6.10. The break Statement

A break statement terminates the nearest enclosing while, do while, for, or switch statement. Execution resumes at the statement immediately following the terminated statement. For example, the following loop searches a vector for the first occurrence of a particular value. Once it's found, the loop is exited:

     vector<int>::iterator iter = vec.begin();
     while (iter != vec.end()) {
        if (value == *iter)
             break; // ok: found it!
        else
             ++iter; // not found: keep looking
     }// end of while
     if (iter != vec.end()) // break to here ...
         // continue processing

In this example, the break terminates the while loop. Execution resumes at the if statement immediately following the while.

A break can appear only within a loop or switch statement or in a statement nested inside a loop or switch. A break may appear within an if only when the if is inside a switch or a loop. A break occurring outside a loop or switch is a compile-time error. When a break occurs inside a nested switch or loop statement, the enclosing loop or switch statement is unaffected by the termination of the inner switch or loop:

     string inBuf;
     while (cin >> inBuf && !inBuf.empty()) {
         switch(inBuf[0]) {
         case '-':
             // process up to the first blank
             for (string::size_type ix = 1;
                         ix != inBuf.size(); ++ix) {
                   if (inBuf[ix] == ' ')
                        break; // #1, leaves the for loop
                   // ...
             }
             // remaining '-' processing: break #1 transfers control here
             break; // #2, leaves the switch statement
         case '+':
             // ...
         } // end switch
         // end of switch: break #2 transfers control here
     }  // end while

The break labeled #1 terminates the for loop within the hyphen case label. It does not terminate the enclosing switch statement and in fact does not even terminate the processing for the current case. Processing continues with the first statement following the for, which might be additional code to handle the hyphen case or the break that completes that section.

The break labeled #2 terminates the switch statement after handling the hyphen case but does not terminate the enclosing while loop. Processing continues after that break by executing the condition in the while, which reads the next string from the standard input.

Exercises Section 6.10 Exercise 6.19: The first program in this section could be written more succinctly. In fact, its action could be contained entirely in the condition in the while. Rewrite the loop so that it has an empty body and does the work of finding the element in the condition. Exercise 6.20: Write a program to read a sequence of strings from standard input until either the same word occurs twice in succession or all the words have been read. Use a while loop to read the text one word at a time. Use the break statement to terminate the loop if a word occurs twice in succession. Print the word if it occurs twice in succession, or else print a message saying that no word was repeated.