Very often, people will make an observation and then change their subsequent actions based on that observation. For example, someone going out or a walk might go through the following thought process:
If it looks like it might rain I will take an umbrella otherwise I will leave my umbrella at home.
Such decision making is at the heart of computer programming. Very often, a program will either receive some data or perform a calculation and then change the actions it performs based on that occurence. For example, a program that calculated student grades might want to output a message "Congratulations on your good work!" if a student had done very well, but display a different message if the student's grades were low.
C allows decisions to be made using the if statement. The general form of the if statement is as follows:
if ( condition )
{
/*if the condition is true,
do whatever commands are contained
within the braces*/
}
The condition must evaluate to either true or false. The following operators can be used in a
condition statement:
|
Operator |
Example |
Description |
== |
a == b |
This is the equivalence operator. It is true if a and b have the same value and false otherwise. |
|
< |
a < b |
The less than operator is true if a is less than b and false otherwise. |
|
> |
a > b |
This is the greater than operator. It is true if a is greater than b and false otherwise. |
|
<= |
a <= b |
The less than or equal to operator is true as long as b is not greater thana. |
|
>= |
a >= b |
The greater than or equal to operator is true as long as b is not less thana. |
|
! |
!a |
The "not" or negation operator changes the truth value of what follows it. For the example, the condition !a is true if a is false. |
The code snippet below shows an example of how an if statement could be used:
#include <stdio.h>
int main(int argc, char* argv[])
{
/*create some test values for our if condition*/
int a, b;
a = 10;
b = -1;
/*make a decision based on the values of a and b*/
if(a > b)
{
/*if a is greater than b, than execute this command*/
printf("The bigger number is a!\n");
}
return 0;
}
In the next section, we will look at more complex examples of the if statement and the condition term