if

What is if...else statement in Python?

Decision making is required when we want to execute a code only if a certain condition is satisfied.

The statement is used in Python for decision making.if…elif…else

Python if Statement Syntax

if test expression:
    statement(s)

Here, the program evaluates the and will execute statement(s) only if the test expression is.test expressionTrue

If the test expression is , the statement(s) is not executed.False

In Python, the body of the statement is indicated by the indentation. The body starts with an indentation and the first unindented line marks the end.if

Python interprets non-zero values as . and are interpreted as .TrueNone0False

Flowchart

Example

Python if Statement

# If the number is positive, we print an appropriate message
                
num = 3
if num > 0:
print(num, "is a positive number.")
print("This is always printed.")

num = -1
if num > 0:
print(num, "is a positive number.")
print("This is also always printed.")

When you run the program, the output will be:

3 is a positive number
This is always printed
This is also always printed.

In the above example, is the test expression.num > 0

The body of is executed only if this evaluates to .ifTrue

When the variable is equal to 3, test expression is true and statements inside the body of are executed.numif

If the variable is equal to -1, test expression is false and statements inside the body of are skipped.numif

The statement falls outside of the block (unindented). Hence, it is executed regardless of the test expression.print()if

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if-else

Python if...else Statement

Syntax of if...else

if test expression:
    Body of if
else:
    Body of else

The statement evaluates and will execute the body of only when the test condition is .
if..elsetest expressionifTrue

If the condition is , the body of is executed. Indentation is used to separate the blocks.
Falseelse

Flowchart

Example

# Program checks if the number is positive or negative
# And displays an appropriate message
                    
num = 3
                    
# Try these two variations as well.
# num = -5
# num = 0
                    
if num >= 0:
    print("Positive or Zero")
else:
    print("Negative number")

Output

Positive or Zero

In the above example, when is equal to 3, the test expression is true and the body of is executed and the of else is skipped.numifbody

If is equal to -5, the test expression is false and the body of is executed and the body of is skipped.numelseif

If is equal to 0, the test expression is true and body of is executed and of else is skipped. numifbody

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if-elif-else

Python if...elif...else Statement

Syntax of if...elif...else

if test expression:
    Body of if
elif test expression:
    Body of elif
else:
    Body of else

The is short for else if. It allows us to check for multiple expressions. elif

If the condition for is , it checks the condition of the next block and so on. ifFalseelif

If all the conditions are , the body of else is executed.False

Only one block among the several blocks is executed according to the condition. if...elif...else

The block can have only one block. But it can have multiple blocks. ifelseelif

Flowchart

Example

'''In this program,
we check if the number is positive or
negative or zero and
display an appropriate message'''
                        
num = 3.4
                        
# Try these two variations as well:
# num = 0
 # num = -4.5
                        
if num > 0:
    print("Positive number")
elif num == 0:
    print("Zero")
else:
    print("Negative number")

Output

Positive number

When variable is positive, is printed.numPositive number

If is equal to 0, is printed.numZero

If is negative, is printed.numNegative number

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Nested if

We can have a statement inside another statement. This is called nesting in computer programming.if...elif...elseif...elif...else

Any number of these statements can be nested inside one another. Indentation is the only way to figure out the level of nesting. They can get confusing, so they must be avoided unless necessary.

Example

'''In this program, we input a number
check if the number is positive or
negative or zero and display
an appropriate message
This time we use nested if statement'''
                
num = float(input("Enter a number: "))
if num >= 0:
    if num == 0:
        print("Zero")
    else:
        print("Positive number")
else:
    print("Negative number")

Output 1

Enter a number: 5
Positive number

Output 2

Enter a number: -1
Negative number

Output 3

Enter a number: 0
Zero

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