5.4.1. The while Statement

A while statement repeatedly executes a target statement as long as a condition is true. Its syntactic form is

while (condition)
      statement

In a while, statement (which is often a block) is executed as long as condition evaluates as true. condition may not be empty. If the first evaluation of condition yields false, statement is not executed.

The condition can be an expression or an initialized variable declaration (§ 5.2, p. 174). Ordinarily, the condition itself or the loop body must do something to change the value of the expression. Otherwise, the loop might never terminate.

Using a while Loop

A while loop is generally used when we want to iterate indefinitely, such as when we read input. A while is also useful when we want access to the value of the loop control variable after the loop finishes. For example:

vector<int> v;
int i;
// read until end-of-file or other input failure
while (cin >> i)
    v.push_back(i);
// find the first negative element
auto beg = v.begin();
while (beg != v.end() && *beg >= 0)
    ++beg;
if (beg == v.end())
    // we know that all elements in v are greater than or equal to zero

The first loop reads data from the standard input. We have no idea how many times this loop will execute. The condition fails when cin reads invalid data, encounters some other input failure, or hits end-of-file. The second loop continues until we find a negative value. When the loop terminates, beg is either equal to v.end(), or it denotes an element in v whose value is less than zero. We can use the state of beg outside the while to determine further processing.