Short answer: To find the maximal list in a list of lists, you need to make two lists comparable. How? With the key argument of the max() function. The key argument is a function that takes one input (a list) and returns one output (a numerical value). The list with the largest numerical value is returned as the maximum of the list of lists.
Problem: Say you have a list of lists (nested list) and you want to find the maximum of this list. It’s not trivial to compare lists—what’s the maximum among lists after all? To define the maximum among the inner lists, you may want to consider different objectives.
- The first element of each inner list.
- The i-th element of each inner list.
- The sum of inner list elements.
- The maximum of inner list elements.
- The minimum of inner list elements.
Example: Given list of lists [[1, 1, 1], [0, 2, 0], [3, 3, -1]]. Which is the maximum element?
- The first element of each inner list. The maximum is
[3, 3, -1]. - The i-th element of each inner list (
i = 2). The maximum is[1, 1, 1]. - The sum of inner list elements. The maximum is
[3, 3, -1]. - The maximum of inner list elements. The maximum is
[3, 3, -1]. - The minimum of inner list elements. The maximum is
[3, 3, -1].
So how do you accomplish this?
Solution: Use the max() function with key argument.
Syntax: The max() function is a built-in function in Python (Python versions 2.x and 3.x). Here’s the syntax:
max(iterable, key=None)
Arguments:
| Argument | Description |
|---|---|
iterable | The values among which you want to find the maximum. In our case, it’s a list of lists. |
key | (Optional. Default None.) Pass a function that takes a single argument and returns a comparable value. The function is then applied to each element in the list. Then, the method find the maximum based on the key function results rather than the elements themselves. |
Let’s study the solution code for our different versions of calculating the maximum “list” of a list of lists (nested list).
lst = [[1, 1, 1], [0, 2, 0], [3, 3, -1]] # Maximum using first element print(max(lst, key=lambda x: x[0])) # [3, 3, -1] # Maximum using third element print(max(lst, key=lambda x: x[2])) # [1, 1, 1] # Maximum using sum() print(max(lst, key=sum)) # [3, 3, -1] # Maximum using max print(max(lst, key=max)) # [3, 3, -1] # Maximum using min print(max(lst, key=min)) # [1, 1, 1]
Try it yourself in our interactive code shell:
Related articles:
- How to Find the Minimum of a List of Lists in Python?
- Python List Methods [Overview]
- Python List sort() – The Ultimate Guide
- Python Lists – Everything You Need to Know to Get Started
- Key with Maximum Value in Dict
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