1 Description

• Relational, equality and logical operators are used to form logic expressions.

• Logic expressions are those expressions whose value is either true or false

  • (as opposed to arithmetic expressions whose value is a number)

• Relational and equality operators usually compare two numbers and return a value of true or false (forming a logic expression).

• Logical operators combine logical values of true or false into a logical expression.

• Relational operators and logical operators are often used together in logic expressions.

  • Logical operators often combine logical expressions formed from relational operators.

• Because of this, it is important to understand the precedence relationship between relational and logical operators

  • Precedence is used by the compiler to decide in order in which the expression is evaluated.

• Logical expressions are important in the conditional expression in if, while, for, and do-while statements

2 Examples

a > b && b == c || c < d

(a > b && b > c) || c < d //do first two relational operators first, then the last, value is true
a > b && (b >c || c < d)  // do last two relational operators first, then the first, value is false

• Do not confuse the logical operator “==” ( double equal signs) with the assignment operator “=” (single equals sign). It can lead to logical errors that the compiler will not catch. (click for experiment about this)

• Use pararenthesis even if not necessary to make your logical expressions easier to read.

• When trying to decide whether you need a simple comparison (‘<’ and ‘>’) or a compound one ( “<=” and “>=”) mentally run the program for a simple case (e.g. like executing the loop 0 or 1 time). Off by one errors are common in logical expressions.

  Always mentally check the boundary conditions.

5 Experiment

Difference between the assignment and equality

#include <iostream>

using namespace std;

int main()
{
    int i = 4;
    int j = 5;

    if ( i = j) {
        cout << "equal" << endl;
    } else {
        cout << "NOT equal" << endl;
    }
    return 0;
}

Hypothesis: What will this program print out?

• equal?

• NOT equal?

If you aren’t sure, compile and run the program to see.

What is the value of “i” after the “if” statement executes? (you can modify the experiment to output the value)