5.7. break and continue Statements

In addition to the selection and repetition statements, C++ provides statements break and continue to alter the flow of control. The preceding section showed how break can be used to terminate a switch statement's execution. This section discusses how to use break in a repetition statement.

break Statement

The break statement, when executed in a while, for, do...while or switch statement, causes immediate exit from that statement. Program execution continues with the next statement. Common uses of the break statement are to escape early from a loop or to skip the remainder of a switch statement (as in Fig. 5.10). Figure 5.13 demonstrates the break statement (line 14) exiting a for repetition statement.

 1   // Fig. 5.13: fig05_13.cpp
 2   // break statement exiting a for statement.
 3   #include <iostream>
 4   using std::cout;
 5   using std::endl;
 6
 7   int main()
 8   {
 9      int count; // control variable also used after loop terminates
10
11      for ( count = 1; count <= 10; count++ ) // loop 10 times
12      {
13         if ( count == 5 )                     
14            break; // break loop only if x is 5
15
16         cout << count << " ";
17      } // end for
18
19      cout << "\nBroke out of loop at count = " << count << endl;
20      return 0; // indicate successful termination
21   } // end main

1 2 3 4
Broke out of loop at count = 5

When the if statement detects that count is 5, the break statement executes. This terminates the for statement, and the program proceeds to line 19 (immediately after the for statement), which displays a message indicating the control variable value that terminated the loop. The for statement fully executes its body only four times instead of 10. Note that the control variable count is defined outside the for statement header, so that we can use the control variable both in the loop's body and after the loop completes its execution.

continue Statement

The continue statement, when executed in a while, for or do...while statement, skips the remaining statements in the body of that statement and proceeds with the next iteration of the loop. In while and do...while statements, the loop-continuation test evaluates immediately after the continue statement executes. In the for statement, the increment expression executes, then the loop-continuation test evaluates.

Figure 5.14 uses the continue statement (line 12) in a for statement to skip the output statement (line 14) when the nested if (lines 11–12) determines that the value of count is 5. When the continue statement executes, program control continues with the increment of the control variable in the for header (line 9) and loops five more times.

 1   // Fig. 5.14: fig05_14.cpp
 2   // continue statement terminating an iteration of a for statement.
 3   #include <iostream>
 4   using std::cout;
 5   using std::endl;
 6
 7   int main()
 8   {
 9      for ( int count = 1; count <= 10; count++ ) // loop 10 times
10      {
11         if ( count == 5 ) // if count is 5,
12            continue;      // skip remaining code in loop
13
14         cout << count << " ";
15      } // end for
16
17      cout << "\nUsed continue to skip printing 5" << endl;
18      return 0; // indicate successful termination
19   } // end main

1 2 3 4 6 7 8 9 10
Used continue to skip printing 5

In Section 5.3, we stated that the while statement could be used in most cases to represent the for statement. The one exception occurs when the increment expression in the while statement follows the continue statement. In this case, the increment does not execute before the program tests the loop-continuation condition, and the while does not execute in the same manner as the for.

Good Programming Practice 5.8

Some programmers feel that break and continue violate structured programming. The effects of these statements can be achieved by structured programming techniques we soon will see, so these programmers do not use break and continue. Most programmers consider the use of break in switch statements acceptable.

Performance Tip 5.5

The break and continue statements, when used properly, perform faster than do the corresponding structured techniques.

Software Engineering Observation 5.1

There is a tension between achieving quality software engineering and achieving the best-performing software. Often, one of these goals is achieved at the expense of the other. For all but the most performance-intensive situations, apply the following rule of thumb: First, make your code simple and correct; then make it fast and small, but only if necessary.