2.6. Decision Making: Equality and Relational Operators

This section introduces a simple version of C++'s if statement that allows a program to take alternative action based on the truth or falsity of some condition. If the condition is met, i.e., the condition is true, the statement in the body of the if statement is executed. If the condition is not met, i.e., the condition is false, the body statement is not executed. We'll see an example shortly.

Conditions in if statements can be formed by using the equality operators and relational operators summarized in Fig. 2.8. The relational operators all have the same level of precedence and associate left to right. The equality operators both have the same level of precedence, which is lower than that of the relational operators, and associate left to right.

Fig. 2.8. Equality and relational operators.

Standard algebraic equality or relational operator	C++ equality or relational operator	Sample C++ condition	Meaning of C++ condition
Relational operators
>	>	x > y	x is greater than y
<	<	x < y	x is less than y
	>=	x >= y	x is greater than or equal to y
	<=	x <= y	x is less than or equal to y
Equality operators
=	==	x == y	x is equal to y
	!=	x != y	x is not equal to y

Common Programming Error 2.4

Confusing the equality operator == with the assignment operator = results in logic errors. The equality operator should be read "is equal to," and the assignment operator should be read "gets" or "gets the value of" or "is assigned the value of." Some people prefer to read the equality operator as "double equals." As we discuss in Section 5.9, confusing these operators may not necessarily cause an easy-to-recognize syntax error, but may cause extremely subtle logic errors.

The following example uses six if statements to compare two numbers input by the user. If the condition in any of these if statements is satisfied, the output statement associated with that if statement is executed. Figure 2.9 shows the program and the input/ output dialogs of three sample executions.

Lines 6–8

using std::cout; // program uses cout
using std::cin; // program uses cin
using std::endl; // program uses endl

are using declarations that eliminate the need to repeat the std:: prefix as we did in earlier programs. Once we insert these using declarations, we can write cout instead of std::cout, cin instead of std::cin and endl instead of std::endl, respectively, in the remainder of the program. [Note: From this point forward in the book, each example contains one or more using declarations.]

Good Programming Practice 2.6

Place using declarations immediately after the #include to which they refer.

Lines 13–14

int number1; // first integer to compare
int number2; // second integer to compare

declare the variables used in the program. Remember that variables may be declared in one declaration or in separate declarations.

The program uses cascaded stream extraction operations (line 17) to input two integers. Remember that we are allowed to write cin (instead of std::cin) because of line 7. First a value is read into variable number1, then a value is read into variable number2.

The if statement in lines 19–20

if ( number1 == number2 )
   cout << number1 << " == " << number2 << endl;

compares the values of variables number1 and number2 to test for equality. If the values are equal, the statement in line 20 displays a line of text indicating that the numbers are equal. If the conditions are true in one or more of the if statements starting in lines 22, 25, 28, 31 and 34, the corresponding body statement displays an appropriate line of text.

Notice that each if statement in Fig. 2.9 has a single statement in its body and that each body statement is indented. In Chapter 4 we show how to specify if statements with multiple-statement bodies (by enclosing the body statements in a pair of braces, { }, creating what is called a compound statement or a block).

 1   // Fig. 2.9: fig02_09.cpp
 2   // Comparing integers using if statements, relational operators
 3   // and equality operators.
 4   #include <iostream> // allows program to perform input and output
 5
 6   using std::cout; // program uses cout
 7   using std::cin; // program uses cin  
 8   using std::endl; // program uses endl
 9
10   // function main begins program execution
11   int main()
12   {
13      int number1; // first integer to compare
14      int number2; // second integer to compare
15
16      cout << "Enter two integers to compare: "; // prompt user for data
17      cin >> number1 >> number2; // read two integers from user
18
19      if ( number1 == number2 )                       
20         cout << number1 << " == " << number2 << endl;
21
22      if ( number1 != number2 )
23         cout << number1 << " != " << number2 << endl;
24
25      if ( number1 < number2 )
26         cout << number1 << " < " << number2 << endl;
27
28      if ( number1 > number2 )
29         cout << number1 << " > " << number2 << endl;
30
31      if ( number1 <= number2 )
32         cout << number1 << " <= " << number2 << endl;
33
34      if ( number1 >= number2 )
35         cout << number1 << " >= " << number2 << endl;
36
37      return 0; // indicate that program ended successfully
38
39   } // end function main

					  

Enter two integers to compare: 3 7
3 != 7
3 < 7
3 <= 7

Enter two integers to compare: 22 12
22 != 12
22 > 12
22 >= 12

Enter two integers to compare: 7 7
7 == 7
7 <= 7
7 >= 7

Common Programming Error 2.5

Placing a semicolon immediately after the right parenthesis after the condition in an if statement is often a logic error (although not a syntax error). The semicolon causes the body of the if statement to be empty, so the if statement performs no action, regardless of whether or not its condition is true. Worse yet, the original body statement of the if statement now becomes a statement in sequence with the if statement and always executes, often causing the program to produce incorrect results.

Figure 2.10 shows the precedence and associativity of the operators introduced in this chapter. The operators are shown top to bottom in decreasing order of precedence. Notice that all these operators, with the exception of the assignment operator =, associate from left to right. Addition is left-associative, so an expression like x + y + z is evaluated as if it had been written (x + y) + z. The assignment operator = associates from right to left, so an expression such as x = y = 0 is evaluated as if it had been written x = (y = 0), which, as we'll soon see, first assigns 0 to y, then assigns the result of that assignment—0—to x.