Key takeaways

 

1. The comparison (otherwise known as relational) operators are used to compare values. The table below illustrates how the comparison operators work, assuming that x = 0, y = 1, and z = 0:

OperatorDescriptionExample

==returns True if operands' values are equal, and False otherwise

!=returns True if operands' values are not equal, and False otherwise

x != y # True

x != z # False

>True if the left operand's value is greater than the right operand's value, and False otherwise

x > y # False

y > z # True

<True if the left operand's value is less than the right operand's value, and False otherwise

x < y # True

y < z # False

≥True if the left operand's value is greater than or equal to the right operand's value, and False otherwise

x >= y # False

x >= z # True

y >= z # True

≤True if the left operand's value is less than or equal to the right operand's value, and False otherwise

x <= y # True

x <= z # True

y <= z # False

2. When you want to execute some code only if a certain condition is met, you can use a conditional statement:

•  a single if statement

x = 10

if x == 10: # condition
    print("x is equal to 10")  # Executed if the condition is True.

• a series of if statements

x = 10

if x > 5: # condition one
    print("x is greater than 5")  # Executed if condition one is True.

if x < 10: # condition two
    print("x is less than 10")  # Executed if condition two is True.

if x == 10: # condition three
    print("x is equal to 10")  # Executed if condition three is True.
    
Each if statement is tested separately.

• an if-else statement, e.g.:

x = 10

if x < 10:  # Condition
    print("x is less than 10")  # Executed if the condition is True.

else:
    print("x is greater than or equal to 10")  # Executed if the condition is False.

• The if-elif-else statement, e.g.:

x = 10

if x > 5:  # True
    print("x > 5")

if x > 8:  # True
    print("x > 8")

if x > 10:  # False
    print("x > 10")

else:
    print("else will be executed")

 

Each if is tested separately. The body of else is executed if the last if is False.

• The if-elif-else statement, e.g.:

x = 10

if x == 10:  # True
    print("x == 10")

if x > 15:  # False
    print("x > 15")

elif x > 10:  # False
    print("x > 10")

elif x > 5:  # True
    print("x > 5")

else:
    print("else will not be executed")

 

If the condition for if is False, the program checks the conditions of the subsequent elif blocks – the first elif block that is True is executed. If all the conditions are False, the else block will be executed.

• Nested conditional statements, e.g.:

x = 10

if x > 5:  # True
    if x == 6:  # False
        print("nested: x == 6")
    elif x == 10:  # True
        print("nested: x == 10")
    else:
        print("nested: else")
else:
    print("else")