Computer Science Programming Basics in Ruby

4.4 If-Then-Else Statements

Ruby uses an if statement for basic conditional control flow.

Gem of Wisdom

An If-Then-Else statement or its equivalent is found in every programming language.

The basic form of an if statement is shown in Example 4-3.

Example 4-3. If statement

    1 if (condition)
    2 	# section 1
    3 end

The condition is a logical expression, as described previously; in Example 4-3, section 1 is executed when the condition evaluates to true, and it is skipped when the condition evaluates to false. An example of using the if statement is shown in Example 4-4. Given a number from the user, the program determines if the value is even. If it is, the program will print “Even” to the screen.

Example 4-4. Determine if the number is even

    1 # if a number is even, print out "Even"
    2 puts "Enter a number"	# print message
    3 number = gets.to_i	# get number as an integer
    4 if (number % 2 == 0)	# check if even
    5 	puts "Even"
    6 end

Assume the user inputs a value of 11. In this case, number will be equal to 11 when we try to evaluate the expression number % 2 == 0. This means that the flow option, shown on line 5, will not execute because the expression 11 % 2 == 0 will evaluate to false since 11 % 2 is equal to 1, not 0. Thus, nothing will be printed to the screen. If the user enters 10, then 10 % 2 == 0 will evaluate to true, meaning puts ''Even'' will be executed; the screen will display “Even.”

The if statement can be extended with the use of the else. The general form for the ...