/* This is a multi-line
comment. The compiler will
ignore this, but it is
useful for putting notes
in the source code */
#include <stdio.h>
// This is a single line comment
void main(void)
{
int i;
for(i=0;i<10;i++) // Loop from 0 to 9
printf("%d\n",i);
}

The top of the program is an example of a multi-line comment. They start with /* and terminate with */ , anything in between is ignored by the compiler so you can put whatever you like in there to make the program more readable.

A single line comment starts with // and is not terminated. Anything after the // on the same line is ignored by the compiler, so again it can be used to place meaningful comments into the source code. There are two single line comments in the program above, the 2nd is included to show that comments can be placed to the right hand side of actual code lines without affecting the program.

It is normal practice with comments to place a comment at the start of a program saying briefly what the program does. Purists say that every single line of code should have a comment explaining it !

Now down to the main new feature of this program the for loop. Firstly notice a variable called i has been defined as an integer, this variable is used in the for loop to hold the current index count. This is probably the reason why so many programmers use a variable called i for this type of thing. You are free though to define a variable of any name and type.

The line for(i=0;i<10;i++) looks complicated, but it isn't. It will make the next line or block execute a number of times. The format of the line consists of three parts, the first part i=0 is the starting value for the variable i. The second part i<10 is the condition that, when true continues repeating, and the third part i++ is the increment or decrement. What the line is really saying is "start the value of i at 0, execute the next code block while the value of i is less than 10, each time the code block is executed add 1 to i "

Below are some more examples of the for loop :

for(i=9;i>=0;i--)

This time the value of i will start at 9 and have 1 deducted from it each loop ( i-- ), it will continue looping while the value of i is greater than or equal to 0.

for(timesthree=0;timesthree<=60;timesthree+=3)

This line will go through the 3 times table, starting at 0 and with each loop adding 3 to the timesthree variable, until timesthree is greater than 60.