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Boolean expressions, part 1: Compare operators

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• Compare operators
  • Compare operators

    Compare operators compare 2 numerical values and return a Boolean (logical) value A compare operator will return the value true if the test is successful A compare operator will return the value false if the test is unsuccessful

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  • Compare operators in Java:

    Operator symbol	Example	Meaning
    <	a < b	Returns true if a < b, otherwise returns false
    <=	a <= b	Returns true if a ≤ b, otherwise returns false
    >	a > b	Returns true if a > b, otherwise returns false
    >=	a >= b	Returns true if a ≥ b, otherwise returns false
    ==	a == b	Returns true if a is equal to b, otherwise returns false
    !=	a != b	Returns true if a is not equal to b, otherwise returns false

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• Example program: test divisibility
  • Problem description:

    Write a Java program that reads in a number a and a number b The program print a message when a is divisible by b

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  • Algorithm:

    A number a is divisible by the number b if and only if: The remainder of the division a/b is equal to 0

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  • Java program:

    import java.util.Scanner; public class Divisible { public static void main(String[] args) { int a, b; Scanner in = new Scanner(System.in); // Construct Scanner object System.out.print("Enter a: "); a = in.nextInt(); // Read in number into a System.out.print("Enter b: "); b = in.nextInt(); // Read in number into b if ( (a % b) == 0 ) System.out.println(a + " is divisible by " + b); } }

    Explanation:

    The expression (a % b) == 0 will: First compute the remainder of the division a/b Then compare the result (i.e., the remainder of the division) to the value 0

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Divisible01.java To run:          java Divisible01

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• Comparing integer and floating point values
  • Automatic conversion rule for compare operators:

    The same automatic conversion rules used for arithmetic operators apply for compare operators The automatic conversion rules for arithmetic operators were summarized on this webpage: click here

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• Priority of the compare operators
  • For practical purposes, you can assume that:

    All compare operators have the same priority This is because you cannot have back to back compare operations Example: this is illegal a < b == c Because you cannot have back to back compare operations, there is no need to decide which one has higher priority.

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  • Priority ranking of the compare operators against the previously discussed operators:

    Priority level	Operator(s)	Description	Associativity
    1	( )	Brackets
    2	(int)   −	Casting, negation	Right to left
    3	++, --	Increment, decrement
    4	*   /   %	Multiple, divide, remainder	Left to right
    5	+   -	Add, subtract	Left to right
    6	<   <=   >   >=     ==   !=	Compare operators
    7	=   +=   -=   ...	Assignment operators	Right to left

    Reference: click here

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  • Example 1:

    boolean a; Statement: a = 3 > 1; Operators in statement: = > Executed as follows: a = 3 > 1; // > has higher priority than = a = true;

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  • Example 2:

    boolean a; Statement: a = 3 + 4 <= 5 - 2; Operators in statement: = + <= - Executed as follows: a = 3 + 4 <= 5 - 2; // + and - has highest priority a = 7 <= 3; // <= has higher priority than = a = false;

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• Commonly made mistakes with compare operators
  • Mistake 1: using to compare equality

    Example:

    if ( a = 0 ) { ... } else { ... }

    The = symbol represents the assignment operator

    You must use == to compare equality (no spaces between the == characters)