Python: Find maximum element index in a list

Question

I have a list

arr = [0, 1, 45, 2, 40, 3, 70, 4, 45, 5, 6, 7, 8, 9]

in which I'm trying to find the position/index of the maximum element from 3 consecutive elements using below code:

for i in range (0, len(arr)-3):
    print(arr.index(max(arr[i : i+3])))

When i goes to position 7, it gives incorrect result.

Result should be:

2 2 2 4 6 6 6 8 8 11 12

But is instead

2 2 2 4 6 6 6 2 2 11 12

What does "maximum element from 3 consecutive elements" mean? Is the result you show there correct, or incorrect? If incorrect, what should the result be? Please read How to Ask. — Chris
– Chris, Commented Aug 15, 2020 at 19:05
It's not really worth trying to do this in base Python. Pandas would be easier. See ^ — roganjosh
– roganjosh, Commented Aug 15, 2020 at 19:06
@TedKleinBergman I'm kinda inclined to agree, but you've got a decent bit of rep from your answer :P I think the sliding window is easier to implement (and faster) using other tools, and the dupe status will prevent other answers that I suspect will be sub-par — roganjosh
– roganjosh, Commented Aug 15, 2020 at 19:22

Ted Klein Bergman · Accepted Answer · 2020-08-15 19:15:34Z

7

That's because there's two 45's and index returns the first occurrence. You can pass a start and end argument to tell the index method from which indices to start looking from.

for i in range (0, len(arr)-3):
    print(arr.index(max(arr[i : i+3]), i, i+3))

Or alternatively:

for i in range (0, len(arr)-3):
    sliced_array = arr[i : i+3]
    print(i + sliced_array.index(max(sliced_array)))

edited Aug 15, 2020 at 19:15

answered Aug 15, 2020 at 19:08

Ted Klein Bergman

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6 Comments

alani Over a year ago

Nice. The alternative, which I won't bother to post as a separate answer, is to save the slice (arr[i:i+3]) in a separate variable and do i + the_slice.index(max(the_slice))

Ted Klein Bergman Over a year ago

@alaniwi True. I'll add that. Thanks!

alani Over a year ago

Incidentally, it shouldn't need that end argument. (Not that it does any harm.)

Ted Klein Bergman Over a year ago

@alaniwi True, but having the end argument will reduce the amount of elements the index method have to compare

alani Over a year ago

@TedKleinBergman It will tell it when to give up if it hasn't yet found a match, but it is already constrained to find a match before it gets to the point where it could give up.

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