Python Control Statement

Python Indentation

Sample JAVA Program

To define a block of code, in most of the programming languages like C, C++, Java. It uses braces { }

In Python , the code block can be defined using indentation.

Indentation Syntax : Tab or four(4) whitespaces

Python Control Statements

It is used in Python for decision making.It is required when we want to execute a code only if a certain condition is satisfied.

Python is having 4 types of control statement

if
if else
if elif else
nested if

1. if statement

If the boolean expression evaluates to TRUE, then the block of statement(s) inside the if statement is executed.

If boolean expression evaluates to FALSE, then the first set of code after the end of the if statement(s) is executed.

Note: Python interprets non-zero values as True. None and 0 are interpreted as False.

Syntax:

if test expression:
    statement(s) ( Tab or 4 white space)

Example

# Program to check whether the user age is below 18 or not
user_age = 15

if(user_age <= 18):
    print("User age is 18 or younger")
    
Output:
User age is 18 or younger

# Program to check whether the user age is below 18 or not
user_age = 20 # Execute twice with both the conditions

if(user_age <= 18):
    print("User age is 18 or younger")  
    
print("This line will be executed always")    

Output:
This line will be executed always

2. if ... else Statement

The if..else statement evaluates test expression and will execute body of if only when test condition is True.

If the condition is False,then body of else is executed

Syntax:

if test expression:    
    Body of if "statement(s)" ( Tab or 4 white space)
else:     
    Body of else "statement(s)" ( Tab or 4 white space)

Example

# Program to check whether the user age is below 18 or not
user_age = 22

if(user_age <= 18):
    print("User age is 18 or younger")
else:
    print("User age is above 18")

Output:
User age is above 18

# Program to verify whether the number is positive or not
num = 10 # Execute twice with negative value
if num > 0:
    print("Positive number")
else:
    print("Negative Number")
    
Output:
Positive number

3. if...elif...else Statement

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.

Note: The if block can have only one else block. But it can have multiple elif blocks.

Syntax:

if test expression:
    Body of if "statement(s)" ( Tab or 4 white space)
elif test expression:    
    Body of elif "statement(s)" ( Tab or 4 white space)
else:     
    Body of else "statement(s)" ( Tab or 4 white space)

Example:

# Program to verify whether the number is positive or not
num = 0

if num > 0:
    print("Positive number")
elif num == 0:
    print("ZERO")
else:
    print("Negative Number") 
    
Output:
ZERO

# Program to verify total number of digits
num = 100
if 9 < num < 99:
    print("Two digit number")
elif 99 < num < 999:
    print("Three digit number")
elif 999 < num < 9999:
    print("Four digit number")
else:
    print("Number is not in the mentioned range")
    
Output:
Three digit number

Note: Python does not provide switch or case statements as in other languages, but we can use if..elif...statements to simulate switch

# Program to display the month name
month_num = 12

if month_num == 1:
    print("January Month")
elif month_num == 2:
    print("February Month")
elif month_num == 3:
    print("March Month")
elif month_num == 4:
    print("April Month")
elif month_num == 5:
    print("May Month")
elif month_num == 6:
    print("June Month")
elif month_num == 7:
    print("July Month")    
elif month_num == 8:
    print("August Month")
elif month_num == 9:
    print("September Month")    
elif month_num == 10:
    print("October Month")
elif month_num == 11:
    print("November Month")   
elif month_num == 12:
    print("December Month")
else:
    print("Enter a valid number")
    
Output:
December Month

4. Nested if Statements

There may be a situation when you want to check for another condition after a condition resolves to true. In such a situation, you can use the nested if construct.

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

Note: 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

Syntax:

if expression1:
   statement(s)
   if expression2: # Nested if
      statement(s)
   elif expression3:
      statement(s)
   elif expression4:
      statement(s)
   else:
      statement(s)
else:
   statement(s)

Example:

# Program to verify whether the number is positive or not
num = 20
if num >= 0:
    if num == 0:
        print("Zero")
    else:
        print("Positive number")
else:
    print("Negative number")
    
Output:
Positive number