How can I manually (with an explicit loop) find the index of the maximum value in a list? [duplicate]

Question

I am able to get the maximum value in the array with an explicit loop:

def main():
    a = [2,1,5,234,3,44,7,6,4,5,9,11,12,14,13]
    max = 0
    for number in a:
        if number > max:
            max = number
    print(max)

if __name__ == '__main__':
    main()

How can I get the index (position) of that value?

“without using max() of course” – I did understand that correctly, you don’t want to use the built-in function, right? — poke
– poke, Commented Jul 17, 2012 at 21:07
Yeah, I was hoping for an answer like Recursed's below. I'm trying to learn programming the hard way first before I can use the built-in functions ;) — Shankar Kumar
– Shankar Kumar, Commented Jul 17, 2012 at 21:10
It also depends on the definition of "built-in functions", as (for instance) if number > max could be considered using the builtin method __gt__ of the built-in type list :) Seriously though - why can't you just use the built-in max - just an intellectual exercise or self-torture? — Jon Clements
– Jon Clements, Commented Jul 17, 2012 at 21:12
Yeah, I'm just a beginner trying to become a "smarter" programmer! — Shankar Kumar
– Shankar Kumar, Commented Jul 17, 2012 at 21:14
So, am I right in thinking that you're aware that max is the correct, concise and efficient method for this in Python, but you're after ways of how not to do it? — Jon Clements
– Jon Clements, Commented Jul 17, 2012 at 21:17

ovgolovin · Accepted Answer · 2012-07-17 21:15:49Z

30

In my code I would use this:

>>> max(enumerate(a),key=lambda x: x[1])[0]
3

answered Jul 17, 2012 at 21:15

ovgolovin

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2 Comments

Andrew Clark Over a year ago

Alternatively, max(range(len(a)), key=lambda i: a[i])

Joel Cornett Over a year ago

Or even: max(zip(a, range(len(a)))[1] although if two elements are equal, this will return the element with the highest index, while yours will return the element with the lowest index.

Jon Clements · Accepted Answer · 2012-07-17 21:02:59Z

23

A simple one liner of:

max( (v, i) for i, v in enumerate(a) )[1]

This avoids having to .index() the list after.

answered Jul 17, 2012 at 21:02

Jon Clements

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3 Comments

inspectorG4dget Over a year ago

As a note, if the list has duplicates, this will return the largest index at which the max element resides. (5,3) < (5,4) returns True

mara004 Over a year ago

@inspectorG4dget I think this can be amended by making the index negative.

mara004 Over a year ago

@JonClements The downside of this is unnecessary comparisons of indices if the same value occurs several times consecutively in an ascending sequence. e.g. 1, 3, 3, 3, 5 would compare i 1 < 2 and 2 < 3, although we'd only need to keep track of the current max value's index instead of comparing anything.

Levon · Accepted Answer · 2012-07-17 21:42:42Z

12

Update:

max_idx = -1
max_val = a[0]
for i in xrange(1, len(a)):
    if a[i] > max_val:
        max_val = a[i]
        max_idx = i

This doesn't shadow built-in function max(), and also will give correct answers for lists that consist of only negative values.

Previous solution

a.index(max(a))

will do the trick.

Built-in function max(a) will find the maximum value in your list a, and list function index(v) will find the index of value v in your list. By combining them, you get what you are looking for, in this case the index value 3.

Note that .index() will find the index of the first item in the list that matches, so if you had several identical "max" values, the index returned would be the one for the first.

For more information:

In the spirit of "Simple is better than complex." (Zen of Python)

edited Jul 17, 2012 at 21:42

answered Jul 17, 2012 at 21:01

Levon

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2 Comments

Levon Over a year ago

@ShankarKumar Sorry, I read your question too quickly initially, I updated my answer just FYI.

Wayne Workman Over a year ago

I am not dealing with negative numbers in my scenario, your previous solution is a perfect and simple solution.

Rob Wagner · Accepted Answer · 2012-07-17 21:04:01Z

8

If you aren't allowed to use the built in index() function, just iterate with an index, instead of using a foreach loop.

for i in range(len(a)):
    if a[i] > max:
        max = a[i]
        maxIndex = i

answered Jul 17, 2012 at 21:04

Rob Wagner

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9 Comments

Shankar Kumar Over a year ago

Thanks, this was what I was looking for. Something that didn't use any built-in functions! :)

Jon Clements Over a year ago

@ShankarKumar Being a pedant range and len are builtin functions :)

Joel Cornett Over a year ago

I might use enumerate, instead of range.

Levon Over a year ago

This shadows built-in function max (which may not be a problem since we aren't suppose to use it .. :-) but probably not good practice, and wouldn't work correctly for lists that only contain negative values

Rob Wagner Over a year ago

@Levon You shouldn't assign max to zero before the list anyway, you should set it to the first item in the list.

|

AlexP · Accepted Answer · 2014-10-01 14:05:22Z

6

Use the argmax method of the numpy.array object.

import numpy as np
np.array(a).argmax()

answered Oct 1, 2014 at 14:05

AlexP

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Comments

poke · Accepted Answer · 2012-07-17 21:04:33Z

1

You can use enumerate to also give you an index while iterating through a list:

>>> a = [2, 1, 5, 234, 3, 44, 7, 6, 4, 5, 9, 11, 12, 14, 13]
>>> maxIndex, maxNumber = 0, 0
>>> for index, number in enumerate(a):
        if number > maxNumber:
            maxIndex = index
            maxNumber = number

>>> maxIndex, maxNumber
(3, 234)

answered Jul 17, 2012 at 21:04

poke

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Comments

Mohamed Emad · Accepted Answer · 2017-05-16 13:59:15Z

1

this is way simpler

x.index(max(x)) #where x is your list

answered May 16, 2017 at 13:59

Mohamed Emad

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1 Comment

José Tomás Tocino Over a year ago

The OP specifically said without using max().

MoRe · Accepted Answer · 2012-07-17 21:03:04Z

0

Use the index(x) function. See the documentation here http://docs.python.org/tutorial/datastructures.html

def main():
    a = [2,1,5,234,3,44,7,6,4,5,9,11,12,14,13]
    max = 0
    for number in a:
        if number > max:
            max = number
    max_index = a.index(max)
    print max

However, this is not as fast as other suggested answers (e.g. using enumerate). Simple though.

answered Jul 17, 2012 at 21:03

MoRe

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

-3

If you like powerfull code you would like this :) If you just have integer numbers you can substitute float by int.

maximum= max(map(float,[2,1,5,234,3,44,7,6,4,5,9,11,12,14,13]))

If you have your input in a text file do this:

file.txt

2 1 5 234 3 44 7 6 4 5 9 11 12 14 13

maximum= max(map(float,(open('file.txt', 'r').readline()).split()))

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 1, 2013 at 4:33

Carlos Neves

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