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Example Program: Valid Input Checking

The do-while statement is useful for checking that input from a user lies in a valid range and repeatedly requesting input until it is within range. This is illustrated in the following portion of C++ program:

bool accept;      // indicates if value in range
float x;          // value entered
float low, high;  // bounds for x

// assume low and high have suitable values
do {
    cout << "Enter a value (" << low <<" to "
         << high << "):";
    cin >> x;
    if (low <= x && x <= high)
      accept = true;
    else
      accept = false;
  }
while (!accept);
Download program.

Note the use of the logical operator not (!) operating on the boolean value, to invert its truth value.

Another way of controlling the loop is to assign the value of the condition directly to accept. At first sight, this may appear strange, but the condition is already being evaluated as either true or false, so it makes sense to replace the if-else statement with

accept = low <= x && x <= high;


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