $i <br>";
    }
    echo "After loop i = $i" ;
?>

    Try it!

  • The for loop is primarily used for counting as shown in the example above
  • The execution sequence of a for loop is:
  1. When for statement reached -- initialization executes
  2. If the test condition evaluates to true:
    1. Execute statements in the body of the loop
    2. When the end of loop body is reached, execute the update statement
    3. Return to step 2
  3. Otherwise, the loop is finished so continue with statements after the loop

More Information

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8.2.3: Using the while and do-while Loop

  • The simpler while loop has the following syntax: 
    while (test) {
   // statements to repeat
}
  • Where test is the condition to evaluate at the start of each iteration
  • For example: 
    $count = 0;
while ($count < 5) {
    echo "count = $count<br>";
    $count = $count + 1;
}
  • Because of its simpler form, the while loop is considered a more general purpose loop
  • However, we still have to include the four parts of a loop design:
    1. Loop body -- which statements to repeat
    2. Test condition -- when to loop and when to stop
    3. Test update -- what must change to exit the loop
    4. Initialization code -- how to start the loop correctly

The do-while Loop

  • A variation on the while statement is the do-while, which tests the condition at the end of the loop body: 
    do {
   // statements to repeat
} while (test); //test condition
  • Where test is the condition to evaluate at the end of each iteration
  • For example: 
    $count = 0;
do {
    echo "count = $count<br>";
    $count = $count + 1;
} while ($count < 5);
  • Testing at the end ensures a minimum of at least one iteration
  • Thus we use the do-while loop when we want to ensure at least one iteration

More Information

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8.2.4: Nesting Loops

  • We can have several statements in a loop
  • One of these nested statements can be another loop
  • When nesting, it is important to use indentation to show the nesting of the loops
  • The following example shows several loop examples
  • We use the loops to print the rows and columns of an HTML table
  • The first loop prints the table heading
  • Following this is a nested loop
  • The outer loop iterates through each row of the nested loop
  • The nested inner loop prints each column of the table
  • To see the loop click here

Example of Nested Loop

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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<html>
<head><title>Loopy Table</title></head>
<body>
<h1>Loopy Table</h1>
<table border="1" style="text-align:right;">
<?php
$start = 1;
$maxCol = $start + 9;
$maxRow = $start + 9;

echo "<tr>\n";
echo "<th>x</th>\n";
// Print the heading
for ($col = 1; $col < $maxCol; $col++) {
  echo "<th>&nbsp;&nbsp;&nbsp;$col</th>\n";
}
echo "</tr>\n";

// Outer loop to iterate through each row
for ($row = 1; $row < $maxRow; $row++) {
  echo "<tr>\n";
  echo "<td><b>$row</b></td>\n";
  // Inner loop prints each column
  for ($col = 1; $col < $maxCol; $col++) {
      $value = $row * $col;
      echo "<td>$value</td>\n";
   }
  echo "</tr>\n";
}
?>
</table>
</body>
</html>

Increment and Decrement Operators

  • Notice the ++ operator in the previous example 
    $row++
  • The the ++ operator is an arithmetic shortcut called the increment operator
  • The increment operator (++) adds 1 to a variable's value
  • A similar operator is the decrement operator (--) which subtracts 1 from a variable's value

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Exercise 8.2

In this exercise we explore how to use loops in PHP.

Specifications

  1. Copy the following PHP script into a text editor, save the file in your phptest directory as loopy.php, and then view the file as a Web page in your browser. 
    <?php
    $max = 5;
    for ($i = 0; $i < $max; $i = $i + 1) {
        echo "i = $i <br>";
    }
    echo "After loop i = $i" ;
?>

    The page should display the numbers from 0 to 4.

  2. Change the code in loopy.php to use a while loop rather than a for loop.

    Both for loops and while loops can perform the same tasks.

  3. Submit your final loopy.php file to Blackboard as part of assignment 8.

As time permits, be prepared to answer the Check Yourself questions in the section: 8.2.5: Summary.

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8.2.5: Summary

  • Loops are used to repeat sections of code
  • Whenever we work with a loop, we need to consider these things:
    1. Loop body -- which statements to repeat
    2. Test condition -- when to loop and when to stop
    3. Test update -- what must change to exit the loop
    4. Initialization code -- how to start the loop correctly
  • The while loop is the most basic repetition statement
  • A more compact repetition statement is the for loop 
    for (initialization; condition; increment) {
   //loop body
}
  • It is legal code to have one loop nested inside another
  • We looked at an example that built an HTML table

Quick Quiz

  1. When using a while loop, the loop body will run for as long as a specific condition is met.


  2. What will be output after the following statements have been executed? 
    $count;
for ($count = 1; $count < 4; $count++) {
    print ($count + " ");
}


  3. When working with a counting loop, you must always the counting variable.

Check Yourself

  1. What are PHP's three general purpose looping statements? (8.2.1)
  2. What are the four things to consider when working with loops? (8.2.2)
  3. What part of a loop is repeated? (8.2.2)
  4. What is the purpose of a loop test condition? (8.2.2)
  5. What is the purpose of the loop initialization code? (8.2.2)
  6. What is the code for a for statement that uses a counter variable named counter that starts with the value 0 and continues up to and including 100 in increments of 10? (8.2.2)
  7. What is the syntax of a while statement? (8.2.3)
  8. True of false? A use for a nested loop is to print the rows and columns of a table. (8.2.4)

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8.3: Using Arrays

Learner Outcomes

At the end of the lesson the student will be able to:

  • Write code to initialize arrays
  • Access array elements
  • Sometimes useful to group related data values into a list
  • Can use a variable type called an array to store data in a list
  • Then able to operate on the array data as a group

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8.3.1: About Arrays

Array: a list of data values where each individual value is access by an index

  • Arrays are a type of data structure -- a way to organize data
  • An array is used to process a collection of data as a group like
    • Scores
    • Temperatures
  • Why do we need arrays?
  • Imagine keeping track of 5 test scores, or 100, or 1000 in a program
    • How would you name all the variables?
    • How would you process each of the variables?
  • Arrays provide a convenient solution to these problems
  • Entire array referred to by a single identifier
  • Each element uniquely identified by an index

Example Array

  • One-dimensional array is like single row of data in spreadsheet or table

    $score[0] $score[1] $score[2] $score[3] $score[4]
    90 95 87 89 98

  • Top row shows the indices for an array named "$score"
  • Individual items are referenced by the index inside the square brackets [ ]
  • By default, array indexes start at the number 0
  • PHP uses a type of array known as an associative array, or "ordered map"
  • This means that indexes can be strings as well as numbers
  • In the exercise we will explore how to create and use arrays in PHP

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8.3.2: Creating and Populating Arrays

  • Like other PHP variables, arrays do not need to be declared in advance
  • For example, to create an array named numbers with three elements: 
    $numbers[0] = "zero";
$numbers[1] = "one";
$numbers[2] = "two";
  • After assignment, you can use array element like any other variable 
    echo "$numbers[0], $numbers[1], $numbers[2]";
  • By default, arrays indexes start at zero (0)
  • Thus you can skip the numbers in the square brackets like this: 
    $numbers[] = "zero";
$numbers[] = "one";
$numbers[] = "two";
  • If you do not assign your own index, PHP automatically assigns the next highest index starting a 0
  • The following examples assigns three values one after another without specifying an index:

    <?php
    // Creating an array
    $numbers[] = "zero";
    $numbers[] = "one";
    $numbers[] = "two";

    // Accessing values
    echo "$numbers[0], $numbers[1], $numbers[2]";
    ?>

    Try it!.

Using the Array Function

  • Another way to create arrays is with the array() function.
  • By default, the array() function numbers array indexes starting at 0
  • For example:

    <?php
    // Creating an array
    $numbers = array("zero", "one", "two");

    // Accessing values
    echo "$numbers[0], $numbers[1], $numbers[2]";
    ?>

    Try it!.

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8.3.3: Selecting Array Items

  • Individual array elements are accessed using the array name and an index
  • Use the array operator [] to specify the index
  • Indexes should be an integer expression or a string
  • Arrays indexes can be literal values or a variable

<?php
$weekDay = array("Sunday", "Monday", "Tuesday",
    "Wednesday", "Thursday", "Friday", "Saturday");

echo "$weekDay[0]<br>$weekDay[1]<br>";
echo "$weekDay[2]<br>$weekDay[3]<br>";
echo "$weekDay[4]<br>$weekDay[5]<br>";
echo $weekDay[6];
?>

Try it!.

  • If you wish, you can set one or more index values for your array

<?php
$weekDay = array(1=>"Sunday", "Monday", "Tuesday",
    "Wednesday", "Thursday", "Friday", "Saturday");

echo "$weekDay[1]<br>$weekDay[2]<br>";
echo "$weekDay[3]<br>$weekDay[4]<br>";
echo "$weekDay[5]<br>$weekDay[6]<br>";
echo $weekDay[7];
?>

Try it!.

  • Above example sets the first index to the number 1
  • Subsequent assignment of values use the maximum index + 1
  • If you try to access an index beyond those defined, PHP returns NULL

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8.3.4: Using Loops with Arrays

  • Loops are often used as a way to iterate (audio) through array items
  • For example, the following code uses a loop to list each item in an array
  • Uses the count function to return the number of items in the array

    <?php
    $weekDay     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    for (    $i = 0; $i < count($weekDay); $i++) {
      echo     "$weekDay[$i]<br>";
    }
    ?>

    Try it!.

  • Note the use of $i++ as a shortcut for adding one to a variable
  • The statement: 
    $i++;
    is equivalent to: 
    $i = $i + 1;
  • Similarly, the statement: 
    $i--;
    is equivalent to: 
    $i = $i - 1;

Using foreach

  • Another way to access each element is the foreach loop statement
  • Syntax: 
    foreach ($arrayName as $varName) {
   //loop body
   ...
}
  • Where $arrayName is the name of the array and $varName is the variable that each array item is assigned to in order
  • The foreach loop automatically assigns $varName the next value in the array for each loop iteration
  • Note that foreach works only with arrays

Example Using foreach

  • The following code displays the contents of the array named $dayName

    <?php
    $weekDay     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    foreach (    $weekDay as $day) {
      echo     "$day<br>";
    }
    ?>

    Try it!.

  • Note that each array item is accessed and displayed in order

^ top

8.3.5: Associative Arrays

  • Arrays indices in PHP can be strings rather than numbers
    • Known as an associative array or ordered maps
  • The string value is used to look up the location of the data value
  • You can use such an array as a cross reference between names and values

Creating Associative Arrays

  • The following code creates an associative array 
    $course['cis154'] = 'Parrish';
$course['cis156'] = 'Griffith';
$course['cis165'] = 'Parrish';
  • Also, you can create associative arrays with the array function
  • You must specify the index name followed by => and the value
  • For instance, we can create a cross-reference between month names and number of days in the month 
    $months = array('Jan'=>31, 'Feb'=>28, 'Mar'=>31,
  'Apr'=>30, 'May'=>31, 'Jun'=>30, 'Jul'=>31,
  'Aug'=>31, 'Sep'=>30, 'Oct'=>31, 'Nov'=>30,
  'Dec'=>31);
  • Then we can access an array element like this: 
    $days = $months['Nov'];
  • This is similar syntax to arrays we have used so far
  • The main difference is the use of string values within the square brackets
  • When used with the foreach statement, you can access both the index name and item name 
    foreach ($months as $name=>$days) {
  print "Index = $name and value = $days<br>";
}
  • Also, you can change the value of any array item by assigning it a new value 
    $course['cis156'] = 'Griffin';

Example of an Array with String indexes

<?php
$months = array('Jan'=>31, 'Feb'=>28, 'Mar'=>31,
  'Apr'=>30, 'May'=>31, 'Jun'=>30, 'Jul'=>31,
  'Aug'=>31, 'Sep'=>30, 'Oct'=>31, 'Nov'=>30,
  'Dec'=>31);

foreach ($months as $name=>$days) {
  print "Index = $name and value = $days<br>";
}
?>

Try it!.

^ top

8.3.6: Functions for Working With Arrays

  • Like the PHP mathematical functions we looked at in lesson 6.2.7, PHP has functions that work with arrays
  • Some functions, like print_r() and var_dump() work not only with arrays but other variable types as well

Using print_r() and var_dump()

  • Functions print_r() and var_dump() display information about a variable
  • You use these functions for debugging when you want to understand the state of a variable
  • For example:

    <?php
    $weekDay     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    echo     "<pre>";
    print_r($weekDay);
    var_dump($weekDay);
    echo     "</pre>";
    ?>

    Try it!.

  • The pre tags show the formatting of the function output

Using count() and sizeof()

  • If you want to know how may elements are in an array, you can use either count() or sizeof()
  • For example:

    <?php
    $weekDay     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    echo     "The count of elements in the array is ";
    echo     count($weekDay);
    echo     "<br>The size of the array is ";
    echo     sizeof($weekDay);
    ?>

    Try it!.

  • Function sizeof() is an alias for count()

Using isset() and unset()

  • Sometimes you want to remove an element from an array
  • In this case you can use the unset() function like this:

    <?php
    $weekDay     = array("Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday");

    if (isset(    $weekDay[1])) {
        echo     "Blue Monday<br>\n";
    } else {
        echo     "No Monday<br>\n";
    }
    echo     "Removing Monday's<br>\n";
    unset(    $weekDay[1]); // Goodbye Monday
    if (isset($weekDay[1])) {
        echo     "Blue Monday<br>\n";
    } else {
        echo     "No Monday<br>\n";
    }
    ?>

    Try it!.

  • Notice that function isset() is testing for the presence of the array element

More Information

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Exercise 8.3

In this exercise we explore how to use arrays in PHP.

Specifications

  1. Copy the following PHP script into a text editor, save the file in your phptest directory as arrays.php, and then view the file as a Web page in your browser. 
    <html>
<head><title>PHP Script</title></head>
<body>
<?php
    // Enter your PHP code here

?>
</body>
</html>
  2. We will first look at how to create an array in PHP. Copy the following code between the PHP tags and then save and reload (refresh) the page: 
    $numbers[0] = "zero";
$numbers[1] = "one";
$numbers[2] = "two";

echo "$numbers[0], $numbers[1], $numbers[2]";

    You should see the words, "zero, one, two", displayed on the page. Like other PHP variables, you do not need to declare arrays before assigning them values. For more information see section 8.3.2: Creating and Populating Arrays.

  3. By default, PHP numbers array indexes starting at 0. Replace the previous code with the following, where the numbers have been removed from the square brackets in the assignment statements: 
    $numbers[] = "zero";
$numbers[] = "one";
$numbers[] = "two";

echo "$numbers[0], $numbers[1], $numbers[2]";

    When assigning a value, PHP will automatically increment the indexes, starting from 0, if we do not assign our own index values. For more information see section 8.3.2: Creating and Populating Arrays.

  4. Another way to create arrays is with the array() function. Replace the previous code with the following: 
    $numbers = array("zero", "one", "two");

echo "$numbers[0], $numbers[1], $numbers[2]";

    By default, the array() function numbers array indexes starting at 0. For more information see section 8.3.2: Creating and Populating Arrays.

  5. If we want to start array numbering somewhere other than 0 while using the array() function, we can specify the starting location using an arrow (=>) before the first item. To see this, replace the previous code with the following: 
    $numbers = array(1=>"zero", "one", "two");

echo "$numbers[1], $numbers[2], $numbers[3]";

    Notice how we access each array element after it is created using square brackets. For more information see section 8.3.3: Selecting Array Items.

  6. Since array indexes are numbers, we can access each element with an index variable and a counting loop. Replace the previous code with the following: 
    $colors = array("red", "green", "yellow", "orange");
for ($i = 0; $i < 4; $i++) {
    echo "$colors[$i]<br>\n";
}

    The index variable $i is used to control the counting loop. For more information see section 8.3.4: Using Loops with Arrays.

  7. Using a loop to access every element of an array is very common. Because of this PHP has a special loop called the foreach loop. Replace the previous code with the following: 
    $colors = array("red", "green", "yellow", "orange");
foreach ($colors as $item) {
    echo "$item<br>\n";
}

    The foreach loop access each array item in order. For more information see section 8.3.4: Using Loops with Arrays.

  8. Arrays indices in PHP can be strings rather than numbers. To use arrays with strings, we need to map the key (index) to the value as shown below. To see the associative array in action, replace the previous code with the following: 
    $colors = array("apple"=>"red", "kiwi"=>"green");
$colors["banana"] = "yellow";
$colors["orange"] = "orange";

foreach ($colors as $fruit=>$color) {
    echo "$fruit color is $color<br>\n";
}

    Notice that the array function uses the arrow (=>) to map the key to the value (key=>value). You can map keys to values using square brackets and the assignment operator as well. For more information see section 8.3.5: Associative Arrays.

  9. If you want to see every element of an array for debugging, you do not need to use a loop. PHP provides two functions for this: print_r() and var_dump(). You can see how they work by adding after the foreach loop: 
    echo "<pre>";
print_r($colors);
var_dump($colors);
echo "</pre>";
    The pre tags show the formatting of the function output. For more information on these and other functions see section 8.3.6: Functions for Working With Arrays.

  10. Submit your final arrays.php file to Blackboard as part of assignment 8.

As time permits, be prepared to answer the Check Yourself questions in the section: 8.3.7: Summary.

^ top

8.3.7: Summary

  • Arrays are convenient ways to process a list of data
  • Like other PHP variables, arrays do not need to be declared in advance
  • Just type the variable name followed by square brackets
  • You can assign a value to a single array element 
    $myArray[] = "someValue";
  • After assignment, you can use array element like any other variable 
    echo $myArray[0];
  • An array function uses more compact code to initialize arrays 
    $myArray = array("zero", "one", "two");
  • Loops are often used as a way to iterate through array items
  • PHP has a special type of loop for use with arrays: foreach
  • You can declare arrays in a form for processing in you PHP scripts
  • Place square brackets '[]' after the element name
  • Arrays indices in PHP can be strings rather than numbers
  • Allows cross-referencing, among other uses: 
    $months = array('Jan'=>31, 'Feb'=>28, 'Mar'=>31,
  'Apr'=>30, 'May'=>31, 'Jun'=>30, 'Jul'=>31,
  'Aug'=>31, 'Sep'=>30, 'Oct'=>31, 'Nov'=>30,
  'Dec'=>31);
  • You can use print_r() and var_dump() to view information about an array 
    var_dump($months);

Quick Quiz

  1. In an array, each individual data value has an that distinguishes it from other values in the array.
  2. Given the code $myArray = array("first", "second");, which of the following prints the word "first"?


  3. A good way to display information about arrays or other variables is to use var_dump($varName).


Check Yourself

  1. What is an array? (8.3.1)
  2. Why do we use arrays? (8.3.1)
  3. What is an array index? (8.3.1)
  4. What are two ways you can create and initialize an array? (8.3.2)
  5. What statement would you use to create an array named $colors? (8.3.2)
  6. What statements would you use to create an array named $dayNames and initialize it with the abbreviations for the days of the week, starting with "Sun" and going through "Sat"? (8.3.2)
  7. What expression would you use to return the third value from the array dayNames? (8.3.3)
  8. How do you display an array using a loop? (8.3.4)
  9. True of false? You can use string values as the index of an array. (8.3.5)
  10. What is a good way to display information about an array for debugging purposes? (8.3.6)
  11. If you want to know how many elements are in an array, what function can you use? (8.3.6)
  12. If you want to remove an element from an array, what function can you use? (8.3.6)

^ top

8.4: Arrays, Forms and Meta-data

Learner Outcomes

At the end of the lesson the student will be able to:

  • Fetch data from MySQL in arrays
  • Create HTML tables
  • Display MySQL data in HTML tables
  • Use result set metadata

^ top

8.4.1: Using Arrays for Multiple Form Values

  • Multiple checkboxes in a form can have the same name 
    <input type="checkbox" name="pref[]" value="ASP"> ASP
<input type="checkbox" name="pref[]" value="JSP"> JSP
  • Also, a selection list can be set up to accept multiple values using the multiple attribute 
    <select name="sections" multiple>
  • Arrays can help to generate checkboxes and selection lists
  • In addition, arrays can help to process values received from checkboxes and selection lists sending multiple values
  • Both generating and processing form elements using an array are shown in the following examples
  • Note the use of square brackets ([]) in the form name
  • You need the square brackets in the form name to tell PHP to process the items as an array

Example Checkboxes with the Same Name

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head><title>Languages</title></head>

<body>
<h3>Make your vote</h3>
<form action="langscript.php" method="post">
<p>Preferences:
<?php
$prefs = array("ASP","JSP","PHP","Other");
foreach ($prefs as $item) {
  print '<input type="checkbox" name="pref[]" value="';
  print $item.'"> '.$item;
}
?>
<p><input type="submit" value="Click to Vote!"></p>
</form>
</body>
</html>

Try it!.

Example Script Processing Form Data with Multiple Values

<html>
<head>
<title>Your Vote Counts!</title>
</head>

<body>
<h3>Your Vote Counts!</h3>
<?php

if (empty($_REQUEST['pref'])) {
  print "Oh no! Please vote!";
} else {
  print "<p>You voted for: <ul>";
  foreach ($_REQUEST['pref'] as $item) {
     print("<li>$item</li>");
  }
  print "</ul>";
}
?>
</body>
</html>

Notes on the Code

  • Note the use of the empty() function to test if a variable has a value assigned 
    if (empty($_REQUEST['pref']))
  • We use empty() to test if the expected data was filled in by the user
  • Another useful function for testing variables is isset()
    • Tests to see if a variable exists or not
  • Also note that print_r() and var_dump() are good functions to use when debugging forms
  • For example, to view all the variable data sent from a browser use: 
    var_dump($_REQUEST);
  • Insert the above code near the top of your script that receives the form data to see all the form values submitted by the user
  • In addition, notice that the textbook has an example of using an array for a selection list

More Information

^ top

8.4.2: Fetching Data from MySQL in Arrays

  • You can use arrays to extract MySQL data from a result set
  • PHP provides mysql_fetch_array() to read result set data into arrays 
    $row = mysql_fetch_array($result, MYSQL_BOTH);
  • Similar functions are:
  • Using this technique, PHP can extract an entire row of data from the result set
  • Each column (field) of the row is contained in the elements of the array
  • Usually, these functions provide better performance than mysql_result()

Checking the Result Set

  • PHP provides other useful functions for processing result set data
  • You can use mysql_num_rows() to count the number of rows in query results 
    $numRows = mysql_num_rows($result);
  • Also, you can use mysql_num_fields() to count the number of columns in query results 
    $numRows = mysql_num_fields($result);
  • With loops, you can use these functions to make a query and display the results

Example Using mysql_fetch_assoc()

  • Let us create a PHP script by copying the following code into a text editor
  • Save the file as datafetch.php in the htdocs/phptest directory of your localhost Web server

<?
require_once("includes/dbconvars.php");

// Open the connection
$dbCnx = mysql_connect($dbhost, $dbuser, $dbpwd)
    or die("Could not connect: ".mysql_error());
mysql_select_db($dbname, $dbCnx)
    or die("Could not select db: ".mysql_error());

// Make the query
$sql = "SELECT * FROM products";
$result = mysql_query($sql)
    or die("Query failed: ".mysql_error());

// Count number of rows returned
$numRows = mysql_num_rows($result);
echo "<p>Number of rows: $numRows</p>\n";

// Outer loop copies each row into $row
while ($row = mysql_fetch_assoc($result)) {
    // Inner loop displays each column of $row
    echo "<p>";
    foreach ($row as $item) {
        echo "$item ";
    }
    echo "</p>\n";
}

// Clean up
mysql_free_result($result);
mysql_close($dbCnx);
?>

Try it!

Notes on the Code

  • Note that the outer loop copies each row of data into $row
  • $row is an array, which means we can iterate through all the columns using a loop
  • As long as mysql_fetch_assoc() returns a row, the test condition of the loop evaluates to true
  • The inner loop displays each column of data using a foreach loop
  • What do you think of the layout of the data?

More Information

^ top

8.4.3: Displaying MySQL Data in Tables

  • You can use HTML tables to neatly format MySQL data
  • Using our previous example, we add table tags in the correct locations

For Example

<?
require_once("includes/dbconvars.php");

// Open the connection
$dbCnx = mysql_connect($dbhost, $dbuser, $dbpwd)
    or die("Could not connect: ".mysql_error());
mysql_select_db($dbname, $dbCnx)
    or die("Could not select db: ".mysql_error());

// Make the query
$sql = "SELECT * FROM products";
$result = mysql_query($sql)
    or die("Query failed: ".mysql_error());

// Count number of rows returned
$numRows = mysql_num_rows($result);
print "<p>Number of rows: $numRows</p>\n";

// Show the results in a table
print "<table border=\"1\">\n";
while ($row = mysql_fetch_assoc($result)) {
    echo "<tr>\n";
    foreach ($row as $item) {
        echo "<td>&nbsp;$item</td>\n";
    }
    echo "</tr>\n";
}
print "</table>\n";

// Clean up
mysql_free_result($result);
mysql_close($dbCnx);
?>

Try it!

^ top

8.4.4: Adding Column Names with Meta-data

  • One thing missing from the table is names for the columns
  • You could add this information by hand
  • However, a better way is to use MySQLs meta-data

    Meta-data: data about data.

  • MySQL keeps data about its databases, table structure and records
  • For example: database names, table names, column names, column types, etc.
  • When you make a query, the result set contains information about the data
  • This information is known as the result set metadata
  • PHP provides a number of functions to access the result set metadata
  • One function is called mysql_field_name() which returns the name of the specified column (field) from the result set
for ($i = 0; $i < $count; $i++) {
    print "<th>".mysql_field_name($result, $i)."</th>\n";
}
  • Another function is mysql_field_table, which returns the name of the specified table for a column (field) from the result set 
    mysql_field_table($result, 0)
  • You can use this meta-data to provide table names and field names

<?
require_once("includes/dbconvars.php");

// Open the connection
$dbCnx = mysql_connect($dbhost, $dbuser, $dbpwd)
    or die("Could not connect: ".mysql_error());
mysql_select_db($dbname, $dbCnx)
    or die("Could not select db: ".mysql_error());

// Make the query
$sql = "SELECT * FROM products";
$result = mysql_query($sql, $dbCnx)
    or die("Query failed: ".mysql_error());

// Show the table name
echo "<h4>Table: ";
echo mysql_field_table($result, 0);
echo "</h4>\n";

echo "<table border>\n";
// Show the column names as table headings
echo "<tr>\n";
$count = mysql_num_fields($result);
for ($i = 0; $i < $count; $i++) {
    echo "<th>&nbsp;";
    echo mysql_field_name($result, $i);
    echo "</th>\n";
}
echo "</tr>\n";

// Show the result set data using HTML tables
while ($row = mysql_fetch_assoc($result)) {
    echo "<tr>\n";
    foreach ($row as $item) {
        echo "<td>&nbsp;$item</td>\n";
    }
    echo "</tr>\n";
}
echo "</table>\n";

// Clean up
mysql_free_result($result);
mysql_close($dbCnx);
?>

Try it!

More Information

^ top

Exercise 8.4

In this exercise we explore how to use arrays with MySQL. For this exercise you will need the Artzy database installed, which we completed in Exercise 2.1.

Specifications

  1. Verify your computer has the following connection information in a file named dbconvars.php in a subdirectory named includes of the phptest directory: 
    <?php
$dbhost = "localhost";
$dbuser = "root";
$dbpwd = "";
$dbname = "artzy";
?>

    If needed, change the values assigned to $dbuser and $dbpwd to match your MySQL installation. In the classroom, $dbuser = root; and $dbpwd = password;. For more information see lesson 6.3.1: Connecting to a MySQL Database.

  2. Copy the following PHP script into a text editor, save the file in your phptest directory as prodselect.php, and then view the file as a Web page in your browser. 
    <html>
<head><title>Product Selection</title></head>
<body>
<h1>Product Selection</h1>
<form action="prodreport.php" method="GET" name="cust">
<p>Select a Product:
<?php
// Enter your PHP code here

?>
</p>
<p><input type="submit" value="Submit"></p>
</form>
</body>
</html>

    Notice that we have declared a form to which we will add a selection list using data from the Artzy database.

  3. Inside the PHP tags, add the PHP statements to connect to the Artzy database: 
    require_once("includes/dbconvars.php");

// Open the connection
$dbCnx = mysql_connect($dbhost, $dbuser, $dbpwd)
    or die("Could not connect: ".mysql_error());
mysql_select_db($dbname, $dbCnx)
    or die("Could not select db: ".mysql_error());

    For more information see lesson 6.3.1: Connecting to a MySQL Database.

  4. There are two parts to an option tag, the value and words to display. We want to retrieve both parts from our database. Add the following code after all the other PHP statements: 
    // Make the query
$sql = "SELECT ID, ProductName FROM products";
$result = mysql_query($sql)
    or die("Query failed: ".mysql_error());

    For more information on selection lists see lesson 7.2.4: Other Form Elements.

  5. From the result set returned by the query, we want to build a selection list. Add the following code after all the other PHP statements:

    Code for selection control

    Notice that we are using one of the PHP functions that extract data from a result set into an array. For more information see section 8.4.2: Fetching Data from MySQL in Arrays.

  6. Finally, add the code to free the result set memory and close the MySQL connection after all the other PHP statements: 
    // Clean up
mysql_free_result($result);
mysql_close($dbCnx);

    For more information see lesson 6.3.3: Cleaning Up.

  7. Reload (refresh) the Web page and verify you see the following form:

    Verify the selection list displays the six products from the Artzy database. If you do not see the correct results then compare your source code against the listing at the end of this exercise. Also, view the source and notice the value and content displayed in the selection list.

  8. To create the report page, copy the following PHP script into a text editor, and save the file in your phptest directory as prodreport.php 
    <html>
<head><title>Product Report</title></head>
<body>
<h1>Product Report</h1>
<?php
var_dump($_REQUEST); // replace this statement

?>
</body>
</html>
  9. Select a product on your prodselect.php page and press the submit button. Verify that you see a page like the following:

    The exact "prodselect" value will depend on which product you selected. If your do not see the report then check your work and correct the problem before continuing. If you cannot correct the problem after a reasonable time, then ask the instructor for help.

  10. Replace the var_dump() function call with the following statement to retrieve the value of "prodselect" submitted by the form: 
    $id = $_REQUEST["prodselect"];

    For more information see lesson 7.2.5: Processing Form Data with PHP.

  11. Inside the PHP tags, add the PHP statements to connect to the Artzy database after the previous PHP code: 
    require_once("includes/dbconvars.php");

// Open the connection
$dbCnx = mysql_connect($dbhost, $dbuser, $dbpwd)
    or die("Could not connect: ".mysql_error());
mysql_select_db($dbname, $dbCnx)
    or die("Could not select db: ".mysql_error());

    For more information see lesson 6.3.1: Connecting to a MySQL Database.

  12. Since we know the ID of the selected product, we can tailor our query to retrieve only the one product. Add the following code after all the other PHP statements: 
    // Make the query
$sql = "SELECT ProductName AS 'Product Name',
ProductDescription As 'Product Description',
Price, InStock AS Quantity
FROM products WHERE ID = $id";
$result = mysql_query($sql)
    or die("Query failed: ".mysql_error());

    Notice that we are using two column aliases in our SELECT statement. For more information see lesson 2.3.4: Using an Alias.

  13. From the result set returned by the query, we want to build a table from the data. Add the following code after all the other PHP statements:

    Code for table

    Notice that we are using meta data from the result set to display the column names. For more information see section 8.4.4: Adding Column Names with Meta-data.

  14. Finally, add the code to free the result set memory and close the MySQL connection after all the other PHP statements: 
    // Clean up
mysql_free_result($result);
mysql_close($dbCnx);

    For more information see lesson 6.3.3: Cleaning Up.

  15. Reload (refresh) the prodreport.php page and verify you see a table of data displayed. If you do not see the table, then verify your work including reentering the form data. If you do not see the correct results then compare your source code against the listing at the end of this exercise.
  16. Submit both your final prodselect.php and prodreport.php files to Blackboard as part of assignment 8.

Check Yourself

As time permits, be prepared to answer these questions. You can find more information by following the links after the question.

  1. True of false? Checkboxes and selection lists can submit multiple values to a form processing script. (8.4.1)
  2. What PHP functions can you use to extract and entire row of data from a result set? (8.4.2)
  3. In what format is the extracted returned from these functions? (8.4.2)
  4. What is meta-data? (8.4.4)

Listing of prodselect.php

Code for selection page

Listing of prodreport.php

Code for report page

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8.4.5: Summary

  • In this section we looked at PHP functions for extracting data from a result set into an array
  • Using this technique, an entire row of data is returned from the result set
  • Each column (field) of the row is contained in the elements of the array
  • In addition, we looked at using HTML tables to format the retrieved data
  • We also found that we could use the meta data functions to label the columns of our table

Quick Quiz

  1. Meta data is data about data.


  2. To find out how many rows of data were returned in a results set, you use the function .
  3. Which of the following is not one of the three functions for placing a row of data from a results set into an array?


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Wrap Up

Due Next:
A8-Query and Report (4/20/09)
Quiz 8 and Discussion Chapter 9 (4/20/09)
When class is over, please shut down your computer if it is on

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Last Updated: May 21 2009 @12:09:10