List max() Method

Question

I have read this. I have also read documentation about max() method, but I don't understand what is the difference between these two

len(max(name_of_the_list))

len(max(name_of_the_list, key=len))

if we want to get the longest item in our list.

Only the later will output the length of the longest item in the list. — Code-Apprentice, Aug 24 '14 at 07:01

Anshul Goyal · Accepted Answer · 2014-08-24T07:12:01.340

4

The first will compare the strings, and whichever is found alphabetically greatest, output its length (default behavior of max).

The second will use the string's length as a key for the max function, and find the string with maximum length, and output its length. Here, the len refers to the python builtin len function.

So both of these are not the same. Since you want to get the longest item in the list, use the second one as it is correct.

As a demo, consider the following

>>> name_of_the_list = ["abcdefgh", "ijkl"]
>>> max(name_of_the_list)
"ijkl"
>>> len(max(name_of_the_list))
4
>>> max(name_of_the_list, key=len)
"abcdefgh"
>>> len(max(name_of_the_list, key=len))
8

edited Aug 24 '14 at 07:12

answered Aug 24 '14 at 06:57

Anshul Goyal

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The OP didn't specify what the elements of the list are. Using strings as an example makes the discussion much more concrete. – Code-Apprentice Aug 24 '14 at 07:02
I assumed they were strings because otherwise the `len` can throw `TypeError` in second example. Also, it doesn't make much sense to compare strings to integers or such. – Anshul Goyal Aug 24 '14 at 07:04
@ mu 無 How can I know which one is "alphabetically greater". Can you describe this term? I mean, not in code, but me as a person, what is criteria for that? – Muhamed Huseinbašić Aug 24 '14 at 07:06
1

@MuhamedHuseinbašić ["alphabetically greater"](http://en.wikipedia.org/wiki/Alphabetical_order) will be the same as [dictionary ordering](http://en.wikipedia.org/wiki/Dictionary_order) or [lexicographical ordering](http://en.wikipedia.org/wiki/Lexicographical_order). – Anshul Goyal Aug 24 '14 at 07:10

enrico.bacis · Answer 2 · 2014-08-24T07:06:10.493

The function max will use the function cmp defined by the type to check which one is the greatest. With the parameter key you can override this behavior specifying the function that need to be applied in order to compute the score of an item, in base of which max will return the maximum.

max(name_of_the_list, key=len)

This will return the longest item in the list (the one whose len returns the biggest number).

If name_of_the_list contains strings, omitting the key results in using the default cmp function defined for the strings which is the alphabetical comparison.

The max function will return the element which has the biggest key function, so if you wrap the call in another len(...) you are getting the length of that element. In order to obtain that, it means that you don't need the information about the element anymore but only the maximum of the keys, so you could do:

max(len(x) for x in name_of_the_list)

of also the equivalent using map:

max(map(len, name_of_the_list))

This way, you will tell python to compute the length of the elements and then return the max of these lengths.

List max() Method

2 Answers2