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5 5 Decision Making
Decision Making

in Python
in Python

Pre-Processing

• Conditional Statements • If Statement
• If … Else Statement • Nested If … Else Statement
• If … Elif … Else Ladder • Indentation of Statements in Python

The statements in a program are usually executed in sequence. But, sometimes we may need to decide
between two courses of actions, depending on a specific situation that arises during the execution of
the program. Decision making is the process of evaluating a given condition and deciding the course of
action to follow based on the result of the condition. Start
For example, deciding whether a person is eligible to vote or note,
Is
depending on his/her age is an example of decision making. If the age of Input Number Test Condi on 1 No
True?
the person is 18 years or above, he/she will be declared eligible to vote, N
else he/she will be declared ineligible to vote. Is No Yes
Consider the example of accepting a number from the user and deciding N > 0 ?
Is
whether the number is positive or negative. We know that a number is Yes No Test Condi on 1 else block
Execute
positive if it is greater than or equal to 0, else it is negative. Thus, we need Print N is Print N is True?
Nega ve
Posi ve
to check the condition to see if the variable containing the number is Yes
greater than 0 or not. Stop Execute
The flow chart given in the image illustrates the decision-making involved in the process of checking if block
whether a number is positive or not. Start
Is
CONDITIONAL STATEMENTS Input Number Test Condi on 1 No
Is
N
Yes
The statements that let us take decisions in a Python program are called Conditional statements. These True?
Execute
Test Condi on
if block
True?
Is
statements work on the basis of specific conditions. The flow of program execution is directed to different Test Condi on 1 Yes Yes if block 3
Execute
Is
No
sets of statements on the basis of the evaluation of the condition. N > 0 ? True?
No
Python provides several decision-making statements. Let us learn about them. Yes No No Is Execute
Test Condi on 1 else block
IF STATEMENT Print N is Print N is True?
Nega ve
Posi ve
Is
Execute
Execute
The if statement is the simplest conditional statement. It evaluates a test condition. If the test condition if block 3 Yes Test Condi on 2 Yes if block 3
Yes
True?
Stop
evaluates to True, the block of statements under if is executed. If the test condition evaluates to False, Execute
the block of statements under if is bypassed and the control is passed to the statement after the if block. if block
No
The statement used with the if statement is called Selection statements. Is Yes Execute
Is
The syntax for using the if statement is: Test Condi on if block Test Condi on 3
True?
Is
if <test condition>: Test Condi on Yes Execute True?
No
set of statements True? if block Test Condi on 1 Yes Execute
Is
No
Execute
other statements else block No Execute if block 3
True?
else block
No
57
Is
Execute Yes Yes Execute 64
if block 3 Test Condi on 2 if block 3
True?
No
Is
Test Condi on Yes Execute Is
if block
True? Test Condi on 3
True?
No
No
Execute
else block
Execute
else block
64