Python Find Shortest List in Dict of Lists

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Problem Formulation

💬 Programming Challenge: Given a dictionary where the values are lists of varying sizes. Find and return the shortest list!

Here’s an example:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}
# Goal: [1]

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

💬 Bonus challenge: Find only the key that is associated with the shortest list in the dictionary.

Here’s an example:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}
# Goal: 1

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 from the dictionary values. Call min(d.values(), key=len) to return the shortest list in d.values() using the built-in len() function to associate the weight of each list, so that the shortest list will be the minimum.

Here’s an example:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}

print(min(d.values(), key=len))
# [1]

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

👉 Recommended Tutorial: Python Find Longest List in Dict

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

To get the key mapping to the shortest list value in a dictionary, use min(d, key=lambda x: len(d[x])).

Explanation: You set the key argument of min() to a lambda function that maps each dictionary key x to the length of the associated value obtained with len(d[x]). This way, you use the length of the mapped values as weights to determine the “minimum”, i.e., the shortest list.

💡 Note: When you call min(d) Python will automatically replace it with min(d.keys()), so it iterates over the keys of the dictionary.

Here’s an analogous example:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}

print(min(d, key=lambda x: len(d[x])))
# 1

The key with shortest list in the dictionary is 4.

👉 Recommended Tutorial: A Complete Guide to Python Dictionaries

Method 4: Shortest List in Dict Values Using List Comprehension

You can also get the length of the shortest list from the dictionary values by combining a generator expression or list comprehension with the min() function without key using the following expression: min(len(val) for val in d.values())

Like so:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}

print(min(len(val) for val in d.values()))
# 1

Note that this returns the length of the shortest list in the dict values, not the key or the list itself.

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 key-value pairs 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:

d = {1: [1, 2, 3],
     2: [1, 2],
     3: [1],
     4: [1, 2, 3, 4]}

# Initialize variables with dummy values
k_min, v_min = -1, []
i = 0

for key,val in d.items():
    if len(val) < len(v_min) or i==0:
        k_min, v_min = key, val
    i += i

print("Shortest key and value:", str(k_min) + ',' + str(v_min))
# Shortest key and value: 3,[1]

So many lines of code! 😅 I hate it.

Summary

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

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!

👉 Recommended Tutorial: Python Find Longest List in List