6. Python If Conditions

Decision making is needed when you want to execute a block of code on particular condition, only if that condition is satisfied.

For decision making we're going to use Python if..elif..else statement.

6.1 Python if Statement

if condition:

statement1
statement2
...............

The program evaluates the condition. When the condition is satisfied or True the statement1, statement2, and etc will be executed, but when the expression is False then the statement will be not executed. Example

n = -3

if n % 2 != 0:
    print(n,"is odd number.")

print("After if statement")

if n > 0:
    print(n,"is postive.") 
print("After if statement")

Remember that condition with non zero value Python interpereter will treat it as True, while None and 0 are treated as False. Example

if None:
    print("This block will not executed.")
n = 0
if n:
    print("This block will not executed.")
print("This is the only one will be printed.")

6.2 Python if..else Statement

if condition:
if_statement
..............
else:
else_statement
..............
In Python if..else statement, if the condition evaluate to True then the block if statement will be executed otherwise the else statement is executed. Example

n = 3
if n % 2 == 0:
    print(n, "is even number")
else:
    print(n, "is odd number")

a, b = 5, 7
if a > b:
    print(a, "is greater than", b) 
else:
    print(b, "is greater than", a)

You can make the program above with one line using Python ternary operator. Example

n = 3
if n % 2 == 0:
    print(n, "it's even")
else:
    print(n, "it's odd")

a, b = 5, 7
if a > b:
    print("a is greater") 
else:
    print("b is greater")  

print("Using Python ternary operator:")
print("it's even") if n % 2 == 0 else print("it's odd")
print("a is greater") if a > b else print("b is greater")

6.3 Python if..elif..else Statement

if condition:
block_statement
elif condition:
block_statement
......
......
elif condition:
block_statement
else:
block_statement

In Python elif is short of else if. It allows the program to evaluate conditions.

When condition if is False then the elif block_statement will be executed, but if all elif condition turn to be False then the block else statement will be executed. Example

# Try these options
# ch = 'a'
# ch = 'b'
# ch = 'c'
ch = 'd'

if ch == 'a':
    print("it's 'a'")
elif ch == 'b':
    print("it's 'b'")
elif ch == 'c':
    print("it's 'c'")
else:
    print("it's 'd'")

6.4 Python Nested if Statements

When there are some if_elif_else statements inside if_elif_else statement, it is called nested if Statement. Example

n = -35
if n > 0:
    if n % 2 == 0:
        print("n is positive and even number.")
    else:
        print("n is positive and odd number.")
else:
    if n % 2 == 0:
        print("n is negative and even number.")
    else:
        print("n is negative and odd number.")