Learn python:usage of if...else Statement.

Python if...else Statement

Decision making is required when we want to execute a code only if a certain condition is satisfied. The if…elif…else statement is used in Python for decision making.

Python if Statement Syntax

if test expression:
    statement(s)

Here, the program evaluates the test expression and will execute statement(s) only if the text expression is True. If the text expression is False, the statement(s) is not executed. In Python, the body of the if statement is indicated by the indentation. Body starts with an indentation and the first unindented line marks the end. Python interprets non-zero values as True. None and 0 are interpreted as False.

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.")

output

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

In the above example, num > 0 is the test expression. The body of if is executed only if this evaluates to True. When variable num is equal to 3, test expression is true and body inside body of if is executed. If variable num is equal to -1, test expression is false and body inside body of if is skipped. The print() statement falls outside of the if block (unindented). Hence, it is executed regardless of the test expression.

Python if...else Statement Syntax of if...else

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

The if..else statement evaluates test expression and will execute body of if only when test condition is True. If the condition is False, body of else is executed. Indentation is used to separate the blocks.

Example of if...else

# 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 num is equal to 3, the test expression is true and body of if is executed and body of else is skipped. If num is equal to -5, the test expression is false and body of else is executed and body of if is skipped. If num is equal to 0, the test expression is true and body of if is executed and body of else is skipped.

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Python if...elif...else Syntax of if...elif...else

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

The elif is short for else if.It allows us to check for multiple expressions. If the condition for if is False, it checks the condition of the next elif block and so on. If all the conditions are False, body of else is executed. Only one block among the several if...elif...else blocks is executed according to the condition. The if block can have only one else block. But it can have multiple elif blocks.

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Example of if...elif...else

# 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 num is positive, Positive number is printed. If num is equal to 0, Zero is printed. If num is negative, Negative number is printed

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Python Nested if statements

We can have a if...elif...else statement inside another if...elif...else statement. This is called nesting in computer programming. Any number of these statements can be nested inside one another. Indentation is the only way to figure out the level of nesting. This can get confusing, so must be avoided if we can.

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Python Nested if 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

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