In computer programming, the if
statement is a conditional statement. It is used to execute a block of code only when a specific condition is met. For example,
Suppose we need to assign different grades to students based on their scores.
- If a student scores above 90, assign grade A
- If a student scores above 75, assign grade B
- If a student scores above 65, assign grade C
These conditional tasks can be achieved using the if
statement.
Python if Statement
An if
statement executes a block of code only if the specified condition is met.
Syntax
if condition:
# body of if statement
Here, condition is a boolean expression, such as number >5
, that evaluates to either True
or False
.
- If
condition
evaluates toTrue
, the body of theif
statement is executed. - If
condition
evaluates toFalse
, the body of theif
statement will be skipped from execution.
Let's look at an example.
Example: Python if Statement
number = int(input('Enter a number: '))
# check if number is greater than 0
if number > 0:
print(f'{number} is a positive number.')
print('A statement outside the if statement.')
Sample Output 1
Enter a number: 10 10 is a positive number. A statement outside the if statement.
If user enters 10, the condition number > 0
evalutes to True
. Therefore, the body of if
is executed.
Sample Output 2
Enter a number: -2 A statement outside the if statement.
If user enters -2, the condition number > 0
evalutes to False
. Therefore, the body of if
is skipped from execution.
Indentation in Python
Python uses indentation to define a block of code, such as the body of an if
statement. For example,
x = 1
total = 0
# start of the if statement
if x != 0:
total += x
print(total)
# end of the if statement
print("This is always executed.")
Here, the body of if
has two statements. These statements execute only if x != 0
evaluate to True
.
You will get an error if you write the above code like this:
# Error code
x = 1
total = 0
if x != 0:
total += x
print(total)
Here, we haven't used indentation after the if
statement. In this case, Python thinks our if
statement is empty, which results in an error.
Python if...else Statement
An if
statement can have an optional else
clause. The else
statement executes if the condition in the if
statement evaluates to False
.
Syntax
if condition:
# body of if statement
else:
# body of else statement
Here, if the condition
inside the if
statement evaluates to
- True - the body of
if
executes, and the body ofelse
is skipped. - False - the body of
else
executes, and the body ofif
is skipped
Let's look at an example.
Example: Python if…else Statement
number = int(input('Enter a number: '))
if number > 0:
print('Positive number')
else:
print('Not a positive number')
print('This statement always executes')
Sample Output 1
Enter a number: 10 Positive number This statement always executes
If user enters 10, the condition number > 0
evalutes to True
. Therefore, the body of if
is executed and the body of else
is skipped.
Sample Output 2
Enter a number: 0 Not a positive number This statement always executes
If user enters 0, the condition number >
0 evalutes to False
. Therefore, the body of if
is skipped and the body of else
is executed.
Python if…elif…else Statement
The if...else
statement is used to execute a block of code among two alternatives.
However, if we need to make a choice between more than two alternatives, we use the if...elif...else
statement.
Syntax
if condition1:
# code block 1
elif condition2:
# code block 2
else:
# code block 3
Let's look at an example.
Example: Python if…elif…else Statement
number = -5
if number > 0:
print('Positive number')
elif number < 0:
print('Negative number')
else:
print('Zero')
print('This statement is always executed')
Output
Negative number This statement is always executed
Here, the first condition, number > 0
evaluates to False
. In this scenario, the second condition is checked.
The second condition, number < 0
evaluates to True
. Therefore, the statements inside the elif
block is executed.
In the above program, it is important to note that regardless the value of number
variable, only one block of code will be executed.
Python Nested if Statements
It is possible to include an if
statement inside another if
statement. For example,
number = 5
# outer if statement
if number >= 0:
# inner if statement
if number == 0:
print('Number is 0')
# inner else statement
else:
print('Number is positive')
# outer else statement
else:
print('Number is negative')
Output
Number is positive
Here's how this program works.
More on Python if…else Statement
if
Statement
In certain situations, the if
statement can be simplified into a single line. For example,
number = 10
if number > 0:
print('Positive')
This code can be compactly written as
number = 10
if number >0: print('Positive')
This one-liner approach retains the same functionality but in a more concise format.
if...else
Python doesn't have a ternary operator. However, we can use if...else
to work like a ternary operator in other languages. For example,
grade = 40
if grade >= 50:
result = 'pass'
else:
result = 'fail'
print(result)
can be written as
grade = 40
result = 'pass' if number >= 50 else 'fail'
print(result)
We can use logical operators such as and
and or
within an if
statement.
age = 35
salary = 6000
# add two conditions using and operator
if age >= 30 and salary >= 5000:
print('Eligible for the premium membership.')
else:
print('Not eligible for the premium membership')
Output
Eligible for the premium membership.
Here, we used the logical operator and
to add two conditions in the if
statement.
We also used >=
(comparison operator) to compare two values.
Logical and comparison operators are often used with if...else
statements. Visit Python Operators to learn more.