3: Flow of Control

What We Will Cover Reminders: Please silence all cell phones and pagers When class is over, please shut down your computer if it is on. Continuations 3.1: Making Decisions 3.1.1: Flow of Control 3.1.2: Test Conditions and Relational Operators 3.1.3: Using the if-else Statement 3.1.4: Nested if Statements 3.1.5: Testing Multiple Conditions 3.1.6: Using Multiway else-if Statements 3.1.7: Using switch Statements 3.1.8: Conditional (Ternary) Operators 3.1.9: Summary Exercise 3.1 3.2: Repetition 3.2.1: About Loops 3.2.2: The while and do-while Loops 3.2.3: The for Loop 3.2.4: Nested Loops 3.2.5: Using break and continue Statements 3.2.6: Looping Applications 3.2.7: Common Loop Pitfalls 3.2.8: Summary Exercise 3.2 3.3: Strings 3.3.1: About Strings and Characters 3.3.2: String Methods 3.3.3: Comparing Strings 3.3.4: Converting Strings 3.3.5: StringBuffer Class 3.3.6: Parsing Strings Exercise 3.3 Wrap Up Assignments Due Next: A2-Metabolic Energy (2/24/10) A3-ASCII Shapes (3/3/10) Illuminations Questions from last class? Homework Questions? A2-Metabolic Energy (2/24/10) Homework Discussion Questions Did anyone have difficulty developing the conversion algorithm? How easy was it to use variables and arithmetic statements? How easy was it to use CheckStyle? ^ top 3.1: Making Decisions Learner Outcomes At the end of the lesson the student will be able to: Formulate relational expressions to produce boolean values Assemble complex boolean expressions Generate if and if-else statements Create multiway if-else statements Recognize switch statements and conditional operators ^ top 3.1.1: Flow of Control The term flow of control refers to the order in which a computer executes instructions You can write any program with just three control-flow elements: Sequence - continue with the next instruction Sequence is the default operation A program automatically goes on to the next statement unless directed otherwise Selection - a choice between at least two options Either execute some instruction based on a condition Or continue with the next instruction Selection statements include: if if-else switch Repetition - a loop (repeat a block of code) At the end of the loop: Either go back and repeat the block of code Or continue with the next instruction after the block Repetition statements include: for do-while while We will start with selection statements ^ top 3.1.2: The Value of a Relationship To make a selection (or repetition statement) we need to code a test condition The computer uses the test to decide whether or not to execute the statement One way to code a test condition is to use a relational expression Relational Expressions In addition to arithmetical operations, all computers can compare numbers Many decisions can be reduced to choosing between two numbers For instance, did the user enter a number equal to the correct guess Expressions that compare numbers are called relational expressions Simple relational expressions have one relational operator comparing two operands: You can see the relational operators of Java in the following table Relational Operators Math Name Java Examples   Result Notes = Equal to == 5 == 10 2 == 2 false true Do not confuse with = which is assignment. ≠ Not equal to != 5 != 10 2 != 2 true false The ! is like the line that "crosses through" the equal sign. < 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 Be careful not to write =<. Code the symbols in the order people normally say them. > 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 Be careful not to write =>. Code the symbols in the order people normally say them. Comparing Characters Character data can be evaluated using relational operators as well Comparing characters works because Java stores characters as numbers using Unicode Unicode is a 16-bit numerical code for letters and symbols covering most of the world's writing systems Letters nearer to the start of the alphabet have lower numerical values For comparisons, Java takes the 16-bit numerical code and converts it to an int Thus you can use a numerical comparison to decide the alphabetical order of characters Example Program Comparing Characters 1 2 3 4 5 6 7 8 9 10 11 12 public class CompareChar { public static void main(String[] args) { System.out.println("'A' == 'A': " + ('A' == 'A')); System.out.println("'A' != 'A': " + ('A' != 'A')); System.out.println("'A' < 'B': " + ('A' < 'B')); System.out.println("'C' < 'B': " + ('C' < 'B')); System.out.println("'A' <= 'X': " + ('A' <= 'X')); System.out.println("'7' > 'A': " + ('7' > 'A')); System.out.println("'a' > 'A': " + ('a' > 'A')); System.out.println("'a' < 'X': " + ('a' < 'X')); } } More Information Summary of Operators: from the Java Tutorial ^ top 3.1.3: Using the if-else Statement The if-else statement chooses between two alternatives based on a test condition The condition must evaluate to either true or false If a condition is true then do this Otherwise it is false so do something else Syntax: if (test) { statements1 } else { statements2 } Where: test: the test condition to evaluate statements1: the statements to execute if the test evaluates to true statements2: the statements to execute if the test evaluates to false For clarity, if and else are written on different lines than the statements and the statements are indented Note that the else clause can be omitted entirely if (test) { statements1 } The following example shows an if-else statement Example of an If-Else Statement import java.util.Scanner; public class GradeApp { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I show pass or fail based" + " on a score.\nEnter your score: "); int score = input.nextInt(); if (score >= 60) { System.out.println("You passed!"); } else { System.out.println("Sorry, you failed."); } } } About those Curly Braces Technically, the if and else affects only the single statement that follows We can use curly braces to make the single statement into a block of statements This allows us to put any number of statements within the block Though curly braces are not always required, the best practice is to include them Programming Style: Placement of Braces Different practices for placing curly braces in a compound statement Many programmers prefer placing curly braces on separate lines However, Java standard is to place the opening brace on the same line as shown above In practice, you should use the style dictated by your company's policy Or your professor's instructions For the acceptable styles for this course see: Curly Braces ^ top 3.1.4: Nested if Statements You can include if statements within other if statements Each inner if statement is evaluated only if outer condition is met Example of Nested if Statements import java.util.Scanner; public class GradeApp2 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I show a grade based on a" + " score.\nEnter your score: "); int score = input.nextInt(); if (score >= 60) { if (score >= 70) { if (score >= 80) { if (score >= 90) { System.out.println("A"); } else { System.out.println("B"); } } else { System.out.println("C"); } } else { System.out.println("D"); } } else { System.out.println("Sorry, you got an F"); } } } Nested conditionals can be confusing if too deep Rule of thumb: no more than three deep "Dangling Else" Problem A potential problem with nested if statements is the dangling else problem if (test1) if (test2) Statement1; else Statement2; Indentation is misleading and should be: if (test1) if (test2) Statement1; else Statement2; Common mistake is not matching the else with the correct if Rule: else always matches the last unmatched if Avoid this confusion by using both curly braces and indentation to show the grouping: if (test1) { if (test2) { Statement1; } else { Statement2; } } ^ top 3.1.5: Testing Multiple Conditions One reason to use nested if statements is for testing multiple conditions However, an often better way is to combine conditions using logical (Boolean) operators Logical operators allow you to consider multiple cases and to change conditions AND Operation One such operator is "and", which is spelled && in Java For example, the following is true if score >= 80 and score < 90 (score >= 80) && (score < 90) When two boolean expressions are connected using && the whole expression is true only if both conditions are true However, if either condition is false the whole expression is false OR Operation Also, you can combine two boolean expressions using the "or" operator, spelled || in Java For example, the following is true if score < 0 or score > 100 (score < 0) || (score > 100) When two boolean expressions are connected using || the whole expression is true if either condition is true Only if both conditions are false is the entire expression false NOT Operation Additionally, you can negate any expression using ! operator To use it, place the entire expression in parenthesis and place the ! operator in front For example, to get a age between 0 and 100, we can negate our previous example: !((score < 0) || (score > 100)) Example Using Logical operators import java.util.Scanner; public class GradeApp3 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I show a grade based on a" + " score.\nEnter your score: "); int score = input.nextInt(); if (score >= 90 && score < 100) { System.out.println("You got an A"); } if (score >= 80 && score < 90) { System.out.println("You got a B"); } if (score >= 70 && score < 80) { System.out.println("You got a C"); } if (score >= 60 && score < 70) { System.out.println("You got a D"); } if (score >= 0 && score < 60) { System.out.println("Sorry, you got an F"); } if (score < 0 || score > 100) { System.out.println("Invalid score"); } } } Truth Table for ! Operator If expr is... Then ! expr is... Example Result true false !true false false true !(5 < 2) true Truth Table for && Operator If expr1 is... And expr2 is... Then expr1 && expr2 is... Example Result true true true 5 < 10 && 5 > 2 true true false false 5 < 10 && 5 < 2 false false true false 5 > 10 && 5 > 2 false false false false 5 > 10 && 5 < 2 false Truth Table for || Operator If expr1 is... || expr2 is... Then expr1 || expr2 is... Example Result true true true 5 < 10 || 5 > 2 true true false true 5 < 10 || 5 < 2 true false true true 5 > 10 || 5 > 2 true false false false 5 > 10 || 5 < 2 false Short-Circuit (Lazy) Evaluation Sometimes Java can take a shortcut when evaluating a logical operator If the left side of an && is false, you know the whole condition is false Similarly, if the left side of an || is true, you know the whole condition is true A common use of short-circuit evaluation is to prevent a division by zero error as shown in the following example If you always want complete evaluation then you can use | for "or" and & for "and" operations Example of Short-Circuit Evaluation import java.util.Scanner; public class ShortCircuit { public static void main(String[] args) { final double BAD = .001; Scanner input = new Scanner(System.in); System.out.print("Enter number of bugs: "); int bugs = input.nextInt(); System.out.print("Enter total lines of code: "); int total = input.nextInt(); if (total != 0 && (double) bugs / total > BAD) { System.out.println("Too many bugs!"); } else { System.out.println("Good quality code."); } } } ^ top 3.1.6: Using Multiway else-if Statements Sometimes the order of the tests is important When order is important, a useful construction is an if ... else-if statement An if statement is nested inside an else clause Test conditions are checked sequentially until finding the first true condition Once found, the program executes the code within the associated block and skips the remainder of the if-else chain You can code a final else clause as a default condition If none of the other conditions are true then execute the default code Note that if the order of tests were reversed in the following code we would get the wrong results Example of a Multiway if-else-if Chain import java.util.Scanner; public class GradeApp4 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I show a grade based on a" + " score.\nEnter your score: "); int score = input.nextInt(); if (score >= 90) { System.out.println("You got an A"); } else if (score >= 80) { System.out.println("You got a B"); } else if (score >= 70) { System.out.println("You got a C"); } else if (score >= 60) { System.out.println("You got a D"); } else { System.out.println("Sorry, you got an F"); } } } Note the formatting of if-else-if statements The else clauses are aligned rather than indented This stops each else-if from creeping to the right ^ top 3.1.7: switch Statements The switch statement gives us an alternative to an if-else-if chain Executes a section of code depending on value of a variable The general syntax is: switch (integerExpression) { case label1: statements break; case label2: statements break; ... case labeln: statements break; default: statements } Where: integerExpression: an arithmetic expression that resolves to an integer number labelx: a numeric constant statements: the statements to execute when the condition is met Any number of case labels can be placed in any order Any value that does not match starts executing with the statement after default Execution continues until the end of the switch statement or a break statement The break statement causes an immediate exit from the switch statement Just as case identifies possible starting points, break determines end points Example of a switch Statement: import java.util.Scanner; public class GradeApp5 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I show a grade based on a" + " score.\nEnter your score: "); int score = input.nextInt(); switch (score / 10) { case 10: case 9: System.out.println("You got an A"); break; case 8: System.out.println("You got a B"); break; case 7: System.out.println("You got a C"); break; case 6: System.out.println("You got a D"); break; default: System.out.println("Sorry, an F"); } } } Notice that each case other than the last contains a break statement Ensures that the switch statement is exited after a matching case is found When to Use switch Statements You can only use switch statements with integer expressions and each case must be a constant In addition, switch statements only work for exact matches (==) Thus, switch statements are inherently less useful than if-else statements Also, the syntax is no clearer than if-else statements Note that there is a reason for the limitations of the switch statement Many years ago a compiler could generate more efficient code (using jump tables or binary searches) only within the limitations of the switch statement However, modern compilers are quite capable of optimizing if-else statements to the same degree Thus, we have no reason to ever use a switch statement, depending on your compiler On the other hand, there are reasons to avoid using a switch statement Every branch of the switch statement must be terminated by a break statement If the break statement is missing, the program falls through and executes the next case without testing There are rare uses for this fall through behavior, such as printing the words for the song, The Twelve Days of Christmas However, according to a study by Peter van der Linden, reported in his book, Expert C Programming, p. 38, the falling through behavior is needed less than 3% of the time Thus, the default behavior is wrong 97% of the time Forgetting to type the break statement is a very common error and the source of many bugs So one has to ask oneself, "Why use an inferior programming statement that causes more problems than it solves?" ^ top 3.1.8: Conditional Ternary Operators Another conditional statement is the conditional ternary operator Provides a compact if-else structure Syntax: operand1 ? operand2 : operand3; First operand must be a conditional expression that evaluates to true or false If operand1 is true, return operand2; otherwise return operand3. Example Comparing if-else with a Conditional Ternary Operator Following is an if-else statement: if (n1 > n2) { max = n1; } else { max = n2; } Equivalent using the conditional operator: max = (n1 > n2) ? n1 : n2; When to Use Conditional (Ternary) Operators Shorthand style rarely used in professional programing Almost always clearer to use if-else statements You can use it for the Java version of the Obfuscated C Code Contest How not to format your code: arachnid.c ^ top 3.1.9: Summary Selection is a choice between at least two options Selection statements include: if if-else switch Single statements are expanded into compound statements using blocks Test conditions can be created using relational expressions such as: == != < <= > >= All relational expression evaluate to a boolean value: true or false You can build complex conditions using logical operators: ! && || Nested if statements are legal but often confusing Which if does an else match up with? Use curly braces to clarify Better yet, avoid using nested if statements In contrast, multiway if ... else-if statements are very useful if statements are nested in the else clause Programs execute the code when the first condition matches Often used to create menus Java has an older style switch statement that is sometimes used Inherently more limited than if-else statements Also, Java has a conditional (ternary) operator that is occasionally used Check Yourself To what two values must a test condition always evaluate? True of false? An elegant way to choose among multiple alternatives is to nest if statements in an else clause. True of false? Order never matters in a sequence of if and else statements. True of false? A switch statement is a more powerful solution to multiple if-else-if statements. When does an AND (&&) of two or more conditions evaluate to true? When does an OR (||) of two or more conditions evaluate to false? What is the effect of the NOT (!) operator? True of false? A && B is the same as B && A for any Boolean conditions A and B. ^ top Exercise 3.1 Take one minute to prepare an answer the following questions. What will happen if we attempt to compile and run the following code? class Dangling { public static void main(String[] args) { if (true) if (false) System.out.println("first"); else System.out.println("second"); } } Which of the following boolean expressions tests to see if x is between 2 and 15 (including 2 and 15)? (x <= 15 || x >= 2) (2 <= x || x <= 15) (x >= 2 && x <= 15) (2 <= x <= 15) ^ top 3.2: Repetition Learner Outcomes At the end of the lesson the student will be able to: Use while and do-while Loops Use for Loops Generate the appropriate repetition structure for a Java program Diagnose common looping problems ^ top 3.2.1: About Loops Loops are used to repeat sections of code General loop structure: Initialization code Loop condition -- evaluated during the loop Loop body Java has 3 looping statements: for while do-while You can use any loop statement in place of another loop statement With minor changes to your code ^ top 3.2.2: The while and do-while Loops The simplest looping structure is the while statement A while statement has a test condition and a body, like an if statement Before each execution of the loop body, the while statement checks the test condition If the test condition evaluates to true, then our program executes the body of the loop Syntax: while (test) { statement1 statement2 ... } Where: test: the test condition to evaluate statementX: the statements to execute while the test remains true For example: String repeat = "y"; while ("y" == repeat) { // ... statements to repeat repeat = input.next(); } Note that if the test is initially false then loop body never executes do-while Loop A variant of the while loop is the do-while loop Syntax: do { // statements to repeat } while (test); //test condition Where test is the condition to evaluate at the end of each iteration For example: String repeat = "whatever"; do { // ... statements to repeat repeat = input.next(); } while ("y" == repeat); The important difference between the while and do-while is when the loop test is checked The body of the do-while loop always executes at least one time The following example uses both while and do-while loops Loop Examples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 import java.util.Scanner; public class PlayAgain { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("Let's play two exciting games!"); String repeat = "y"; while (repeat.equalsIgnoreCase("y")) { System.out.println("\nPlaying the first game!"); System.out.print("Play again? (y/n) "); repeat = input.next(); } System.out.println("\nLet's play another game!"); repeat = "whatever"; do { System.out.println("\nPlaying the second game!"); System.out.print("Play again? (y/n) "); repeat = input.next(); } while (repeat.equalsIgnoreCase("y")); System.out.println("\nThanks for playing!"); } } ^ top 3.2.3: The for Loop When we design a loop, we need to consider these things: Loop body -- which statements to repeat Test condition -- when to loop and when to stop Test update -- what must change to exit the loop 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 for loop is primarily used for counting: for (int i = 0; i < 10; i++) { System.out.println(i); } You can see an example below for Loop Example 1 2 3 4 5 6 7 8 public class CounterFor { public static void main(String[] args) { int count; for (count = 0; count < 5; count++) { System.out.println(count); } } } Note that code is indented in the loop body for readability Execution Sequence The execution sequence of a for loop is: When for statement reached -- initialization executes If the test condition evaluates to true: Execute statements in the body of the loop When the end of loop body is reached, execute the update statement Return to step 2 Otherwise, the loop is finished so continue with statements after the loop Which Loop Statement to Use Generally, we use for statements when counting with a loop For non-counting loops where we want to ensure at least one iteration, use a do-while statement Otherwise, use the simpler while statement Notice that all loops have the same test condition This makes it possible always to convert one looping statement into another with some adjustment to other code Thus, choosing which loop statement to use is more a matter of elegance than a requirement ^ top 3.2.4: Nested Loops More advance looping applications often have loops nested within other loops For example, you may use a nested loop to print a table of values The following example shows a simple table created with nested loops Let's follow the execution sequence before checking the result Example of Nested Loops 1 2 3 4 5 6 7 8 9 public class Nested { public static void main(String[] args) { for (int outer = 1; outer < 4; outer++) { for (int inner = 1; inner < 4; inner++) { System.out.println(outer + " " + inner); } } } } Tracing the Variables Below is a trace of the variables for the inner and outer loops Note that the outer loop changes only after the inner loop is finished Memory  Screen   int outer   int inner      1   1 1 1     2 1 2     3 (end of loop)   1 3   2   1 2 1     2 2 2     3 (end of loop)   2 3   3 (end of loop)     1 3 1     2 3 2     3 (end of loop)   3 3 Nested Loop Example: Drawing Squares Another example of a nested loop is to draw a square using stars ("*") The code for drawing a square consists of two loops, one nested inside the other: for (int i = 0; i < size; i++) { for (int j = 0; j < size; j++) { System.out.print("*"); } System.out.println(); } The code is easier to read from the inside out The inner loop draws a line of characters across the screen from left to right At the end of the inner loop we print a newline to end the line of characters The outer loop controls the number of lines to draw Note how the inner loop is indented to make the structure clear The inner loop is controlled by the outer loop, which is why it is indented The inner loop in turn controls other statements, which ends up indenting yet again Whenever you type an opening brace ({) you should indent the next character Whenever you type a closing brace (}) you should remove the indentation (outdent) Example Program to Draw Squares 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 import java.util.Scanner; public class Square { public static void main(String[] args) { System.out.println("I draw squares for you!\n"); System.out.print("Enter width of square: "); Scanner input = new Scanner(System.in); int size = input.nextInt(); for (int row = 0; row < size; row++) { for (int col = 0; j < size; j++) { System.out.print("*"); } System.out.println(); } } } You can make the square hollow by using an if statement inside the inner loop to decide when to print characters Develop test conditions that allow the characters to print only at the start and end of the columns and rows Use an else clause to print spaces when not printing characters ^ top 3.2.5: Using break and continue Statements The break statement is an integral part of the switch statement Another use the break statement is to terminate loops The continue statement is similar except that instead of exiting a loop, it jumps to the end of the loop The causes the loop to skip the rest of the current iteration and start the next iteration The following code shows an example of break and continue Note that using break and continue statements with loops is generally consider poor programming style The reason we are reviewing them is to see what NOT to do Example Using break and continue Statements public class BreakContinue { public static void main(String[] args) { int count = 0; while (true) { count++; if (count == 3) continue; System.out.println(count); if (count >= 5) break; } System.out.println("After count= " + count); } } When to Use break and continue Statements Use break statements in switch statements only For this course, never use break or continue statements in looping constructs: Poor coding habit Loops should only have one exit point Note that there is an interesting story about a major problem caused by the use of break statements On January 15, 1990, a misused break statement caused an AT&T 4ESS telephone switch to fail The failure propagated through the entire U.S. network, rendering it nearly unusable for about nine hours A programmer had used a break to terminate an if statement However, break cannot be used to break out of an if Therefore the program execution broke out of the enclosing switch statement instead Perhaps more importantly, using a break statement makes it difficult to prove program correctness in critical software ^ top 3.2.6: Looping Applications Let us look at some ways loops are commonly used Counter-Controlled Loops Use this pattern when you know how many times to repeat a section of code The key is a variable called the counter: Initialize the counter variable to 0 Set the loop condition to check for the maximum count Adjust the counter variable inside the body of the loop We saw this in our previous example: public class CounterFor { public static void main(String[] args) { int count; for (count = 0; count < 5; count++) { System.out.println(count); } } } A variation is to start at the maximum value and count down Ask Before Repeating Another pattern is to ask the user whether or not to repeat a loop Often used to allow the user to control the flow of a program For example: import java.util.Scanner; public class SumAskFirst { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("I sum numbers"); double total = 0.0; // Loop initialization do { System.out.print("Enter a number: "); double num = input.nextDouble(); total += num; System.out.print("Another number? (y/n): "); } while (input.next().charAt(0) == 'y'); System.out.println("Total: " + total); } } For a long list, this approach can be tiring Oftentimes, a better solution is to use a sentinel-controlled loop Sentinel-Controlled Loops Use a sentinel when you can use input data to tell the program when to stop Looping continues as long as the data value read is not a sentinel value When a sentinel value is read, the loop exits For example: import java.util.Scanner; public class SumSentinel { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("I sum numbers.\n" + "Enter 0 to exit"); double total = 0.0; // Loop initialization double num = 0.0; do { System.out.print("Enter a number: "); num = input.nextDouble(); total += num; } while (num != 0); System.out.println("Total: " + total); } } num is the sentinel variable The sentinel value is the number: 0 The program ends when the user enters the sentinel value Menu Systems Loops are often used to create a simple text menu system A menu allows the user to select one item from a list A loop keeps the menu system operating A series of conditional or case statements act on the choice made You can see an example in the following code 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 import java.util.Scanner; public class Menu { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("Menus R Us!"); int choice = 1; while (choice != 0) { System.out.println("\n0. Exit program"); System.out.println("1. First item"); System.out.println("2. Second item"); System.out.print("Choice: "); choice = input.nextInt(); if (choice == 1) { System.out.println("Doing first item"); } else if (choice == 2) { System.out.println("Doing second item"); } else if (choice != 0) { System.out.println("\nInvalid choice!"); } } System.out.println("\nGoodbye!"); } } ^ top 3.2.7: Common Loop Pitfalls Infinite Loops Common error: unintentional infinite loop For example, what is wrong with the following code? 1 2 3 4 5 6 7 8 public class InfiniteLoop1 { public static void main(String[] args) { int count = 0; while (count < 10) { System.out.println(count); // trace } } } Empty Statements Remember that statements are terminated by a semicolon Is the following a legal statement? ; Known as an empty or null statement Empty statements are a common source of infinite loops For example, what is wrong with the following code? 1 2 3 4 5 6 7 8 9 public class InfiniteLoop2 { public static void main(String[] args) { int count = 0; while (count < 10); { System.out.println(count); // trace count++; } } } Off-By-One Errors A common looping problem is the off-by-one error Often a less-than is confused with a less-than-or-equal-to For example, to sum the even numbers between one and 10, what is wrong with: 1 2 3 4 5 6 7 8 9 public class OffByOne { public static void main(String[] args) { int sum = 0; for (int i = 1; i < 10; i += 2) { sum += i; } System.out.println(sum); } } Tracing Variables One good way to discover loop errors is to trace key variables Tracing variables means watching them change as the program executes You can insert temporary output statements in your program For instance, we had trace statements in the infinite loop examples above What variables would we trace for the off-by-one error example? ^ top 3.2.8: Summary Loops are used to repeat sections of code General loop structure: Initialization code Loop condition -- evaluated during the loop Loop body Several common uses of a loop: Counter-controlled loops Ask Before Repeating Sentinel-controlled loops It is legal code to have one loop nested inside another Several problems that can occur when using a loop including: infinite loops: loops that "never" stop Off-by-one errors: start or stop at a wrong value Thoroughly test loops -- especially at the boundaries of the loop test Tracing variables is a good way to discover loop errors ^ top Exercise 3.2 Take one minute to prepare an answer the following: Recall the code to draw a solid square shown below. What changes would we make to draw a hollow square instead? import java.util.Scanner; public class Square { public static void main(String[] args) { System.out.println("I draw squares for you!\n"); System.out.print("Enter width of square: "); Scanner input = new Scanner(System.in); int size = input.nextInt(); for (int row = 0; row < size; row++) { for (int col = 0; j < size; j++) { System.out.print("*"); } System.out.println(); } } } ^ top 3.3: Strings Learner Outcomes At the end of the lesson the student will be able to: Use Strings in Java programs Convert Strings to primitive types Convert other types to Strings Parse Strings ^ top 3.3.1: About Strings and Characters Recall that a string is a sequence of individual characters that are numbered The following diagram shows how the string Hello is stored Note that the first character of a string is position 0, not 1 String Variables are Objects String data types are actually objects in Java A String variable does not store the characters of the string Instead, a String variable stores the location of a String object in memory For instance: String message; message = new String("Hello"); The first line declares a reference variable of type String The second line creates a String object in memory and assigns the object location to the reference variable The keyword new creates the storage for an object and returns its memory location You can combine the two steps into one line: String message = new String("Hello"); When you use a String variable, like in the statement above, the value of the variable is not displayed Instead, Java finds the location of the object and uses the object itself This is known as dereferencing the variable Unlike some languages, Java automatically dereferences variables when needed Thus, when we write a statement like: System.out.println(message); The content of the object referred to by the String variable gets displayed It is common to refer to a reference variable as if it contains a value This is a convenient mental shorthand However, it is important to understand that this is technically incorrect A reference variable always contains the memory address of where an object is stored We will see a consequence of reference variables shortly ^ top 3.3.2: String Methods Because a String is a class type, it has methods associated with it The syntax for calling a method of a String object is: stringName.methodName(arguments) Where: stringName: the name of the string variable methodName: the name of the method arguments: the input values, if any Once you create a string, you can use its methods Some of the more commonly used methods are shown below For a complete list, see the API for class String Some Commonly-Used Methods of Class String Method Description length() Returns the number of characters in the string as an int. charAt(index) Returns a char at the specified index. equals(other) Compares two strings for equality and returns true if they are equal and false otherwise. equalsIgnoreCase(other) Compares two strings for equality, ignoring upper or lower case, and returns true if they are equal and false otherwise. indexOf(text) Returns the index of the first occurrence of text in the string or -1 if not present. substring(begin, end) Returns a new string with all the characters from begin up to, but not including, end. toLowerCase() Returns a new string with all characters in lowercase. toUpperCase() Returns a new string with all characters in uppercase. trim() Returns a new string with all whitespace removed from the beginning and end. Example Using Some String Methods 1 2 3 4 5 6 7 8 9 10 11 12 13 14 public class StringMethods { public static void main(String[] args) { String greeting = "Hello, World!\n"; System.out.print(greeting); System.out.println(greeting.length()); String sub = greeting.substring(0, 5); System.out.println(sub); // prints "Hello" System.out.print(greeting.toLowerCase()); System.out.print(greeting.toUpperCase()); for (int i = 0; i < greeting.length(); i++) { System.out.println(greeting.charAt(i)); } } } Character Sequence Notice that strings have a sequence: there is a first character, second character, etc. Each character is assigned an index number, starting with 0 and ending with length() - 1: H e l l o , W o r l d ! 0 1 2 3 4 5 6 7 8 9 10 11 12 The charAt() method uses this index in the example: String greeting = "Hello, World!"; for (int i = 0; i < greeting.length(); i++) { System.out.println(greeting.charAt(i)); } The substring(begin, end) method also uses index numbers to select part of a string: String greeting = "Hello, World!"; String sub = greeting.substring(0, 5); System.out.println(sub); // prints "Hello" The begin argument starts with the index number of the first character The end argument is always one past the last character to extract H e l l o , W o r l d ! 0 1 2 3 4 5 6 7 8 9 10 11 12 As another example, we can extract "World" using: String w = greeting.substr(7, 12); System.out.println(w); Which extracts the characters highlighted in the following diagram: H e l l o , W o r l d ! 0 1 2 3 4 5 6 7 8 9 10 11 12 Note that you get a Runtime Exception (error) if you try to select an index lower than 0 or beyond the end of a string For example, the code: char bogus = greeting.charAt(15); produces an error like: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 15         at java.lang.String.charAt(String.java:687)         at StringMethods.main(StringMethods.java:13) ^ top 3.3.3: Comparing Strings "==" does not act like you may think for String objects "==" tests to see if the address of the two objects are the same The reason is that the two strings, s1 and s2, refer to different objects You must use the equals() method to test if the String contents are equal Use equals() when case matters Use equalsIgnoreCase() when case does NOT matter The following example shows the result of using == and the methods equals() and equalsIgnoreCase() Example of Comparing Strings import java.util.*; public class StringComparer { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter string 1: "); String s1 = input.next(); System.out.print("Enter string 2: "); String s2 = input.next(); System.out.println(s1 == s2); System.out.println(s1.equals(s2)); System.out.println(s1.equalsIgnoreCase(s2)); } } More Information Class String: from the Java API String Equality and Interning String interning: from Wikipedia ^ top 3.3.4: Converting Strings Recall that we can easily convert other data types, like int or double, to strings using the + operator If an expression starts with a String type then the compiler converts other types to String automatically For example: System.out.println("The result is: " + 7 + 2); Produces the output: The result is: 72 If you want to add 7 + 2, you need to use parenthesis: System.out.println("The result is: " + (7 + 2)); Explicit Conversions We often need to convert between String to numeric types Java provides a very useful set of methods for converting between strings and primitive types These methods are located in wrapper classes Wrapper class: a class that provides object representation and methods for primitive types. Each of the eight Java primitive types have a wrapper class Each of the classes has conversion methods we can use to convert between a string and the primitive type: The following tables list some commonly used conversion methods Converting Strings to Primitive Values Return Type Conversion Method Example int Integer.parseInt(String) Integer.parseInt("1234") long Long.parseLong(String) Long.parseLong("123456") float Float.parseFloat(String) Float.parseFloat("123.45") double Double.parseDouble(String) Double.parseDouble("123.45") Converting Primitive Values to Strings Type to Convert Conversion Method Example int Integer.toString(int) Integer.toString(1234) long Long.toString(long) Long.toString(123456) float Float.toString(float) Float.toString(123.45) double Double.toString(double) Double.toString(123.45) ^ top 3.3.5: StringBuffer Class After we create a String object, its contents cannot change The immutability of String objects is an important property that we will look at in more depth later However, sometimes we may prefer to work with a mutable (non-constant) string For this we use a StringBuffer instead of a String Like a String object, a StringBuffer represents a sequence of characters However, StringBuffer has mutator methods that can change its sequence of characters For example: StringBuffer str = new StringBuffer("ABCDE"); str.reverse(); Sytem.out.println(str); // displays EDCBA Note that StringBuffer does not return a new String object Instead, it actually modifies the StringBuffer referred to by str More Information Class StringBuffer: from the Java API ^ top 3.3.6: Parsing Strings Sometimes we need to take long strings and divide them into smaller pieces The smaller pieces of the string are referred to as tokens Token: a text element that consists of a sequence of characters such as a word or symbol. The process of splitting a string into tokens is known as parsing: Parse: to split a string of information into its constituent parts. Often strings are split into tokens using whitespace as a delimiter ("divider" character) However, other delimiters are used as well An easy to use technique for tokenizing strings is to use the StringTokenizer class The following shows an example program using a StringTokenizer Example Application Tokenizing a String Using StringTokenizer import java.util.*; class Tokenizer { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a string: "); String data = input.nextLine(); final String DELIMITERS = " ,!.?"; StringTokenizer st = new StringTokenizer(data, DELIMITERS); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); } } } Tokenizing with Scanner Another way to tokenize a string is to use a Scanner By default a Scanner will split strings on whitespace delimiters If you want to split strings on other delimiters, you will need to use a regular expression The following example shows an example program using a Scanner Unfortunately, regular expressions are beyond the scope of this course However, you can study them using the Java Tutorial listed in More Information below Example Application Tokenizing a String Using Scanner import java.util.*; class Tokenizer2 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a string: "); String data = input.nextLine(); final String DELIMITERS = "\\s*\\W+\\s*"; Scanner scanner = new Scanner(data); scanner.useDelimiter(DELIMITERS); while (scanner.hasNext()) { System.out.println(scanner.next()); } } } More Information Class StringTokenizer: from the Java API Parsing: Wikipedia article From StringTokenizer to Scanner: JDC Tech Tip Scanning Text With java.util.Scanner: JDC Tech Tip Lesson: Regular Expressions: from the Java Tutorial ^ top 3.3.7: Summary Strings are a sequence of individual characters and the characters are numbered from 0 to length() - 1: Strings are objects and have methods associated with them We looked at several of these methods including substr() When you compare strings, you need to use the equals() method rather than == string1.equals(string2) You use equalsIgnoreCase() when case does not matter You can convert between a string and a primitive type using one of the wrapper classes for primitive types A String is immutable -- it cannot be changed after it is created If you want to work with a mutable string, then you can use a StringBuffer object: StringBuffer str = new StringBuffer("ABCDE"); str.reverse(); Sytem.out.println(str); // displays EDCBA ^ top Exercise 3.3 Take one minute to prepare an answer the following question: Write a program that determines if a string entered by the user is a palindrome. A palindrome is a string that reads the same forwards and backwards. Ignore case in making the determination. For testing you can use, "Able was I ere I saw Elba", which is palindromic with respect to spacing. One possible answer What could we do to ignore spacing and punctuation in our palindrome tester? ^ top Wrap Up Due Next: A2-Metabolic Energy (2/24/10) A3-ASCII Shapes (3/3/10) ^ top Home | Blackboard | Schedule | Room Policies | Syllabus Help | FAQ's | HowTo's | Links Last Updated: October 05 2010 @14:17:58