3. Arithmetic, Strings and Selection

What We Will Cover Reminders: Please silence all cell phones and pagers Sit at a computer station, start your computer and login When class is over, please shut down your computer. Remember that you lose any files not saved in the student folder. Continuations 3.1: Numbers and Arithmetic 3.1.1: Algorithms Using Numbers and Arithmetic 3.1.2: Arithmetic Exercise 3.1 3.1.3: Mixed-Mode Expressions 3.1.4: Integer Division and Modulus 3.1.5: Mathematical Functions 3.1.6: Summary 3.2: Characters and Strings 3.2.1: Type char 3.2.2: Introduction to Strings 3.2.3: String Variables and Simple I/O 3.2.4: Joining Strings (Concatenation) 3.2.5: String Functions 3.2.6: Output of Hard-to-Print Characters Exercise 3.2 3.2.7: Summary 3.3: Making Selections 3.3.1: Teaching Computers to Make Decisions 3.3.2: Using if Statements 3.3.3: Using if-else Statements 3.3.4: The Value of a Relationship 3.3.5: Comparing Characters and Strings Exercise 3.3 3.3.6: Nested if Statements 3.3.7: Summary 3.4: Pair Programming 3.4.1: Solo Development Problems 3.4.2: Pair Programming 3.4.3: Best Practices 3.4.4: Summary Exercise 3.4 Wrap Up Due Next: A2-Implementing an Algorithm (2/25/10) A3-Three Easy Programs (3/4/10) Continuations Questions from last class or reading? Homework Questions? A2-Implementing an Algorithm (2/25/10) Viewing Blackboard assignment and exercise results Viewing due dates in CodeLab Tutoring resources Homework Discussion Questions What was the hardest part of writing the code to implement your algorithm? How useful was the example executable file for determining how the program operated? How did you keep the line length to 80 characters or less? What was the most informative CodeLab tutorial exercise? Quiz Compiling and Running a Program Using TextPad Many text editors have provision for compiling within the editor We use TextPad in our class room as a text editor Note that you can install TextPad at home To make your in-class exercises easier, we have set up TextPad to compile and run C++ programs We will use the following program for demonstration 1 2 3 4 5 6 7 #include <iostream> using namespace std; int main() { cout << "Hello, World!\n"; return 0; } Compiling with TextPad Load your source code into the active TextPad window Select the Tools menu Select Compile C++ If there are any syntax errors, you will see a page or area showing them Otherwise, you will return to your source code page Running Programs with TextPad Load your source code into the active TextPad window Select the Tools menu Select Run C++ Application Your program will run in a console window ^ top 3.1: Numbers and Arithmetic Learner Outcomes At the end of the lesson the student will be able to: Distinguish between an integer and a floating-point number Write C++ code for arithmetic expressions Infer the type returned from a mixed-mode arithmetic expression Construct expressions that use mathematical functions ^ top 3.1.1: Algorithms Using Numbers and Arithmetic Many problems can be solved using mathematical formulas and numbers Consider the simple problem of adding up all the coins in your pocket to get the total value in dollars What would be an algorithm for solving this problem? To implement this algorithm, we will need to use numbers and arithmetic As we saw before, C++ has two general types of numbers: integers and floating-point An integer is zero or any positive or negative number without a decimal point For example: 0 1 -1 +5 -27 1000 -128 We call numbers like these literal integers because they stand for what they look like A floating-point number is any signed or unsigned number with a decimal point For example: 0.0 1.0 -1.1 +5. -6.3 3234.56 0.33 Note that 0.0, 1.0 and +5. are floating-point numbers, but could be rewritten as integers In C++, both integers and floating-point numbers cannot have any commas or special symbols The following program implements an algorithm for summing coins Which characters are integers and which are floating-point numbers? Program coins.cpp to Sum Coins 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 #include <iostream> using namespace std; int main() { int pennies = 8; int nickels = 5; int dimes = 4; int quarters = 3; double total = pennies * 0.01 + nickels * .05 + dimes * 0.10 + quarters * 0.25; cout << "Total value = " << total << "\n"; return 0; } ^ top 3.1.2: Arithmetic C++ uses the following operators for arithmetic: + for addition - for subtraction * for multiplication / for division % for modulus (remainder after division) As in algebra, multiplication and division are performed before addition and subtraction To change the order of operation, you use parentheses For example: is written as: a + b / 2 is written as: (a + b) / 2 Notes About Arithmetic You can use parentheses to group expressions Anything within the parentheses is evaluated first 2 * (10 + 5) You can have parentheses within parentheses Innermost parenthesis evaluated first (2 * (10 + 5)) Parentheses cannot be used to indicate multiplication Invalid expression: (2)(3) Must use the * operator Programming Style Programming style: add spaces around binary operators 2 + 3, not 2+3 Programming style: no spacing after opening or before closing parenthesis (2 / 3), not ( 2/3 ) We explore how you can use numbers and arithmetic in the following exercise. ^ top Exercise 3.1 Through the miracles of computer science, we will now convert your $1000 computer into a $10 calculator! Along the way, we learn how to work with arithmetic using C++. Specifications Type the following program into a text editor, save it as arithmetic.cpp, and then compile and run the starter program to make sure you typed it correctly. In main(), declare two double variables named a and b, and assign them a value of 7 and 2 respectively. For instance: double a = 7, b = 2; Add a line of code to display the arithmetic expression (a + b) and then recompile and run the program. cout << "a + b = " << a + b << endl; The output when you run the program should look like this: a + b = 9 If you do not see this output, please ask a classmate or the instructor for help. Add three more lines of code like the previous one that computes the expressions: a - b, a * b and a / b. Compile and run your program again and make sure your program now displays the following output: a + b = 9 a - b = 5 a * b = 14 a / b = 3.5 The order of operations matters in C++ just like it does in algebra. Multiplication and division are performed before addition and subtraction. Add the following two statements to your program: cout << "a + b / 2 = " << a + b / 2 << endl; cout << "(a + b) / 2 = " << (a + b) / 2 << endl; Compile and run your program again and compare the output. Your program should now display the following output: a + b = 9 a - b = 5 a * b = 14 a / b = 3.5 a + b / 2 = 8 (a + b) / 2 = 4.5 Note how the output of the two statements is different. You can change the order of operation using parenthesis, just like in algebra. For more information on the order of operations see section: 3.1.2: Arithmetic. As you can see, arithmetic in C++ works much like you would expect. However, there are some mysteries when working with integer variables which we will now explore: Truncation in integer division Modulus (%) operator Truncation in integer division: Change the data type of the two variables from double to int, like this: int a = 7, b = 2; Compile and run your program again and compare the output. Note how the result of the division operation changed. What happened to the decimal part of the result? In programming terms, we say that the decimal part is truncated (cut short). You have to watch out for this in C++ programming or you may get unexpected results in your calculations. For more information see section: 3.1.4: Integer Division and Modulus. Modulus (%) operator: Sometimes we want the integer remainder from an integer division. To see the integer remainder, we use the modulus (%) operator. Add the following statement to your program: cout << "a % b = " << a % b << endl; Compile and run your program again with this added statement. Your program should now display the following output: a + b = 9 a - b = 5 a * b = 14 a / b = 3 a + b / 2 = 8 (a + b) / 2 = 4 a % b = 1 For more information on the modulus operator see section: 3.1.4: Integer Division and Modulus. Mathematical functions: More complex mathematical operations require the use of a function in C++. one such function is sqrt(number) which calculates the square root of the number inside the parenthesis. Add the following statement to your program: cout << "sqrt(a + b) = " << sqrt(a + b) << endl; You program will not compile with this new statement because you must include a library of the mathematical functions. Add the statement: #include <cmath> to the top of your program like this: #include <iostream> #include <cmath> // math function library using namespace std; Compile and run your program again with this added statement. Your program should now compile and display the following output when run: a + b = 9 a - b = 5 a * b = 14 a / b = 3 a + b / 2 = 8 (a + b) / 2 = 4 a % b = 1 sqrt(a + b) = 3 For more information on mathematical functions see section: 3.1.5: Mathematical Functions. Submit your program source code that displays all eight (8) expressions to Blackboard as part of assignment 3. Completed Program Once you are finished, your source code should look like the following: As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 3.1.6: Summary. ^ top 3.1.3: Mixed-Mode Expressions Recall that different data types are stored in different forms An int is stored in 4 bytes A double is stored in 8 bytes The format they are stored in is different as well The computer needs both operands in the same form before it can perform an operation If one operand is different than the other, the compiler converts it to the wider of the two types For example: 2 + 2.3 First number (2) is an int Second number (2.3) is a double C++ will automatically convert and int to a double Then the arithmetic operation can take place to produce a result of 3.3 Remember that the result of arithmetic with an int and a double is a double ^ top 3.1.4: Integer Division and Modulus Dividing two integers can produce unexpected results for the unwary In division, if at least one of the numbers is a floating-point number, the result is a floating point number: 7.0 / 2.0 // 3.4 7 / 2.0 // 3.4 7.0 / 2 // 3.4 However, if both numbers are integers, then the result is an integer: 7 / 2 // 3 The decimal remainder is truncated (cut short, discarded, thrown away) To get the integer remainder of division between two integers, you use the modulus operator: % 7 % 2 // 1 (remainder) 7 % 2 returns 1 because 1 is the remainder when 7 is divided by 2: 3 r 1 2 ) 7 -6 1 remainder For a refresher on remainders see: Long Division with Remainders Check Yourself What is the result of the following arithmetic operations? 9 / 4 17 / 3 14 / 2 9 % 4 17 % 3 14 % 2 ^ top 3.1.5: Mathematical Functions Operators provide only the simplest mathematical operations For more complex operations, we use mathematical functions A C++ function is like a mathematical function that takes an argument ("input") and produces ("returns") a value For example: cout << sqrt(9.0) << endl; In the above example, the input is 9.0 and the sqrt() function returns the square root of the argument C++ has a standard library named cmath that contains many such functions Some of the functions are listed below Note that many of these functions are designed to work with floating-point date types like double For example, pow() will not compile if the first term is not a floating-point number Name Description Example Result fabs absolute value fabs(-3.9) fabs(3.9) 3.9 3.9 exp exponent (ex) exp(1.0) 2.71828 log natural log log(10.0) 3.10259 pow powers (xy) pow(2.0, 3.0) 8 sqrt square root sqrt(4.0) 2 sin sine sin(0) 0 cos cosine cos(0) 1 In addition, the cmath library includes two similar functions: ceil, and floor Name Description Example Result ceil ceiling: round up ceil(3.3) ceil(3.7) 4 4 floor floor: round down floor(3.3) floor(3.7) 3 3 Both return whole numbers, although they are of type double Using Mathematical Functions How are mathematical functions evaluated? Whatever is within the parenthesis of the function call is evaluated first Thus, in the following example, we get the square root of 9.0 cout << sqrt(3.0 * 3) << endl; If the function is used in an arithmetic expression, they are handled just like a number of the type returned For example, in the following, the value 4.0 is stored in the double variable num: double num = 1 + sqrt(3.0 * 3.0); cout << num << endl; Note that the function evaluates the sqrt(3.0 * 3) before adding it to 1.0 Thus functions have a higher precedence than arithmetic operators ^ top 3.1.6: Summary C++ uses the following operators for arithmetic: + for addition - for subtraction * for multiplication / for division % for modulus (remainder) As in algebra, multiplication and division are performed before addition and subtraction To change the order of operation, you use parenthesis The results of integer division are truncated You must use modulus operator (%) to get the remainder value More complex mathematical operations are available using the cmath library cout << sqrt(3.0 * 3) << endl; Check Yourself How can you tell the difference between an integer and a floating-point number? (3.1.1) What are the five operators C++ provides for arithmetic? (3.1.2) What is the first operation performed in the expression: 1 + 2 * 3 / 4 % 5? (3.1.2) What is the data type returned by the following expression? (3.1.3) 3 + 4.5 What happens to the remainder during integer division? (3.1.4) What operator can you use to compute the integer remainder? (3.1.4) What is the result of the following arithmetic expressions? (3.1.4) 5 / 4 5 % 4 What is a function? (3.1.5) What code do you write to calculate the square root of the number 49? (3.1.5) ^ top 3.2: Characters and Strings Learner Outcomes At the end of the lesson the student will be able to: Identify characters and strings from their literal representation Assign characters and strings to variables Concatenate strings Collect characters and strings from user input Output characters and strings ^ top 3.2.1: Type char In addition to numbers, computers can manipulate text and other non-numerical types Values of type char (short for character) are a single letter, number or special symbol You specify a character by enclosing it in single quotes (') The quote marks are not part of the data For example: 'a' 'b' 'Z' '3' 'q' '$' '*' When you use a char data type, you store the character using an ASCII code ASCII is a coding method that assigns a number to every character You can see the codes in an ASCII Table Declaring and Assigning char Variables As with other data types, you must declare char variables before use: char letter; You assign values to a char variable using the equals sign: letter = 'A'; Just like numerical types, you can combine declaration and assignment into one statement: char letter = 'A'; Also like numerical types, you can declare multiple variables on one line: char letter, letterA = 'A', letterB = 'B'; User I/O with Type char Like numbers, you can output type char using cout << char letter = 'A'; cout << letter << 'B' << endl; Also, you can input type char using cin >> cin >> letter; cout << letter << endl; ^ top 3.2.2: Introduction to Strings In addition to single characters, computers can work with text strings For example, in the following the characters between the double quotes are displayed as text: cout << "Hello World!" << endl; Programmers refer to text like this as a string because it is composed of a sequence of characters that we string together C++ provides the string type so we can work with text To work with the string type you need to include the string library: #include <string> using namespace std; This library is included automatically with <iostream> in our version of g++ Strings are enclosed in double quotes, which are not part of the string For example: "Hello" "b" "3.14159" "$3.95" "My name is Ed" Notice that the string "3.14159" could be expressed as a double by removing the quotes However, a computer stores these two values very differently and we must use them in different ways For instance, we cannot multiply the "3.14159" by 2, but we can when it is expressed as a double: cout << "3.14159" * 2; // NO! cout << 3.14159 * 2; // allowed ^ top 3.2.3: String Variables and Simple I/O We declare and assign values to string variables like numeric types For example: string firstName; // declaration firstName = "Edward"; // assignment string lastName = "Parrish"; // declaration + assignment cout << firstName << " " << lastName << endl; Simple I/O with Strings Like numbers, you can output type string using cout << Also, you can input a string using cin >> stringName For example: string name; cout << "Enter your name: "; cin >> name; cout << "You entered: " << name << endl; The cin statement assigns the user input to the string variable name ^ top 3.2.4: Joining Strings (Concatenation) You can join two strings together using the '+' operator The join operation is called concatenation For example: string s1 = "Hello", s2 = "World!"; string s3 = s1 + s2; cout << s3 << endl; The string s3 now has the contents of both s1 and s2 You can also mix string variables and literal strings: string s1 = "Hello", s2 = "World!"; string s3 = s1 + ", " + s2; cout << s3 << endl; One or both strings surrounding the + must be a string variable For instance, the following will NOT work: string greeting = "Hello" + " " + "World!"; // No! However, this is not usually a problem because we can just make one long literal string: string greeting = "Hello World!"; ^ top 3.2.5: String Functions Strings are a special type of variable called objects, which we will study in more detail later in the course An object is a data type that can have functions associated with it These functions are called member functions and are called using dot notation The syntax for calling a member function of a string object is: stringName.functionName(arguments) Where: stringName: the name of the string variable functionName: the name of the member function arguments: the input values, if any Once you create a string variable, you can use its member functions Some Commonly-Used Functions length(): Returns the number of characters in a string string str = "Hello"; cout << "The number of characters is " << str.length() << ".\n"; substr(i, n): Returns a substring of length n starting at index i string greeting = "Hello, World!\n"; string sub = greeting.substr(0, 4); cout << sub << endl; The position numbers in a string start at 0. The last character is always one less than the length of the string H e l l o , W o r l d ! \n 0 1 2 3 4 5 6 7 8 9 10 11 12 13 string w = greeting.substr(7, 5); H e l l o , W o r l d ! \n 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Example Using String Functions Consider the problem of extracting the initials from a person's name What would be an algorithm for solving this problem? To implement this algorithm, we can use the string function substr() The following program implements an algorithm for extracting the intials from a person's name What other technique could you use to extract the initials? Program to Create Initials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 #include <iostream> #include <string> using namespace std; int main() { string first; string middle; string last; cout << "Enter your full name (first middle last): "; cin >> first >> middle >> last; string initials = first.substr(0, 1) + middle.substr(0, 1) + last.substr(0, 1); cout << "Your initials are " << initials << "\n"; return 0; } ^ top 3.2.6: Output of Hard-to-Print Characters We have been using the << operator to output strings string msg = "Hello Mom!"; cout << msg << endl; However, some characters are more difficult to output than others For example, what would the compiler do with the following statement? cout << "Say, "Hey!"" << endl; Some characters cannot be output directly in a string Also, the first 32 ASCII characters are not visible on our monitors Known as control characters because they control the output device Even though these characters are not visible, we sometimes need to use them For example, a newline character We need some way to "print" invisible and hard-to-print characters Escape Sequences C++ can access control codes and hard-to-print characters using escape sequences Backslash (\) directly in front of a certain character tells the compiler to escape from the normal interpretation The following table has some nonprinting and hard-to-print characters: Sequence Meaning \a Alert (sound alert noise) \b Backspace \f Form feed \n New line \r Carriage return \t Horizontal tab \v Vertical tab \\ Backslash \' Single quote \" Double quote \ooo ASCII character in octal notation \xhhh ASCII character in hexadecimal notation Some examples: cout << '\a' << endl; // alert cout << '\n' << endl; cout << "Left \t Right" << endl; cout << "one\ntwo\nthree" << endl; Programming Style No tab characters in your source code We explore how you can use strings in the next exercise. ^ top Exercise 3.2 In this exercise we write an interactive program using strings that runs like this: First name: Ed Last name: Parrish Welcome "Ed Parrish"! Your initials: EP Note that the underlined font shows what is typed by the user. As you work through the exercise, I suggest that you compile after each step so you know where the error is located if you make a mistake. Also, if you get stuck then ask a classmate or the instructor for help. Specifications Copy the following program into a text editor, save it as nameapp.cpp, and then compile and run the starter program to make sure you copied it correctly. #include <iostream> using namespace std; int main() { // Enter your code here return 0; } In main(), declare three string variables, firstName, lastName, fullName, like this: Add two statements: (1) to prompt the user for the data they should enter and (2) to collect the user input and store it in the firstName variable. For instance: cout << "First name: "; cin >> firstName; Add two more statements like these to collect the last name and store the input in the lastName variable. Write a line of code to concatenate (join) the first and last names and assign them to the variable fullName like this: For more information see section: 3.2.4: Joining Strings (Concatenation) Finally, add code to your program to output the fullName variable using cout: cout << "Full name: " << fullName << "!\n"; Compile and run your program to make sure it works correctly like this: First name: Ed Last name: Parrish Full name: Ed Parrish! Some text characters are hard to print, like a double quote ("). To print these we must escape them from their special meaning by putting a backslash (\) in front of them. Put a double quote mark around the full name by changing the line printing the full name to: cout << "Welcome \"" << fullName << "\"!\n"; For more information on escaping characters see section: 3.2.6: Output of Hard-to-Print Characters You can extract parts of a string variable using the substr() function. Extract the first letter from the first and last name to create initials with the following code: string initials = firstName.substr(0, 1) + lastName.substr(0, 1); cout << "Your initials: " << initials << endl; For more information about string functions see: 3.2.5: String Functions Compile and run your program to make sure it works correctly like this: First name: Ed Last name: Parrish Welcome "Ed Parrish"! Your initials: EP Submit your program source code to Blackboard as part of assignment 3. Completed Program Once you are finished, your source code should look like the following: As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 3.2.7: Summary. ^ top 3.2.7: Summary A character is a letter, number or special symbol You make character literals by enclosing a single character in single quotes You declare character variables using char as the data type: char letter = 'A'; Each character is stored as a number, using its ASCII code You can input and output char data using cin and cout like integer data types You make string literals by enclosing characters in double quotes You declare string variables using string as the data type: string s1 = "Hello Mom!"; To concatenate two strings, use the "+" operator: string s2 = s1 + " suffix"; Type string can be input and output with cin and cout You use functions of the string object for some operations We looked at the member functions: length(): Returns the number of characters in a string string s = "Hello"; cout << s.length() << endl; substr(i, n): Returns a substring of length n starting at index i string greeting = "Hello, World!\n"; string sub = greeting.substr(0, 4); cout << sub << endl; Check Yourself Answer these questions to check your understanding. You can find more information by following the links after the question. What type of delimiters are used to enclose single characters of type char? (3.2.1) What code do you write to declare a char variable named myChar and assign it a value of 'A'? (3.2.1) What code do you write to input a single character from the keyboard and store it in a variable named myChar? (3.2.1) What type of delimiters are used to enclose strings? (3.2.2) Are "A" and 'A' the same? Why or why not? (3.2.2) When would you want to use a string variable rather than a char variable? (3.2.2) What code do you write to define a string variable named myString and assign it a value of, "Hi Mom!"? (3.2.3) What code do you write to input a word from the keyboard and store it in a variable named myWord? (3.2.3) What operator is used to join two strings? (3.2.4) For a string variable named str, what code do you write to determine the number of characters in the string? (3.2.5) What function do you use to find a substring in a string variable? (3.2.5) Certain characters like double-quote marks (") are hard to print. How do you output these characters in C++? (3.2.6) What is the escape sequence for a newline? (3.2.6) ^ top 3.3: Making Computers Smart Learner Outcomes At the end of the lesson the student will be able to: Discuss what is meant by the term flow of control Make use of relational expressions to make decisions Implement decisions using if statements Compare numbers, characters and strings Develop strategies for processing input and handling errors ^ top 3.3.1: Teaching Computers to Make Decisions The programs we have worked with so far simply execute a list of instructions We arrange our command statements in a particular order to get the result we want When the program reaches the end of the list of instruction, it stops Except for differences in the input, these programs always work the same way To create more interesting and useful programs, we need our programs to make decisions and carry out different actions For example, if the user enters an incorrect value, we want our program to warn the user with an error message However, if the user enters a correct value, we do not want to display an error message For our programs to make decisions and carry out different actions, we use statements to change the flow of control Flow of control (or control flow) refers to the order in which programs execute instructions By changing the flow of control based on a test condition, we can teach the computer to make decisions We will look at two major ways to change the flow of control: Conditional (selection) statements Loops (repetition) statements In this section, we will look at how to use the conditional statements: if if...else Later will look at more selection statements and how to use the loop statements ^ top 3.3.2: Using if Statements The simplest control-flow statement is an if statement We use an if statement to select whether or not to execute a set of statements An if statement has two parts: a test and a body The body can have zero or more statements The statements in the body execute if and only if the test evaluates to true If the test condition evaluates to false, the computer skips the code Syntax: if (test) { statement1 statement2 ... } Where: test: the test condition to evaluate statementX: the statements to execute depending on the test For example: if (7 == guess) { cout << "*** Correct! ***" << endl; } For clarity: Write the if on a different line than the executed block Indent statements 3-4 spaces within the curly braces You can see an example of an if statement in the following example Example Program With an if Statement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 #include <iostream> using namespace std; int main() { int guess = 0; cout << "I'm thinking of a number between" << " 1 and 10.\nCan you guess it?\n\n" << "Enter your guess: "; cin >> guess; if (7 == guess) { cout << "*** Correct! ***" << endl; } return 0; } About those Curly Braces Technically, the if statement affects only the single statement that follows We use curly braces to make that one statement into a block of statements This allows us to put any number of statements within the body Curly braces are not always required, but the best practice is to always include them ^ top 3.3.3: Using if-else Statements Sometimes we want to choose between two actions If a condition is true then do this Otherwise it is false so do something else To make this type of selection we use an if...else statement Syntax: if (test) { statements1 } else { statements2 } Where: test: the test condition to evaluate statementsX: the statements to execute depending on the test For example: if (7 == guess) { cout << "*** Correct! ***\n"; } else { cout << "Sorry, that is not correct.\n"; cout << "Try again.\n"; } Note that there is no test condition for the else clause The decision on which set of statements to use depends on only one condition Note that you could write an if-else as a pair of complementary if statements instead, like: if (7 == guess) { cout << "*** Correct! ***\n"; } if (7 != guess) { cout << "Sorry, that is not correct.\n"; cout << "Try again.\n"; } However, it is easier and clearer to write an if-else statement instead For clarity, write the if and else parts on different lines than the other statements Also, indent the other statements You can see an example of an if-else statement in the following example Example Program With an if-else Statement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 #include <iostream> using namespace std; int main() { int guess = 0; cout << "I'm thinking of a number between" << " 1 and 10.\nCan you guess it?\n\n" << "Enter your guess: "; cin >> guess; if (7 == guess) { cout << "*** Correct! ***\n"; } else { cout << "Sorry, that is not correct.\n"; cout << "Try again.\n"; } return 0; } Formatting the if Statement It is important to format the if statement professionally if (7 == guess) { cout << "*** Correct! ***\n"; } else { cout << "Sorry, that is not correct.\n"; cout << "Try again.\n"; } Note how the conditional code is indented inside both the if and else portions This lets us easily see which code is conditional and which is not Also note the placement of curly braces Different groups have different practices for placing curly braces for placing curly braces of if and if-else statements In practice, you should use the style dictated by your group's policy Or your professor's instructions For the acceptable styles for this course see my instructions on: Curly Braces ^ top 3.3.4: The Value of a Relationship To make a selection 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 write a relational expression A relational expression uses a relational operator to compare two entities like numbers or variables For example: You can see the relational operators of C++ in the following table Relational Operators Math Name C++ 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. Check Yourself Formulate the following test conditions in C++: x is positive x is negative x is zero x is zero or negative x is not equal to y x is greater than y plus one ^ top 3.3.5: Comparing Characters and Strings Character data can be evaluated using relational operators as well Comparing characters works because C++ stores characters as numbers using ASCII codes Note that letters nearer to the start of the alphabet have lower numerical values Thus a numerical comparison can decide the alphabetical order of characters Example Program Comparing Characters 1 2 3 4 5 6 7 8 9 10 11 12 #include<iostream> using namespace std; int main() { cout << boolalpha; // output true or false cout << "'A' < 'B': " << ('A' < 'B') << endl; cout << "'A' > 'B': " << ('A' > 'B') << endl; cout << "'A' <= 'Z': " << ('A' <= 'Z') << endl; cout << "'X' >= 'Y': " << ('X' >= 'Y') << endl; cout << "'X' == 'X': " << ('X' == 'X') << endl; cout << "'X' != 'X': " << ('X' != 'X') << endl; } Comparing Strings We can compare strings using relational operators as well C++ compares two strings using lexicographical order (a.k.a. alphabetic order) For example, "car" is less than "cat": c a r c a t Also, "car" is less than "card" c a r c a r d Example Program Comparing Strings 1 2 3 4 5 6 7 8 9 10 11 #include<iostream> using namespace std; int main() { string s1, s2; cout << "Enter two strings: "; cin >> s1 >> s2; cout << boolalpha; // output true or false cout << "s1 <= s2: " << (s1 <= s2) << endl; cout << "s1 > s2: " << (s1 > s2) << endl; } More Information ASCII: Wikipedia article Table of ASCII Characters: for converting from ASCII to numbers Binary Numbers: YouTube video from Computer Science Unplugged Binary Number System: Notes on how to use binary numbers String operators: Available operators in C++ ^ top Exercise 3.3 In this exercise we explore the use of relational operators with if statements to create a simple game. Specifications Copy the following program into a text editor, save it as selection.cpp, and then compile and run the starter program to make sure you copied it correctly. #include <iostream> using namespace std; int main() { int guess = 0; cout << "I'm thinking of a number between" << " 1 and 10.\nCan you guess it?\n\n" << "Enter your guess: "; cin >> guess; cout << "You entered: " << guess << endl; // Insert new statements here return 0; } We want to let the user know if they entered a correct value. For this we need to add an if statement such as: if (7 == guess) { cout << "*** Correct! ***" << endl; } Statements inside the curly braces only execute if the test condition in the parenthesis, (7 == guess), evaluates to true. For more information, see section: 3.3.2: Using if Statements. Compile and run your program again and verify the output looks like: I'm thinking of a number between 1 and 10. Can you guess it? Enter your guess: 7 You entered: 7 *** Correct! *** If you rerun the program and enter a number different than 7 (like 9) then the message saying correct will NOT appear. For a friendlier game output, we should give a message when the user enters an incorrect value. For this we need to replace our if statement with an if-else statement like: if (7 == guess) { cout << "*** Correct! ***\n"; } else { cout << "Sorry, that is not correct.\n"; cout << "Please try again.\n"; } Statements inside the curly braces of the else clause only execute if the test condition in the parenthesis, (7 == guess), evaluates to false. For more information, see section: 3.3.3: Using if-else Statements. Compile and run your program again and verify the output looks like: I'm thinking of a number between 1 and 10. Can you guess it? Enter your guess: 9 You entered: 9 Sorry, that is not correct. Rerun and try again. The error message should appear for any number other than the correct guess. One problem with our program is that a user may enter numbers outside the range of 1 through 10. We can test for this condition with one or more if statements. Add this code to your program after the input statement and before the other if statements: if (guess < 1) { cout << "Error: guess must be >= 1\n"; return 1; } Checking user input is a common use of if statements. This type of code is known as input validation or input verification. Compile and run your program again and verify the output looks like: I'm thinking of a number between 1 and 10. Can you guess it? Enter your guess: 0 You entered: 0 Error: guess must be >= 1 The error message should appear for any number that is less than one. You can nest if statements inside either the if or else clause. We can use this when we have a series of tests to make. Replace all the if and if-else statements with the following code: if (guess < 1) { cout << "Error: guess must be >= 1\n"; } else if (guess > 10) { cout << "Error: guess must be <= 10\n"; } else if (guess != 7) { cout << "Sorry, that is not correct.\n"; } else { cout << "*** Correct! ***\n"; } Conditions are checked in order until the one of the conditions evaluates to true. Once one condition evaluates to true, the rest of the tests are skipped. If no test condition evaluates to true then the else clause executes as the default case. For more information, see section: 3.3.6: Nested if Statements. Compile your program and run it several times to see what message you get when you enter each of the following as a guess: 0 7 9 Review the purpose of each line of code with another student in the class. Then add a comment to the top of the file that contains the name of the person with whom you reviewed the code, like: // Reviewed with Jane Programmer Submit your program source code to Blackboard as part of assignment 3. As time permits, read the following sections and be prepared to answer the Check Yourself questions in the section: 3.3.7: Summary. ^ top 3.3.6: Nested if Statements Note that you can include if statements within other if statements The inner if statement is evaluated only if the test condition of the outer if test first evaluates to true The following code shows an example of a nested if statement Example Showing a Nested if Statement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 #include <iostream> using namespace std; int main() { int guess = 0; cout << "I'm thinking of a number between" << " 1 and 10.\nCan you guess it?\n\n" << "Enter your guess: "; cin >> guess; if (guess != 7) { if (guess < 7) { cout << "Your guess is too low.\n"; } else { cout << "Your guess is too high.\n"; } } else { cout << "*** Correct! ***\n"; } return 0; } Nesting in the else Clause You can also nest if statements in the else clause When used this way, the computer can make only only one selection As soon as a condition is found to be true, the rest of the selections are ignored The following code shows an example of a nested else if statement For example: if (guess < 7) { cout << "Your guess is too low.\n"; } else if (guess > 7) { cout << "Your guess is too high.\n"; } else { cout << "*** Correct! ***\n"; } The trick in understanding this type of logic is to remember: You start at the top The computer makes only one selection Once the selection is made and processes, the computer skips the rest of the options Example Showing a Nested else if Statement 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 #include <iostream> using namespace std; int main() { int guess = 0; cout << "I'm thinking of a number between" << " 1 and 10.\nCan you guess it?\n\n" << "Enter your guess: "; cin >> guess; if (guess < 7) { cout << "Your guess is too low.\n"; } else if (guess > 7) { cout << "Your guess is too high.\n"; } else { cout << "*** Correct! ***\n"; } return 0; } Programming Style: Indentation of if-else-if Statements Note the alignment of the nested statements below: if (guess < 7) { cout << "Your guess is too low.\n"; } else { if (guess > 7) { cout << "Your guess is too high.\n"; } else { cout << "*** Correct! ***\n"; } } The above style is WRONG Instead, we use: if (guess < 7) { cout << "Your guess is too low.\n"; } else if (guess > 7) { cout << "Your guess is too high.\n"; } else { cout << "*** Correct! ***\n"; } This shows more clearly that we are making a single choice among multiple alternatives Also, it prevents indentations from cascading to the right as we add more selections ^ top 3.3.7: Summary All selection and repetition statements have a test condition This test condition controls the operation of the statement Relational operators are a common way to create a test condition In this section we looked at how to implement decisions using an if statement An if statement has two parts: a test and a body For example: if (7 == guess) { cout << "*** Correct! ***" << endl; } The if statement executes a block of code only if the test evaluates to true It does nothing if the conditional expression evaluates to false We can execute statements when the condition is false by using an else clause For example: if (7 == guess) { cout << "*** Correct! ***\n"; } else { cout << "Sorry, that is not correct.\n"; } We often implement test conditions using relational operators to create a relational expression Relational operators include: == != < <= > >= We can use these relational operators with numbers, characters and strings One common use of if statements is validation of user input We can test the type of the input by using: if (cin.fail()) We looked at alternate strategies for testing the input type as well In addition to checking the type, good programs validate the input values For instance, if we want the user to enter a number greater than or equal to 1: if (guess < 1) { cout << "Error: guess must be >= 1\n"; return 1; } Also we noted that you can nest if statements within another if or else Check Yourself As time permits, be prepared to answer these questions. You can find more information by following the links after the question. What is meant by the term "flow of control"? (3.3.1) What is the default flow-of-control operation after a statement finishes executing? (3.3.1) What part of an if statement is indented? (3.3.2) What is the value of x after the following code segment? (3.3.2) int x = 5; if (x > 3) { x = x - 2; } else { x = x + 2; } What is wrong with each of the following if statements after executing the code: (3.3.2) int guess; cout << "Enter your guess: "; cin >> guess; if (7 == guess) then cout << "You guessed 7!\n"; if (guess = 7) cout << "You guessed 7!\n"; if ("7" == guess) cout << "You guessed 7!\n"; Does the syntax for an else clause include a test condition? (3.3.3) What is a relational expression and why are they used? (3.3.4) What is the output of the following code fragment? Why? (see textbook pg. 93) cout.precision(17); double r = sqrt(2); if (r * r == 2) { cout << "sqrt(2) squared is 2\n"; } else { cout << "sqrt(2) squared is not 2 but " << (r * r) << endl; } For the above example code fragment, what is a better test condition than (r * r == 2)? Of the following pairs of strings, which comes first in lexicographic order? (3.3.5) "Harry", "Potter" "Harry", "Hairy" "car", "C++" "car", "Car" "car model", "carburetor" True or false? You can nest if statements in either the if clause, the else clause, or both. (3.3.6) If you have a series of test conditions, and only one can be correct, what is the sequence of if and else statement you should follow? (3.3.6) ^ top 3.4: Pair Programming Learner Outcomes At the end of the lesson the student will be able to: Describe the steps for writing a program Discuss the pros and cons of pair programming ^ top 3.4.1: Solo Development Problems Developing programs on your own is often difficult Sometimes you do not know where to start While developing, you run into problems you must solve You can get some help from others without cheating (see Working Together) However, you have to write all the code on your own When you write code, you produce many bugs without noticing them Then you test your program and find it does not work correctly Sometimes, you just cannot see the problem in your program It seems there must be a better way to learn programming One solution is to use pair programming ^ top 3.4.2: Pair Programming Pair programming is a style of programming in which two people work together on one computer at the same time: Exactly two people: not one nor three or more Exactly one computer: not two or more One person enters the code and the other reviews each line of code as it is entered The person using the mouse and keyboard is called the driver The person reviewing is called the navigator In addition to reviewing, the navigator: Analyzes the design and code to prevent errors Looks up reference materials like program syntax Each person "drives" about half the time: Physically get up and move positions when switching roles At most 25% of your time is spent working alone: Any work done alone is reviewed by the other person The objective is to work together and to learn from each other You cannot divide the work into two pieces with each partner working on a separate piece If you are not both engaged in the process, you will not learn the material More information: Pair Programming for Homework Assignments Why Pair Program? Students who pair program report: Higher confidence in a program solution More satisfaction with programming Instructors report higher completion and passing rates Video Explaining Pair Programming Introduction to Pair Programming: a professionally developed video (10 minutes) More Information The Impact of Pair Programming on Student Performance, Perception and Persistence ^ top 3.4.3: Best Practices You may choose any other student in this class for a partner You may NOT program with a person from another class Both of your names appear on the assignment When choosing partners and working together, certain practices help you perform better Pair-programmers are usually more successful when they have similar experience levels However, pair programming can work with partners of different experience levels The more experienced partner must be ready to mentor rather than just develop the program You should find a partner with whom you can easily exchange ideas If you cannot work easily with someone, you should get another partner or work by yourself Getting Along When working with other people, you need to get along with them For an easy 10-minute review of how to get along, see All I Really Need to Know about Pair Programming I Learned In Kindergarten More Information All I Really Need to Know about Pair Programming I Learned In Kindergarten Experimenting with Pair Programming in the Classroom Guidelines for the Use of Pair Programming in a Freshman Programming Class Real programmers use sign language: YouTube Video ^ top 3.4.4: Summary Programming is about solving problems using a computer program Generally, you should develop programs in two phases: Problem-solving phase: the output is a working algorithm Implementation phase: the output is a working program Since defining problems, developing algorithms and implementing algorithms is hard, you can work with a partner But only if you follow the rules of pair programming Use some care when choosing a pair-programming partner to maximize your success In addition, both you and your partner should read: All I Really Need to Know about Pair Programming I Learned In Kindergarten Check Yourself Answer these questions to check your understanding. You can find more information by following the links after the question. What are three problems with solo programming? (3.4.1) What are the rules of pair programming? (3.4.2) Why should you change who is driving when you pair-program? (3.4.2) What are the jobs of the navigator? (3.4.2) How do you make sure that both people learn the material when pair programming? (3.4.2) Why do the rules of pair-programming require you to use only one computer at a time to edit and compile code? (3.4.2) What is the key factor for success in using pair programming? (3.4.3) ^ top Exercise 3.4 In this exercise we see who might be a compatible pair-programming partner for the next homework assignment. Specifications We will count off and divide into separate groups Within your group, exchange names and email addresses, and then write down everyone's information. Next to each name, write the amount of programming knowledge and experience for that student using the following scale: Absolute beginner Some programming knowledge Can program in another programming language Can program in C++ Save all the information you collect in a file named students.txt. Note that you can choose to have one person in your group record the information and email it to all the students in the group. However, every student must submit the same list of students. Submit the students.txt file to Blackboard as part of assignment 3. ^ top Wrap Up Due Next: A2-Implementing an Algorithm (2/25/10) A3-Three Easy Programs (3/4/10) When class is over, please shut down your computer You may complete unfinished exercises at the end of the class or at any time before the next class. ^ top Home | Blackboard | Day Schedule | Eve Schedule Syllabus | Help | FAQ's | HowTo's | Links Last Updated: February 21 2010 @22:24:24