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ITSC 1213 - Introduction to CS 2 How to Design Programs with Object Oriented Programming

Nadia Najjar, Dhruv Dhamani

Section 2.7 Boolean Expressions

Boolean variables or expressions can only have true or false values.
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Subsection 2.7.1 Testing Equality (==)

The operators == and != (not equal) can be used to compare values. They return true or false boolean values.
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Note 2.7.1.

One = sign changes the value of a variable. Two == equal signs are used to test if a variable holds a certain value, without changing its value!
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The following code shows how == is used with primitive types like int.
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Checkpoint 2.7.2.

What will the code below print out? Try to guess before you run it! Note that 1 equal sign (=) is used for assigning a value and 2 equal signs (==) for testing values.
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Subsection 2.7.2 Relational Operators (<, >)

The Relational Operators below in Java are used to compare numeric values or arithmetic expressions. Although some programming languages allow using relational operators like < to compare strings, Java only uses these operators for numbers, and uses the string methods compareTo() and equals() for comparing String values.
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If you have trouble telling < and > apart, think of a number line and think of < and > as arrows; < (less than) points towards 0 and smaller numbers on the number line and > (greater than) points towards the larger numbers on the number line. Or remember that < starts with the smaller (less) point and > starts with the open wide (greater) side. With <= and >=, remember to write the two symbols in the order that you would say them “less than” followed by “or equal to”.
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Coding Exercise
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Checkpoint 2.7.3.

Try to guess what the code below will print out before you run it.
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Check your understanding
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Checkpoint 2.7.4.

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Subsection 2.7.3 Testing with mod (%)

Here are some boolean expressions that are very useful in coding and mod is used in many of them:
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// Test if a number is positive
(number > 0)
//Test if a number is negative
(number < 0)
//Test if a number is even by seeing if the remainder is 0 when divided by 2
(number % 2 == 0)
//Test if a number is odd by seeing if there is a remainder when divided by 2
(number % 2 > 0)
//Test if a number is a multiple of x (or divisible by x with no remainder)
(number % x == 0)

Checkpoint 2.7.5.

Try the expressions containing the % operator below to see how they can be used to check for even or odd numbers. All even numbers are divisible (with no remainder) by 2.
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The modulo operator is a powerful tool in programming. Because of its versatility, it’s important to become familiar with how it works and how it can be applied in different problem-solving scenarios
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  • Use it to check for odd or even numbers (num % 2 == 1) is odd and (num % 2 == 0) is even. Actually, you can use it to check if any number is evenly divisible by another (num1 % num2 == 0)
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  • Use it to get the last digit from an integer number (num % 10 = last digit on right).
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  • Use it to get the number of minutes left when you convert to hours (num % 60).
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  • Use it whenever you have limit in the value, and you need to wrap around to the front if the value goes over the limit (num % limit).
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Subsection 2.7.4 Summary

  • Primitive values and reference values can be compared using relational operators (i.e., == and !=) in Java.
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  • Arithmetic expression values can be compared using relational operators (i.e., <, >, <=, >=) in Java.
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  • An expression involving relational operators evaluates to a Boolean value of true or false.
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