Nested Loops

suits = ["❤︎","♦"]
faces = ['Jack', 'Queen', 'King']

cards = list()
for suit in suits:
    cards.append(faces[0] + ' of ' + suit)
    cards.append(faces[1] + ' of ' + suit)
    cards.append(faces[2] + ' of ' + suit)
cards

['Jack of ❤︎',
 'Queen of ❤︎',
 'King of ❤︎',
 'Jack of ♦',
 'Queen of ♦',
 'King of ♦']

We’ve seen how loops can help us adhere to the DRY principle, but what can we do if we are already using a loop, and there is still repetition in our code?

For example: Let’s say we are trying to obtain all the red (♥️, ♦️) suited face cards from a deck of cards into a list.

We are currently using a loop, but we keep repeating the same faces[#] + ' of ' + suit line.

In these types of situations, we can reduce redundancy by… you guessed it, adding another loop!

suits = ["❤︎","♦︎"]
faces = ['Jack', 'Queen', 'King']

cards = list()
for suit in suits:
    for face in faces: 
        cards.append(face + ' of ' + suit)
cards

['Jack of ❤︎',
 'Queen of ❤︎',
 'King of ❤︎',
 'Jack of ♦︎',
 'Queen of ♦︎',
 'King of ♦︎']

Just like how we reduced the repetition by making the first loop, we can make a second loop within the first one.

This is called a nested loop since we have a loop nested in an existing one.

We enter into the outer loop, where the first element in suits is ♥️.

The next line constructs the second loop, which iterates over the list faces.

The first element in this list is Jack.

The following indented line of code creates a string containing the suit (♥️) and the face (Jack) in the current iterations and append it to the list cards.

Once we have appended this to cards, the inner loop moves onto the second element in the faces list, which has the value Queen.

Notice how we are still on the first element (♥️) in the suit list.

We create a string containing the suit (♥️) and the face (Queen) of this current iteration, which is Queen of ♥️ and add it to the list cards.

Since we finish all the code in the inner loop’s second iteration, we can move on the third element in list faces, which is King.

The string King of ♥️ is added to the cards list.

It’s at this point that we have finished all the elements in the inner loop; where do we go now?

Now that all the elements in faces have been iterated over, can we move on to the next iteration in the outer loop?

This involves of iterating to the next element (♦️) in the list suits.

We now restart iterating over faces again in the inner loop, starting with Jack.

We iterate over each element in faces again, appending the strings 'Jack of ♦', 'Queen of ♦', 'King of ♦' at each iteration until reaching the end of the list faces.

Since we have reached the end of both the lists suits and faces, all the iterations in the inner and outer loops have finished.

We exit the loops and executes the next line of code, which displays the list cards.

cards = list()
for suit in suits:
    print(suit)
    for face in faces: 
        print(face)
        cards.append(face + ' of ' + suit)
cards

❤︎
Jack
Queen
King
♦
Jack
Queen
King
['Jack of ❤︎', 'Queen of ❤︎', 'King of ❤︎', 'Jack of ♦', 'Queen of ♦', 'King of ♦']

By looking at the output, we notice that all the heart(♥️) face cards are added to the list first.

That means the first iteration of the outer loop waits for the inner loop to finish going through all the face elements before starting the next element in suit.

Python then re-iterates over the inner loop again for the second element (♦️) in the outer loop.

The outer loop cannot move onto its next iteration until the inner loop has reached the last item in its collection.

Let’s see exactly what is happening again in the animation below.

Let’s apply what we learned!