PYTHON - Decision Making

Python - Decision Making

Python’s decision making functionality is in its keywords −if..elif…else. Theifkeyword requires aboolean expression, followed by colon (:) symbol. The colon (:) symbol starts an indented block. The statements with the same level of indentation are executed if the boolean expression inif statementisTrue. If the expression is not True (False), theinterpreterbypasses the indented block and proceeds to execute statements at earlier indentation level.

Decision structures evaluate multiple expressions which produce TRUE or FALSE as outcome. You need to determine which action to take and which statements to execute if 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 anynon-zeroandnon-nullvalues as TRUE, and if it is eitherzeroornull, then it is assumed as FALSE value.

Types of Decision Making Statements in Python

Python programming language provides following types of decision making statements. Click the following links to check their detail.

Sr.No.	Statement & Description
1	if statementsAnif statementconsists of a boolean expression followed by one or more statements.
2	if…else statementsAnif statementcan be followed by an optionalelse statement, which executes when the boolean expression is FALSE.
3	nested if statementsYou can use oneiforelse ifstatement inside anotheriforelse ifstatement(s).

Let us go through each decision making briefly −

Single Statement Suites

If the suite of anifclause consists only of a single line, it may go on the same line as the header statement.

Example

Here is an example of aone-line ifclause −

PYTHON CODE

var = 100
if ( var == 100 ) : print ("Value of expression is 100")
print ("Good bye!")

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

PYTHON CODE

Value of expression is 100
Good bye!

if…else statement

In this decision making statement, if theifcondition is true, then the statements within this block are executed, otherwise, theelseblock is executed.

The program will choose which block of code to execute based on whether the condition in theifstatement is true or false.

Example

The following example shows the use of if…else statement.

PYTHON CODE

var = 100
if ( var == 100 ): 
   print ("Value of var is equal to 100")
else:
   print("Value of var is not equal to 100")

On running the above code, it will show the following output −

PYTHON CODE

Value of var is equal to 100

Nested if statements

A nestedifis another decision making statement in which oneifstatement resides inside another. It allows us to check multiple conditions sequentially.

Example

In this example, we will see the use of nested-if statement.

PYTHON CODE

var = 100
if ( var == 100 ):
   print("The number is equal to 100")
   if var % 2 == 0:
      print("The number is even")
   else:
      print("The given number is odd")
elif var == 0:
   print("The given number is zero")
else:
   print("The given number is negative")

On executing the above code, it will display the below output −

PYTHON CODE

The number is equal to 100
The number is even