How to Sort a List of Lists in Descending and Ascending Order?

Sorting a List of Lists in Descending and Ascending Order

Given a list of lists with two elements, the goal is to sort the lists first in descending order based on the first element, and then in ascending order based on the second element.

To achieve this, we can utilize the sort() method along with a custom key function. Here's a solution:

<code class="python">L = [['a',1], ['a',2], ['a',3], ['b',1], ['b',2], ['b',3]]
L.sort(key=lambda k: (k[0], -k[1]), reverse=True)</code>

The key argument of the sort() method specifies a function that determines the sorting order. In this case, we define a lambda function that takes a list k and returns a tuple. The first element of the tuple is the first element of the list k[0], which is used for descending sorting. The second element is the negation of the second element of the list -k[1], which is used for ascending sorting.

By sorting in descending order first, reverse=True ensures that the first element of each list ('a' or 'b') is sorted in reverse alphabetical order. Subsequently, the ascending sorting of the second element results in an overall list sorted as:

[['b', 1], ['b', 2], ['b', 3], ['a', 1], ['a', 2], ['a', 3]]

This effectively achieves the desired sorting criterion.

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