0

so pretty straight through idea, just want to find the most frequent element but I am just lost on exactly what is happening in this small code snippet. I like how this code doesn't import anything, just simply uses the built-ins. But I dont understand exactly how it works. Here it is:

def most_frequent(List):
    return max(set(List), key = List.count)

w3schools defines the python max() as doing the following:

The max() function returns the item with the highest value, or the item with the highest value in an iterable. If the values are strings, an alphabetically comparison is done.

Makes sense so far, so if we have a list like so a_list = [3,2,5,1], then max(a_list) gives us 5. Okay, simple enough.

But in our function why is set being used here? I understand set is a built-in data structure that ensures all data inside it to be unique. And what is key? why are there two parameters here inside like this inside the max()? I've never seen anything like this before. Running something like max(4,2,6,3,2) makes sense but putting the data structure set to enclose the list and assigning something called key and calling count on the list? What in the world is going on here? Can someone please break this down like I am five and explain how we are able to use the max() like this when the definition is just to find the highest value? What is set and key doing here? Is it like a hashtable? Quite lost here would truly appreciate the help.

Thank you

Improve this question
5
  • set is used because it eliminates duplicates. The key parameter to max lets you supply a filter function; it will return the element where that function has its largest value, not just where the element has its largest value. It's a bit tricky. Commented Oct 27, 2021 at 17:47
  • Note, this algorithm is bad, potentially very inefficient, don't use it. Commented Oct 27, 2021 at 17:50
  • "w3schools defines the python max() as doing the following" No. Don't go to w3schools. Just go to the actual documentation. Have you looked at it? What did it say about the key argument to max? Commented Oct 27, 2021 at 17:51
  • @juanpa.arrivillaga Thanks! What's a better approach to find the highest frequency? and where is the documentation for max() Commented Oct 27, 2021 at 18:00
  • @Preston_Jarvis here's the relevant docs. Better would be to create a dictionary of counts, you could just use a collections.Counter object, something like counts = collections.Counter(data) then you just want max(counts.items(), lambda x:x[1]). The above is going to be linear time Commented Oct 27, 2021 at 18:02

1 Answer 1

Reset to default
2

The max documentation states that key is an ordering function, the method gives a new value for each item of the iterable , which will be used for ordering

Without key : natural order

max([4, 3, 5, 6]) 
     4, 3, 5, 6    << values used for finding the max
           ^

With key : order if values given by the key

max([ 4,  3,  5,  6], key=lambda x: -x) 
     -4, -3, -5, -6    << values used for finding the max
          ^

Your case, the list.count, that telles that 2 occurs 2, and that is more than the ones

l =[4, 2, 6, 3, 2]

max([4, 2, 6, 3], key=lambda x: l.count(x))
     1  2  1  1      << values used for max, regarding the
        ^
Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Thanks, still trying to wrap my head around it. W3 is a bad resource, didn't cover that detail at all. Going with the python docs all the way. So if I understand correctly max() can take either a single iterable like such as a list, or an iterable a key for ordering. Not really understanding the ordering part, ordering how? And why use set()? Doesn't that remove duplicates that need to be counter?
@Preston_Jarvis ordering regarding the value given by the key method. The set just allows to find the maximum between less values, and the initial list is used for counter, no matter the set

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.