Selections The if...else Statement

Programing required to make decisions. The if else and  if.. elif..else are used in python to make decisions.

The  if  statement

Syntax

 

if (condition):

    statement

    statement

 

How  if statement  works

Here, First the program evaluates if the condition is True

If the condition is True, non zero value if block is executed

If the condition is False, zero value, None if block is not executed

 

Note:

Indentation in python means spaces (tab or 4 spaces) at the beginning of a code line. Python uses indentation to indicate a block of code.

The block starts with an indentation and the first unindented line of code  marks the end.

 

if Statement Flowchart

Flow chart  if block

 

Example: Python if Statement

name = "Vijay"

 

if name != '':  #Condition is True

         print("Hello ", name)

         print("This is inside the if block")

 print("This is outside the if block")

 

Output

Hello Vijay

This is inside the if block

This is outside the if block

 

Example: Python if Statement

name = ""

 

if name != '':  #Condition is False

        print("Hello ", name)

       print("This is inside the if block")

   

print("This is outside the if block")

 

Output

This is outside the if block

 

The  if.. else  statement

Syntax

if condition:

        statement

        statement

else:

        statement

        statement

Else block

 

 

   

 

 

 

 

 

How  if...else statement  works

First the program evaluates if the condition is True

If the condition is True,  if block is executed.

If the test expression is False, else block executed.

 

if ..else Statement Flowchart

Flow chart if else  statement

Flowchart of if else statement 

Example: Python if Statement

name = "Vijay"

if name != '':

               print("Hello", name)

 else:

     print("Hello Guest")

name = ""

if name != '':

             print("Hello", name)

  else:

     print("Hello Guest")

 

Output

Hello Vijay

Hello Guest

 

if...elif...else Statement

If else statement check only two condition , to check more than two conditions if… elif…else statement is used.

Syntax

if condition:

          if block

elif condition:

         elif block

.

.

.

else:

           else block

Note:

The if…elif..else  block can have only one if ,  else  block, however  it can have multiple elif blocks.

If the condition is False, it checks the elif ‘s  condition, next elif block and so on.

If all the conditions are False, the  else block is executed.

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

 

Flowchart of if...elif...else statement

Flow chart  if elif else statement

 

Example of if...elif...else

number = int(input("Enter a number"))

if(number == 0 ):

         print("Number is Zero")

elif(number % 2 == 0):

         print("Number is Even")

else:

        print("Number is Odd")

Output 1

Enter a number: 0

Number is Zero

Output 2

Enter a number: 1

Number is Odd

Output 3

Enter a number: 2

Number is Even

 

Nested if statements

Nesting is a if...elif...else statement inside another if...elif...else  or  if else statement.

Nested if else Example

​​​​​​​num1 = int(input("Enter number1: "))

num2 = int(input("Enter number2: "))

num3 = int(input("Enter number3: "))

 if( num1 > num2 ):

               if(num1 > num3):

                            print(num1, " is greater")

               else:  

                            print(num3, " is greater")

else:

          if(num2 > num3):

                     print(num2, " is greater")

           else:

                     print(num3, " is greater")

Output 1

Enter number1: 1

Enter number2: 2

Enter number3: 3

3  is greater

Output 2

Enter number1: 30

Enter number2: 20

Enter number3: 10

30  is greater

Output 3

Enter number1: 3

Enter number2: 9

Enter number3: 6

9  is greater