3.2. While Loops¶

As we noted earlier, algorithms require two important control

structures: iteration and selection. Both of these are supported by Python in various forms. The programmer can choose the statement that is most useful for the given circumstance.

For iteration, C++ provides a standard while statement and a very powerful for statement. The while statement repeats a body of code as long as a condition is true. For example,

int counter = 1;
while (counter <= 5) {          /*Use of an interactive method until the
                                 the loop ends   */
    cout << "Hello, world" << endl;
    counter = counter + 1;
}

Console output:
Hello, world
Hello, world
Hello, world
Hello, world
Hello, world

prints out the phrase “Hello, world” five times. The condition on the while statement is evaluated at the start of each repetition. If the condition is true, the body of the statement will execute.

The while statement is a very general purpose iterative structure that we will use in a number of different algorithms. In many cases, a compound condition will control the iteration. A fragment such as

while ((counter <= 10) && (!done)) {
...

would cause the body of the statement to be executed only in the case where both parts of the condition are satisfied due to the and operator (&&). The value of the variable counter would need to be less than or equal to 10 and the value of the variable done would need to be false (not false is true) so that true and true results in true.

Here are some of the logical operators that are useful for true-false boolean statements in C++.

and          - &&

or           - ||

not equal to - !=

not          - !

greater than - >

less than    - <

greater than
or equal to  - >=

less than
or equal to  - <=

These are the same as we saw in earlier chapters.