6.5 while Statement

The while statement is used when the program needs to perform repetitive tasks. The general form of a while statement is:

while (condition)
    statement;

The program repeatedly executes the statement inside the while until the condition becomes false (0). (If the condition is initially false, the statement will not be executed.)

For example, Example 6-2 computes all the Fibonacci numbers that are less than 100. The Fibonacci sequence is:

1 1 2 3 5 8 . . .

The terms are computed from the equations:

1

1

2 = 1 + 1

3 = 2 + 1

5 = 3 + 2

etc.

In general, the Fibonacci sequence is defined as:

f_n = f_{n - 1} + f_{n - 2}

This is a mathematical equation using math-style variable names (f_n). Mathematicians use this very terse style of naming variables, but in programming, terse is dangerous, so we translate these names into something verbose for C++:

Mathematician	Programmer
fn	next_number
fn-1	current_number
fn-2	old_number

So in C++ code, the equation is expressed as:

    next_number = current_number + old_number; 

We want to loop until our current term is 100 or larger. The following while loop will repeat our computation until we reach this limit.:

    while (current_number < 100) 

In our while loop we compute the value of current_number. Next we need to advance one term.

This completes the body of the loop. The first two terms of the Fibonacci sequence are 1 and 1. We initialize our first two terms to these values.

Figure 6-1 shows what happens to the variables during the execution of the program.

Figure 6-1. Fibonacci execution

At the beginning, current_number and old_number are 1. We print the value of the current term. Then the variable next_number is computed (value 2). Next we advance one term by putting current_number into old_number and putting next_number into current_number. This is repeated until we compute the last term and the while loop exits. Example 6-1 shows our algorithm written as C++ code.

Example 6-1. fib/fib.cpp

#include <iostream>

int   old_number;     // previous Fibonacci number
int   current_number; // current Fibonacci number 
int   next_number;    // next number in the series

int main(  )
{
    // start things out 
    old_number = 1;
    current_number = 1;

    std::cout << "1\n"; // Print first number

    while (current_number < 100) {

        std::cout << current_number << '\n';
        next_number = current_number + old_number;

        old_number = current_number;
        current_number = next_number;
    }
    return (0);
}