Nested Conditionals

By using an if/else statement, we produce two branches for our code to follow. We can add more branches to our program flow by combining conditionals.

Let’s see how this works by solving the following problem:

Our first attempt at a solution might look like this:

1
2
3
4
5
6
7

entry = int(input('Enter a whole number: '))

if entry%2 == 0:
   print("EVEN!")

if entry > 0:
   print("POSITIVE")

Console Output

Enter a whole number: 7

POSITIVE

When we enter 7 in the prompt, we want the program to print nothing. However, we see the output POSITIVE. The code doesn’t work as we want. Why not?

Written this way, the two conditionals are separate from each other. The result from one has no influence on the other. Checking entry as even or odd works fine on its own. However, the second check gets carried out whether or not entry%2 == 0 is True or False. Remember, the if condition does not apply after the first unindented line.

We want to check if entry is positive only if the number is even. To do this, we need to put the second conditional inside the first. This code structure is called a nested conditional. The result of the first conditional determines whether or not to consider the second.

Notice that when we put one conditional inside another, the body of the nested conditional is indented by two levels rather than one. In Python, the indentation of an if statement determines if it is nested. For an if statement to run inside another, it must be indented more than the outer conditional.

The correct solution looks like this:

entry = int(input("Enter a whole number: "))

if entry%2 == 0:
   print("EVEN!")

   if entry > 0:
      print("POSITIVE")

Now, run the two different blocks of code in your IDE. You will see that the correctly nested version of the code will only output POSITIVE if the original if condition evaluates as True.

Nesting Also Works With else

In the examples above, we left out the else clauses from the conditionals. Here is how we could add the else clauses back in.

1
2
3
4
5
6
7
8
9

word = input('Please enter a word that is longer than 4 characters: ')

if len(word) == 4:
   print("We need you to think of a word that is longer than 4 characters.")
else:
   if len(word) < 4:
      print("You can think of a longer word than that!")
   else:
      print("Excellent word!")

Check Your Understanding

1
2
3
4
5

num = 7

if num % 2 == 0:
   if num % 2 == 1:
      print("odd")

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14

answer_1 = 'yes'
answer_2 = 'no'


if answer_1 == 'yes':
   if answer_2 == 'yes':
      print("Both of you agree!")
   else:
      print("You two need to work this out.")
else:
   if answer_2 == 'yes':
      print("Stop arguing and work it out.")
   else:
      print("Clean your bathroom anyway!")