On 30 Aug 2020, at 09:03, Jeff Allen <ja.py@farowl.co.uk> wrote:

On 29/08/2020 14:17, Barry Scott wrote:

On 29 Aug 2020, at 13:42, Filipp Bakanov <filipp@bakanov.su> wrote:

I'd like to propose adding argmax and argmin functions to the python list. These functions return the index of a maximum / minimum element of the list. Eg:

a = [1, 4, 2, 3]

print(a.argmax())  # 1

print(a.argmin())  # 0

It's a very popular request (based on stackoverflow https://stackoverflow.com/questions/16945518/finding-the-index-of-the-value-which-is-the-min-or-max-in-python ), and currently there is no elegant way to find it.

What do you think?

Just do this:

>>> a=[1,4,2,3]

>>> min(a)

1

>>> a.index(min(a))

0

>>> a.index(max(a))

1

Barry

This has the drawback of passing twice over the list. The following doesn't, but the complexity somewhat makes Filipp's point:

>>> min((e, i) for i, e in enumerate(a))[1]
0

That is 4x slower then my code for 1,000,000 items.

--------------- a.py --------------

import sys

import time

import random

alg = sys.argv[1]

size = int(sys.argv[2])

x = [random.randint(0, 1_000_000) for _ in range(size)]

start = time.time()

if alg == 'barry':

    m = x.index(min(x))

elif alg == 'jeff':

    m = min((e, i) for i, e in enumerate(x))[1]

end = time.time()

print( alg, end-start, (end-start)/size )

-------------------------

Here is the output I got on my laptop.

barry 0.022754907608032227 2.2754907608032226e-08

barry 0.03325295448303223 3.325295448303223e-08

barry 0.034243106842041016 3.4243106842041016e-08

barry 0.02784109115600586 2.784109115600586e-08

jeff 0.13722586631774902 1.3722586631774904e-07

jeff 0.1359708309173584 1.359708309173584e-07

jeff 0.13658690452575684 1.3658690452575684e-07

I think one would want argmin() and argmax() to work with general iterables, so I wonder if the stdlib would not be a better home than list itself. I half expected it to be an itertools recipe. The advantage of a recipe is that variations such as needing the last occurrence of the minimum are easily accommodated.

Surely its index_min() and index_max() not argmin() and argmax().

Barry

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