Home

The C in C++

Language Translator

The C in C++

Written by Hemanshu Patel

Saturday, 20 October 2007

Article Index
The C in C++
Page 2
Page 3
Page 4
Page 5
Page 6
Page 7
Page 8

Page 3 of 8

The break and continue keywords

Inside the body of any of the looping constructs while, do-while, or for, you can control the flow of the loop using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin a new iteration.

As an example of break and continue, this program is a very simple menu system:

//: C03:Menu.cpp
// Simple menu program demonstrating
// the use of "break" and "continue"
#include <iostream>
using namespace std;

int main() {
  char c; // To hold response
  while(true) {
    cout << "MAIN MENU:" << endl;
    cout << "l: left, r: right, q: quit -> ";
    cin >> c;
    if(c == 'q')
      break; // Out of "while(1)"
    if(c == 'l') {
      cout << "LEFT MENU:" << endl;
      cout << "select a or b: ";
      cin >> c;
      if(c == 'a') {
        cout << "you chose 'a'" << endl;
        continue; // Back to main menu
      }
      if(c == 'b') {
        cout << "you chose 'b'" << endl;
        continue; // Back to main menu
      }
      else {
        cout << "you didn't choose a or b!"
             << endl;
        continue; // Back to main menu
      }
    }
    if(c == 'r') {
      cout << "RIGHT MENU:" << endl;
      cout << "select c or d: ";
      cin >> c;
      if(c == 'c') {
        cout << "you chose 'c'" << endl;
        continue; // Back to main menu
      }
      if(c == 'd') {
        cout << "you chose 'd'" << endl;
        continue; // Back to main menu
      }
      else {
        cout << "you didn't choose c or d!" 
             << endl;
        continue; // Back to main menu
      }
    }
    cout << "you must type l or r or q!" << endl;
  }
  cout << "quitting menu..." << endl;
} ///:~

If the user selects ‘q’ in the main menu, the break keyword is used to quit, otherwise the program just continues to execute indefinitely. After each of the sub-menu selections, the continue keyword is used to pop back up to the beginning of the while loop.

The while(true) statement is the equivalent of saying “do this loop forever.” The break statement allows you to break out of this infinite while loop when the user types a ‘q.’

switch

A switch statement selects from among pieces of code based on the value of an integral expression. Its form is:

switch(selector) {
    case integral-value1 : statement; break;
    case integral-value2 : statement; break;
    case integral-value3 : statement; break;
    case integral-value4 : statement; break;
    case integral-value5 : statement; break;
    (...)
    default: statement;
}

Selector is an expression that produces an integral value. The switch compares the result of selector to each integral value. If it finds a match, the corresponding statement (simple or compound) executes. If no match occurs, the default statement executes.

You will notice in the definition above that each case ends with a break, which causes execution to jump to the end of the switch body (the closing brace that completes the switch). This is the conventional way to build a switch statement, but the break is optional. If it is missing, your case “drops through” to the one after it. That is, the code for the following case statements execute until a break is encountered. Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer.

The switch statement is a clean way to implement multi-way selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value at compile-time. If you want to use, for example, a string object as a selector, it won’t work in a switch statement. For a string selector, you must instead use a series of if statements and compare the string inside the conditional.

The menu example shown above provides a particularly nice example of a switch:

//: C03:Menu2.cpp
// A menu using a switch statement
#include <iostream>
using namespace std;

int main() {
  bool quit = false;  // Flag for quitting
  while(quit == false) {
    cout << "Select a, b, c or q to quit: ";
    char response;
    cin >> response;
    switch(response) {
      case 'a' : cout << "you chose 'a'" << endl;
                 break;
      case 'b' : cout << "you chose 'b'" << endl;
                 break;
      case 'c' : cout << "you chose 'c'" << endl;
                 break;
      case 'q' : cout << "quitting menu" << endl;
                 quit = true;
                 break;
      default  : cout << "Please use a,b,c or q!"
                 << endl;
    }
  }
} ///:~

The quit flag is a bool, short for “Boolean,” which is a type you’ll find only in C++. It can have only the keyword values true or false. Selecting ‘q’ sets the quit flag to true. The next time the selector is evaluated, quit == false returns false so the body of the while does not execute.