Set(a)

Question

a=[7,2,8,1,9]
print(set(a))
Result - [8,1,2,9,7]
Why? A thought result [1,2,7,8,9]

Answer 1

I think it may have been this: https://www.sololearn.com/Discuss/1556797/which-logic-JUMP_LINK__&&__python__&&__JUMP_LINK-is-using-for-sorting-sets

Answer 2

The order in the set depends on the hash value and the hash value for strings is not constant over different runs of the code (other than for integers):

https://code.sololearn.com/cvDIC9X1gpcA/?ref=app

(run this code several times)

Answer 3

Yes, it doesn't even give the same result every time. Try running this code 10 times or so
https://code.sololearn.com/cPAtxp9XJ3sX/?ref=app

In the end I still don't know how Python does it.

Answer 4

Sets are by definition unordered.

Answer 5

Python may have a way of presenting the elements of a set in a specific order, but that's nothing you can to control.

You can not arbitrarily sort a set or create a specific order. You can basically just put stuff in and take it out, and determine if a thing is in it or not.

Answer 6

If you want to know this, you'll have to read Python's source code.

It's part of the implementation, we don't need to know how exactly it works.

Answer 7

Because sets are not used for ordering.

You have all sorts of other container types for it: lists, tuples, bytearrays etc.

Answer 8

Your answers are excellent and I am grateful, but my question is not exhausted. I will look for an answer through experience :) thanks

Answer 9

The basis for all containers in cpython are c-arrays which have a fixed length. So every container has a default size and specific rules when and how to expand. We don't notice any of that because it all happens automatically (hail python ;-)).

In a set the values are placed based on their hash value which for integers is just the integer itself. If the hash value/integer is bigger than the default size of the array, it gets distributed using a modulo based mechanism. This is why you should get a kind of ordered set if you only use lower numbers.

It is probably a bit more complicated than that because you also need a way to prevent collisions but that is about the gist of it.

This said I don't really get your output because if I try it here on SL I get the output you expected to get. On what system did you get the other output?

Answer 10

Kishalaya Saha Flandre Scarlet
It seems like the order of the set is independant of the length of the list and how many times you call it in a python run instance.
It is however different per run instance.

https://code.sololearn.com/cK0zX6aPDhi4/?ref=app

Answer 11

Maybe another way to get a random number.

Answer 12

don,t understand :(( why always result for this is single?

Answer 13

I believe that this function has alternation logic. The result is always the same for my example. what is the logic ?

Answer 14

I understand result is not ordered, but why only one and only so ?

Answer 15

Why we dont need?

Answer 16

I think there was a long discussion about it quite a while ago, but I can't seem to find it.

Maybe Kishalaya Saha knows?

Answer 17

ok. I wil look.later

Answer 18

I have the order of this function associated with the address of the cells in memory .. and your links about hash

Answer 19

I try on other comps! But i have one result. But in HonFu's link are many so examples. Now i read their experiments. they had different results sometimes

Answer 20

Python 3.5 , vista :) 32bit