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I was looking at this python code that I need some explanation with:

arr = [0, 0, 0, 0, 1, 2, 3, 4,5]
arr = arr[next((i for i, x in enumerate(arr) if x != 0), len(arr)):]

This code would remove the leading zeroes from the array, I am trying to understand how it works. I understand that we created an iterator that would iterate over all elements of arr but 0 values, and next would iterate only till length of array (not inclusive).

But how are these indices returned by next, combine to form an array?

Jonas
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Mr Matrix
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1 Answers1

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Let's look at the code step by step. You want to slice off the initial zeros. If you knew the index of the first non-zero element, n, the expression would look like

arr = arr[n:]

That's basically what we have here, with n = next((i for i, x in enumerate(arr) if x != 0), len(arr)).

In general, the two-arg form of next will return the second argument as a marker instead of raising a StopIteration should the iterator run out. That's what the len(arr) is for. If all the elements are zero, the expression becomes

arr = arr[len(arr):]  # obviously empty

If there is a non-zero element, the next call will find its index (enabled with enumerate), and return it.

Mad Physicist
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  • For some reason, I thought next would return an iterator, my bad. This is straightforward. I will mark it as answer for readers. – Mr Matrix Sep 23 '19 at 04:09