Python Find Shortest List in List

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Problem Formulation

💬 Programming Challenge: Given a list of lists (nested list). Find and return the shortest inner list from the outer list of lists.

Here are some examples:

  • [[1], [2, 3], [4, 5, 6]] 👉 [1]
  • [[1, [2, 3], 4], [5, 6], [7]] 👉 [7]
  • [[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]] 👉 [[1], [2], [3]]

Also, you’ll learn how to solve a variant of this challenge.

💬 Bonus challenge: Find only the length of the shortest list in the list of lists.

Here are some examples:

  • [[1], [2, 3], [4, 5, 6]] 👉 1
  • [[1, [2, 3], 4], [5, 6], [7]] 👉 1
  • [[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]] 👉 3

So without further ado, let’s get started!

Method 1: min(lst, key=len)

Use Python’s built-in min() function with a key argument to find the shortest list in a list of lists. Call min(lst, key=len) to return the shortest list in lst using the built-in len() function to associate the weight of each list, so that the shortest inner list will be the minimum.

Here’s an example:

def get_shortest_list(lst):
    return min(lst, key=len)


print(get_shortest_list([[1], [2, 3], [4, 5, 6]]))
# [1]

print(get_shortest_list([[1, [2, 3], 4], [5, 6], [7]]))
# [7]

print(get_shortest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]))
# [[1], [2], [3]]

A beautiful one-liner solution, isn’t it? 🙂 Let’s have a look at a slight variant to check the length of the shortest list instead.

Method 2: len(min(lst, key=len))

To get the length of the shortest list in a nested list, use the len(min(lst, key=len)) function. First, you determine the shortest inner list using the min() function with the key argument set to the len() function. Second, you pass this shortest list into the len() function itself to determine the minimum.

Here’s an analogous example:

def get_length_of_shortest_list(lst):
    return len(min(lst, key=len))


print(get_length_of_shortest_list([[1], [2, 3], [4, 5, 6]]))
# 1

print(get_length_of_shortest_list([[1, [2, 3], 4], [5, 6], [7]]))
# 1

print(get_length_of_shortest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]))
# 3

Method 3: min(len(x) for x in lst)

A Pythonic way to check the length of the shortest list is to combine a generator expression or list comprehension with the min() function without key. For instance, min(len(x) for x in lst) first turns all inner list into length integer numbers and passes this iterable into the min() function to get the result.

Here’s this approach on the same examples as before:

def get_length_of_shortest_list(lst):
    return min(len(x) for x in lst)


print(get_length_of_shortest_list([[1], [2, 3], [4, 5, 6]]))
# 1

print(get_length_of_shortest_list([[1, [2, 3], 4], [5, 6], [7]]))
# 1

print(get_length_of_shortest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]))
# 3

A good training effect can be obtained by studying the following tutorial on the topic—feel free to do so!

👉 Training: Understanding List Comprehension in Python

Method 4: Naive For Loop

A not so Pythonic but still fine approach is to iterate over all lists in a for loop, check their length using the len() function, and compare it against the currently shortest list stored in a separate variable. After the termination of the loop, the variable contains the shortest list.

Here’s a simple example:

def get_shortest_list(lst):
    shortest = lst[0] if lst else None
    for x in lst:
        if len(x) < len(shortest):
            shortest = x
    return shortest


print(get_shortest_list([[1], [2, 3], [4, 5, 6]]))
# [1]

print(get_shortest_list([[1, [2, 3], 4], [5, 6], [7]]))
# [7]

print(get_shortest_list([[[1], [2], [3]], [4, 5, [6]], [7, 8, 9, 10]]))
# [[1], [2], [3]]

print(get_shortest_list([]))
# None

So many lines of code! 😅 At least does the approach also work when passing in an empty list due to the ternary operator used in the first line.

lst[0] if lst else None

If you need a refresher on the ternary operator, you should check out our blog tutorial.

👉 Training Tutorial: The Ternary Operator — A Powerful Python Device

⭐ Note: If you need the length of the shortest list, you could simply replace the last line of the function with return len(shortest) , and you’re done!

Summary

You have learned about four ways to find the shortest list and its length from a Python list of lists (nested list):

  • Method 1: min(lst, key=len)
  • Method 2: len(min(lst, key=len))
  • Method 3: min(len(x) for x in lst)
  • Method 4: Naive For Loop

I hope you found the tutorial helpful, if you did, feel free to consider joining our community of likeminded coders—we do have lots of free training material!

👉 Also, check out our tutorial on How to Find the Minimum of a List of Lists in Python?—it’s a slight variation!

👉Recommended Tutorial: Python Find Longest List in Dict of Lists