1.4.1. The while Statement

A while statement repeatedly executes a section of code so long as a given condition is true. We can use a while to write a program to sum the numbers from 1 through 10 inclusive as follows:

#include <iostream>
int main()
{
    int sum = 0, val  = 1;
    // keep executing the while as long as val is less than or equal to 10
    while (val <= 10)  {
        sum += val;   // assigns sum + val to sum
        ++val;        // add 1 to val
    }
    std::cout << "Sum of 1 to 10 inclusive is "
              << sum << std::endl;
    return 0;
}

When we compile and execute this program, it prints

Sum of 1 to 10 inclusive is 55

As before, we start by including the iostream header and defining main. Inside main we define two int variables: sum, which will hold our summation, and val, which will represent each of the values from 1 through 10. We give sum an initial value of 0 and start val off with the value 1.

The new part of this program is the while statement. A while has the form

while (condition)
    statement

A while executes by (alternately) testing the condition and executing the associated statement until the condition is false. A condition is an expression that yields a result that is either true or false. So long as condition is true, statement is executed. After executing statement, condition is tested again. If condition is again true, then statement is again executed. The while continues, alternately testing the condition and executing statement until the condition is false.

In this program, the while statement is

// keep executing the while as long as val is less than or equal to 10
while (val <= 10)  {
    sum += val;   // assigns sum + val to sum
    ++val;        // add 1 to val
}

The condition uses the less-than-or-equal operator (the <= operator) to compare the current value of val and 10. As long as val is less than or equal to 10, the condition is true. If the condition is true, we execute the body of the while. In this case, that body is a block with two statements:

{
    sum += val;  // assigns sum + val to sum
    ++val;       // add 1 to val
}

A block is a sequence of zero or more statements enclosed by curly braces. A block is a statement and may be used wherever a statement is required. The first statement in this block uses the compound assignment operator (the += operator). This operator adds its right-hand operand to its left-hand operand and stores the result in the left-hand operand. It has essentially the same effect as writing an addition and an assignment:

sum = sum + val; // assign sum + val to sum

Thus, the first statement in the block adds the value of val to the current value of sum and stores the result back into sum.

The next statement

++val;     // add 1 to val

uses the prefix increment operator (the ++ operator). The increment operator adds 1 to its operand. Writing ++val is the same as writing val = val + 1.

After executing the while body, the loop evaluates the condition again. If the (now incremented) value of val is still less than or equal to 10, then the body of the while is executed again. The loop continues, testing the condition and executing the body, until val is no longer less than or equal to 10.

Once val is greater than 10, the program falls out of the while loop and continues execution with the statement following the while. In this case, that statement prints our output, followed by the return, which completes our main program.