How does python max(list) function work?

Question

I have the following code that doesn't work the way it should.

n = int(input())
arr = map(int, input().split())

num=max(arr)
x=list(set(arr))
print (x)

This returns and empty list "[]". However, if I remove the num=max[arr] line from the code, it works as expected.

n = int(input())
arr = map(int, input().split())

x=list(set(arr))
print (x)

And the output is a list of all elements without duplicates. I wanted to use the max() value somewhere else in the program, but it seems to break the list formation. Why does this happen? Is there a basic property of the max function that I'm missing?

Edit: Why are people downvoting this without any answers? I'm fairly new to python and any help would be appreciated. If I made a silly mistake please point that out.

In Python 3.x, map() returns a lazy iterator (rather than an actual list as in earlier versions). When you call max() on this iterator, all of its elements are consumed, making it appear empty for any further operations. Putting ar = list(ar) as the third line in your code would make an actual list out of it, which can be iterated as many times as you want. — jasonharper
– jasonharper, Commented Feb 22, 2018 at 14:39
I was wondering wth was happening since I use mostly python 2.7. You learn something new every day — usernamenotfound
– usernamenotfound, Commented Feb 22, 2018 at 14:41

Patrick Artner · Accepted Answer · 2018-02-22 14:39:25Z

3

n = int(input())  # unused - why put it in your example?
arr = map(int, input().split())  # returns an iterator

num=max(arr)      # consumes the iterator
x=list(set(arr))  # nothing in the iterator anymore
print (x)         # prints nothing

Fix:

n = int(input())  # unused - why put it in your example?
arr = set(map(int, input().split()))  # returns an set named arr

num=max(arr)      # get num as max from set 
print (arr)       # prints the set named arr

In python 2 map behaved differently - for 3 its an iterator. Once consumed, iterators are "empty". You can see for yourself if you print(type(arr)) for the result of your map operation.

Read: map()

answered Feb 22, 2018 at 14:39

Patrick Artner

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1 Comment

Espoir Murhabazi Over a year ago

Really like all the comment and explanations gives in this answer , like a good teacher

dfundako · Accepted Answer · 2018-02-22 14:40:33Z

0

I'm not sure why you need to use map in this case. Another thing is that you will throw errors on your input if the user does not provide a single int since you are trying to convert it. You can take your input, like a string of '1 4 6 23 5', convert it to a list of ints, and then find the max.

n = '1 4 6 23 5'
arr = [int(x) for x in n.split()]
max(arr)

answered Feb 22, 2018 at 14:40

dfundako

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1 Comment

Patrick Artner Over a year ago

how does your solution prevent throwing errors when someone provides a string with non-integers in it?

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