Conditional Operator

6.1 if else

In Python, just like in many programming languages, commands can be executed not only linearly but also selectively based on a certain condition. Such a construction is called a conditional operator. The general form of a conditional operator looks like this:

if condition:
   command1
else:
   command2

If condition is true (True), then command1 is executed. If false (False), then command2 is executed. The commands never execute simultaneously: it's either one or the other.

Important! The child command or commands must(!) be indented by 4 spaces relative to the if and else commands. I'll tell you more about this in the next lecture "Block of Commands".

Examples:

y = 4
if y > 5:
    print("y is greater than 5")
else:
    print("y is not greater than 5")

Another example:

age = int(input("Enter your age:"))
if age >=18:
    print("you are an adult")
else:
    print("go do your homework")

6.2 if without else

There are 2 other forms of the conditional operator – shorthand and extended.

In the shorthand form, the else part is missing.

if condition:
    command

If the condition is true, the command will execute. If it's not true, nothing happens – the program just moves on to execute the rest of the commands, if there are any.

Example:

age = int(input("Enter your age:"))
if age >=21:
    print("Here's your beer!")

6.3 if elif else

The extended form of the if else operator allows you to chain several if else operators together.

Suppose you want to determine which quadrant of the coordinate plane a point with coordinates x and y belongs to. Here's how you can do it using the if and else operators:

x, y = 5, -8
if x > 0 and y > 0:
    print("first quadrant")
else:
    if x < 0 and y > 0:
        print("second quadrant")
    else:
        if x < 0 and y < 0:
            print("third quadrant")
        else:
            print("fourth quadrant")
 

Since not adding indents is not allowed and you want the code to be readable, a special operator elif was created, which turns such if chains into something more readable. It looks like this:

if condition1:
    command1
elif condition2:
    command2
elif conditionN:
    commandN
else:
    commandElse

The keyword elif is a contraction of else if. And the code from the example above can now be written as:

x, y = 5, -8
if x > 0 and y > 0:
    print("first quadrant")
elif x < 0 and y > 0:
    print("second quadrant")
elif x < 0 and y < 0:
    print("third quadrant")
else:
    print("fourth quadrant")

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