Index of array with maximum length

Question

I am trying to get the index of the array with the longest length. I have this array of arrays.

[
 [0,7,3],
 [1],
 [2,6,5,2,9],
 [1,2]
]

The desired outcome is 2 because it is the array with the maximum length. I have this working code:

max_len = -1
index = -1

for i, feature in enumerate(features):
    if max_len < len(feature):
        max_len = len(feature)
        index = I

print(index)

I think that should be a one-liner or a much better and simpler way to do it. Does anyone know a better way to do it?

Thanks!

timgeb · Accepted Answer · 2022-01-31 17:25:52Z

3

That looks like a python list, not a numpy array. You can use l.index(max(l, key=len)) if l is your list.

answered Jan 31, 2022 at 17:25

timgeb

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3 Comments

Shiplu Mokaddim Over a year ago

This is not very efficient. You are iterating the list 2 times. One in index() call and other is in max call.

timgeb Over a year ago

@ShipluMokaddim it is much more efficient than pulling from the enumerate iterator and calling a python lambda for each element. Time it yourself.

Michael Szczesny Over a year ago

@timgeb - benchmarked both solutions with 1000 to 100_000 lists of length 4 to 1000. This solution is ~5x faster.

user101893 · Accepted Answer · 2022-01-31 17:28:24Z

0

This is a one-liner:

vals = [[0,7,3],[1],[2,6,5,2,9],[1,2]]
pd.Series(vals).apply(lambda x: len(x)).idxmax()

You could also do this without pandas:

lens = [len(val) for val in vals]
print(lens.index(max(lens)))

answered Jan 31, 2022 at 17:28

user101893

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Comments

Learning is a mess · Accepted Answer · 2022-01-31 22:35:51Z

0

You can use enumerate and max for this one:

max_length_index, _ = max(enumerate(l), key=lambda index_and_lst  : len(index_and_lst[1]))

edited Jan 31, 2022 at 22:35

answered Jan 31, 2022 at 17:27

Learning is a mess

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2 Comments

timgeb Over a year ago

The key function expects one parameter

Learning is a mess Over a year ago

@timgeb Indeed, looks like I cannot use unrolling here.

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