C If ... Else
Conditions and If Statements
You have already learned that C supports the usual logical conditions from mathematics:
- Less than: a < b
- Less than or equal to: a <= b
- Greater than: a > b
- Greater than or equal to: a >= b
- Equal to a == b
- Not Equal to: a != b
You can use these conditions to perform different actions for different decisions.
C has the following conditional statements:
- Use
if
to specify a block of code to be executed, if a specified condition istrue
- Use
else
to specify a block of code to be executed, if the same condition isfalse
- Use
else if
to specify a new condition to test, if the first condition isfalse
- Use
switch
to specify many alternative blocks of code to be executed
The if Statement
Use the if
statement to specify a block of code to be executed if a condition is true
.
Syntax
if (condition) {
// block of code to be executed if the
condition is true
}
Note that if
is in lowercase letters. Uppercase letters (If or IF) will generate an error.
In the example below, we test two values to find out if 20 is greater than
18. If the condition is true
, print some text:
We can also test variables:
Example explained
In the example above we use two variables, x and y,
to test whether x is greater than y
(using the >
operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".
The else Statement
Use the else
statement to specify a block of code to be executed if the condition is false
.
Syntax
if (condition) {
// block of code to be executed if the
condition is true
} else {
// block of code to be executed
if the condition is false
}
Example
int time = 20;
if (time < 18) {
printf("Good day.");
} else {
printf("Good evening.");
}
// Outputs "Good evening."
Try it Yourself »
Example explained
In the example above, time (20) is greater than 18, so the condition is false
.
Because of this, we move on to the else
condition and print to the screen "Good
evening". If the time was less than 18, the program would print "Good day".
The else if Statement
Use the else if
statement to specify a new condition if the first condition is false
.
Syntax
if (condition1) {
// block of code to be executed if
condition1 is true
} else if (condition2) {
// block of
code to be executed if the condition1 is false and condition2 is true
} else {
// block of code to be executed if the condition1 is false
and condition2 is false
}
Example
int time = 22;
if (time < 10) {
printf("Good morning.");
} else if (time < 20) {
printf("Good day.");
} else {
printf("Good evening.");
}
// Outputs "Good evening."
Try it Yourself »
Example explained
In the example above, time (22) is greater than 10, so the first condition is false
. The next condition, in the
else if
statement, is also false
, so we move on to the else
condition since condition1 and condition2 is both false
- and print to the screen "Good
evening".
However, if the time was 14, our program would print "Good day."
Another Example
This example shows how you can use if..else
to find out if a number is positive or negative:
Example
int myNum = 10; // Is this a positive or negative number?
if (myNum >
0)
printf("The value is a positive number.");
else if (myNum < 0)
printf("The value is a negative number.");
else
printf("The value is 0.");
Try it Yourself »