The program itself is simple, it loops continuously until the user presses the X key on the keyboard.

#include <stdio.h>
#include <conio.h>

void main(void)
{
int done=0;
unsigned char k;
gotoxy(27,8);
printf("Press the x key to exit\n");
while(done==0)
{
k=getch();
if(k=='x'||k=='X')done=1;
}
gotoxy(23,10);
printf("Thank you for using this program");
}

#include <stdio.h>
#include <conio.h>

void main(void)
{
gotoxy(10,5);
printf("Printing here");
}

Keep changing the two values in the gotoxy line, re-compile and run the program to see the effects.

The second new part shows the while loop. Firstly note the variable called done is set to 0 when it is declared. When the program reaches the line while(done==0) it tests to see if the condition done==0 is true. In this case it is, because the program has initialised the done variable to zero, so it executes the next line or block. In this case it's a block of code because there are curly brackets defining it. It needed to be programmed as a block because more than one statement is needed. Within this block, the program gets the key pressed by the user and stores it in the k variable ( k=getch(); ).

It then checks to see if the key pressed is the X key ( if(k=='x'||k=='X') ). This is different to previous examples of the if statement because the or logical operator is used (||). The line can be read as if k equals 'x' or k equals 'X'. If you remember back to the ASCII character set, upper and lower case letters had a different code. If the user of the program has the CAPS LOCK on, then the ASCII code for the upper case X will be stored in the k variable, if not, the ASCII code for the lower case x will be stored there. This program doesn't mind either way as it checks for both by using the || ( 'or' ) logicial operator. More on logical operators in a minute, back to the while loop ..

If the user does press the x key then the value of the done variable is set from 0 to 1. If the user doesn't press the x key then the value of the done variable remains at unchanged at 0. After this line the code block is ended, at this point a test is made on the condition of the while statement at the start of the block. If the condition ( done==0 ) is true, then the program goes back to the top of the block and repeats. If the condition ( done==0 ) is false then it "drops out of the while loop" and continues down. The red arrows in the following graphic show the program flow.

( a true condition is : done == 0, a false condition is : done != 0 )

Besides the "logical or" ( || ), you can use the "logical and" ( && ) in conditions. There is an example below of the logical and being used in an if statement :

if(a>5&&a<10)printf("The value of the variable a is between 6 and 9 inclusive\n");

With a "logical and" only if all the conditions result in true, is the overall result true. The "logical or" will return true if any of the conditions result in true.

The while loop is not the only loop in C, there is another more controlled loop called the for loop, which is covered with a simple example on the next page.

#include <stdio.h>
#include <conio.h>

void main(void)
{
int x=0,y=0,done=0;
unsigned char k;
while(done==0)
{
k=getch();
if((k=='a'||k=='A')&&x>0)x--;
if((k=='s'||k=='S')&&x<78)x++;
if((k=='p'||k=='P')&&y>0)y--;
if((k=='l'||k=='L')&&y<24)y++;
if(k=='x'||k=='X')done=1;
gotoxy(x,y);
printf("*");
}
}