1.2 Second Example
Our second example is of little more practical use than the first, but it introduces a few more programming language elements:
#include <stdio.h>
/* print a few numbers, to illustrate a simple loop */
main()
{
int i;
for(i = 0; i < 10; i = i + 1)
printf("i is %d\n", i);
return 0;
}
As before, the line #include <stdio.h> is boilerplate which is necessary since we're calling the printf function, and main() and the pair of braces {} indicate and delineate the function named main we're (again) writing.
The first new line is the line
/* print a few numbers, to illustrate a simple loop */
which is a comment. Anything between the characters /* and */ is ignored by the compiler, but may be useful to a person trying to read and understand the program. You can add comments anywhere you want to in the program, to document what the program is, what it does, who wrote it, how it works, what the various functions are for and how they work, what the various variables are for, etc.
The second new line, down within the function main, is
int i;
which declares that our function will use a variable named i. The variable's type is int, which is a plain integer.
Next, we set up a loop:
for(i = 0; i < 10; i = i + 1)
The keyword for indicates that we are setting up a ``for loop.'' A for loop is controlled by three expressions, enclosed in parentheses and separated by semicolons. These expressions say that, in this case, the loop starts by setting i to 0, that it continues as long as i is less than 10, and that after each iteration of the loop, i should be incremented by 1 (that is, have 1 added to its value).
Finally, we have a call to the printf function, as before, but with several differences. First, the call to printf is within the body of the for loop. This means that control flow does not pass once through the printf call, but instead that the call is performed as many times as are dictated by the for loop. In this case, printf will be called several times: once when i is 0, once when i is 1, once when i is 2, and so on until i is 9, for a total of 10 times.
A second difference in the printf call is that the string to be printed, "i is %d", contains a percent sign. Whenever printf sees a percent sign, it indicates that printf is not supposed to print the exact text of the string, but is instead supposed to read another one of its arguments to decide what to print. The letter after the percent sign tells it what type of argument to expect and how to print it. In this case, the letter d indicates that printf is to expect an int, and to print it in decimal. Finally, we see that printf is in fact being called with another argument, for a total of two, separated by commas. The second argument is the variable i, which is in fact an int, as required by %d. The effect of all of this is that each time it is called, printf will print a line containing the current value of the variable i:
i is 0
i is 1
i is 2
...
After several trips through the loop, i will eventually equal 9. After that trip through the loop, the third control expression i = i + 1 will increment its value to 10. The condition i < 10 is no longer true, so no more trips through the loop are taken. Instead, control flow jumps down to the statement following the for loop, which is the return statement. The main function returns, and the program is finished.
