Python 3 - Decision Making

Decision making is anticipation of conditions occurring while execution of the program and specifying actions taken according to the conditions.
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 −
Decision making statements in Python
Python programming language assumes any non-zero and non-null values as TRUE, and if it is either zero or null, then it is assumed as FALSE value.
Python programming language provides following types of decision making statements. Click the following links to check their detail.
StatementDescription
if statements
An if statement consists of a boolean expression followed by one or more statements.
It is similar to that of other languages. The if statement contains a logical expression using which 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 : symbol. If boolean expression evaluates to FALSE, then the first set of code after the end of block is executed.

Flow Diagram

Python if statement

Example

#!/usr/bin/python3

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 −
1 - Got a true expression value
100
Good bye!
if...else statements
An if statement can be followed by an optional else statement, which executes when the boolean expression is FALSE.
An else statement can be combined with an if statement. An else statement contains the 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 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

Python if...else statement

Example

#!/usr/bin/python3

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)
In the above exmple, discount is calculated on amount input. Rate of discount is 5% if amount is less than 1000, and 10% if it is above 10000. When the above code is executed, it produces the following result −
Enter amount: 600
Discount 30.0
Net payable: 570.0

Enter amount: 1200
Discount 120.0
Net payable: 1080.0

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 most one statement, there can be an arbitrary number ofelif statements following an if.

syntax

if expression1:
   statement(s)
elif expression2:
   statement(s)
elif expression3:
   statement(s)
else:
   statement(s)
Core Python does not provide switch or case statements as in other languages, but we can use if..elif...statements to simulate switch case as follows −

Example

#!/usr/bin/python3

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 −
Enter amount: 600
Discount 30.0
Net payable: 570.0

Enter amount: 3000
Discount 300.0
Net payable: 2700.0

Enter amount: 6000
Discount 900.0
Net payable: 5100.0

nested if statements
You can use one if or else if statement inside another if or else if statement(s).
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 ifconstruct.
In a nested if construct, you can have an if...elif...else construct inside anotherif...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)

Example:

# !/usr/bin/python3
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 following result:
enter number8
divisible by 2 not divisible by 3

enter number15
divisible by 3 not divisible by 2

enter number12
Divisible by 3 and 2

enter number5
not Divisible by 2 not divisible by 3
Let us go through each decision making briefly −

Single Statement Suites

If the suite of an if clause consists only of a single line, it may go on the same line as the header statement.
Here is an example of a one-line if clause −
#!/usr/bin/python3

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 −
Value of expression is 100
Good bye!

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