What is the "key" for Quicksort [closed]

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My Professor uses the term key comparison when talking about quicksort, but didn't explain what exactly the key is. If we're sorting an array of numbers, is a key a number in the array?

Answer 1

The key is whatever you're sorting on.

If you're just doing a typical sort of an array of numbers, then the key is a number.

If you're sorting, for example, objects describing people, and you're sorting this by their surname, then the key would be the surname.