What Is If Else Statement In Python And How Its Work ?

Python 3 – Decision Making 

 

 

Decision-making is the anticipation of conditions occurring during the execution of a program and specified actions taken according to the conditions. 

Decision structures evaluate multiple expressions, which produce TRUE or FALSE as the outcome. You need to determine which action to take and which statements to execute if the outcome is TRUE or FALSE otherwise.

Following is the general form of a typical decision making structure found in most of the programming languages-

 

 

 

Python programming language assumes any non-zero and non-null values as TRUE, and any zero or null values as FALSE value. 

Python programming language provides the following types of decision-making statements. 

Statement	Description
if statements	An if statement consists of a Boolean expression followed by one or more statements.
if...else statements	An if statement can be followed by an optional else statement, which executes when the boolean expression is FALSE.
nested if statements	You can use one if or else if statement inside another if or else if statement(s).

Let us go through each decision-making statement quickly. 

 #1 IF Statement :

The IF statement is similar to that of other languages. The if statement contains a logical expression using which the data is compared and a decision is made based on the result of the comparison

Syntax 

if expression:
   statement(s) 

If the boolean expression evaluates to TRUE, then the block of statement(s) inside the if statement is executed. In Python, statements in a block are uniformly indented after the : symbol. If boolean expression evaluates to FALSE, then the first set of code after the end of block is executed. 

Example :

var1 = 100
if var1:
   print ("1 - Got a true expression value")
   print (var1)
var2 = 0
if var2:
   print ("2 - Got a true expression value")
   print (var2)
print ("Good bye!") 

When the above code is executed, it produces the following result  -

 

#2 IF...ELIF...ELSE Statements  : 

 

An else statement can be combined with an if statement. An else statement contains a block of code that executes if the conditional expression in the if statement resolves to 0 or a FALSE value. 

The else statement is an optional statement and there could be at the most only one else statement following if. 

Syntax 

The syntax of the if...else statement is- 

if expression:
   statement(s)
else:
   statement(s) 

Flow Diagram : 

 

Example : 

amount=int(input("Enter amount: "))
if amount<1000:
    discount=amount*0.05
    print ("Discount",discount)
else:
    discount=amount*0.10
    print ("Discount",discount)
    
print ("Net payable:",amount-discount) 

When the above code is executed, it produces the following result  -

 

 

#3 The elif Statement : 

The elif statement allows you to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE. 

Similar to the else, the elif statement is optional. However, unlike else, for which there can be at the most one statement, there can be an arbitrary number of elif statements following an if. 

Syntax

if expression1:
   statement(s)
elif expression2:
   statement(s)
elif expression3:
   statement(s)
else:
   statement(s)  

  

Example : 

amount=int(input("Enter amount: "))

if amount<1000:
    discount=amount*0.05
    print ("Discount",discount)
elif amount<5000:
    discount=amount*0.10
    print ("Discount",discount)
else:
    discount=amount*0.15
    print ("Discount",discount) print ("Net payable:",amount-discount)   

When the above code is executed, it produces the following result

 

 

#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. 

In a nested if construct, you can have an if...elif...else construct inside another if...elif...else construct. 

Syntax 

The syntax of the nested if...elif...else construct may be

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

 

Examlpe :

num=int(input("enter number : "))
if num%2==0:
    if num%3==0:
        print ("Divisible by 3 and 2")
    else:
        print ("divisible by 2 not divisible by 3")
else:
    if num%3==0:
        print ("divisible by 3 not divisible by 2")
    else:
        print  ("not Divisible by 2 not divisible by 3")  

When the above code is executed, it produces the following result -