Friday, July 22, 2011

Loops (for, while, do)

the for statement

the for statement has the form:

   // code to execute inside loop
  • initial_value  : sets up the initial value of the loop counter.  
  • test_condition : is the condition that is tested to see if the loop is executed again.  
  • step : describes how the counter is changed on each execution of the loop.
Here is an example:

// The following code adds together the numbers 1 through 10
// this variable keeps the running total

int total=0;

// this loop adds the numbers 1 through 10 to the variable total
for (int i=1; i < 11; i++){
   total = total + i;

So in the preceding chunk of code we have:

initial_condition is int i=0;
test_condition is i < 11;
step is i++;

So, upon initial execution of the loop, the integer variable i is set to 1. The statement total = total + i; is executed and the value of the variable total becomes 1. The step code is now executed and i is incremented by 1, so its new value is 2.
The test_condition is then checked, and since i is less than 11, the loop code is executed and the variable total gets the value 3 (since total was 1, and i was 2. i is then incremented by 1 again.

The loop continues to execute until the condition i&lt11 fails. At that point total will have the value 1+2+3+4+5+6+7+8+9+10 = 55.

the while statement

The while statement has the form:

while(condition) {
   // code to execute

condition : is a boolean statement that is checked each time after the final "}" of the while statement executes. If the condition is true then the while statement executes again. If the condition is false, the while statement does not execute again.
As an example, let's say that we wanted to write all the even numbers between 11 and 23 to the screen. The following is a full C++ program that does that.

// include this file for cout
#include <iostream.h>

int main(){
   // this variable holds the present number
   int current_number = 12;

   // while loop that prints all even numbers between
   // 11 and 23 to the screen
   while (current_number < 23){
      cerr << current_number << endl;
      current_number += 2;
   cerr << "all done" << endl;

The preceding example prints the value of current_number to the screen and then adds 2 to its value. As soon as the value of the variable current_number goes above 23, the while loop exits and the next line is executed.

The output of the preceding program would be:
all done

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