Equal elements in two lists

Question

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Determine if 2 lists have the same elements, regardless of order?
Match two lists of letters in Python

I have list a and list b. So I need to find, if lists have equal elements. I have no idea, but I am thinking about using collections.

I think you're looking for [this](http://stackoverflow.com/questions/8866652/determine-if-2-lists-have-the-same-elements-regardless-of-order). — David, Nov 05 '12 at 22:22
What do you mean by "equal elements"? Does frequency matter? Does order matter? — arshajii, Nov 05 '12 at 22:22
what about a=[2,2,3] and b=[1,2,2]? Should be [2] or [2,2]? What about order? — enrico.bacis, Nov 05 '12 at 22:23

Andrew Clark · Answer 1 · 2012-11-05T22:52:13.920

4

Assuming you don't care about order. There are a few options here, the best solution depends on the contents of your lists. Here are a few possibilities in order of preference (and restrictions):

If there are no duplicates, and all items are hashable:
```
set(a) == set(b)
```
If there are duplicates, and all items are of the same type:
```
sorted(a) == sorted(b)
```
If there are duplicates and the list may have mixed types, and all types are hashable:
```
collections.Counter(a) == collections.Counter(b)
```
If there are duplicates, the list may have mixed types, and some elements may not be hashable (there must be a better way to do this, but what I came up with was to turn each element into a tuple with the string representation of the type, followed by the value):
```
keyfunc = lambda x: (str(type(x)), x)
sorted(a, key=keyfunc) == sorted(b, key=keyfunc)
```

edited Nov 05 '12 at 22:52

answered Nov 05 '12 at 22:25

Andrew Clark

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"wrapping each element in a tuple allows ordering between different types" - are you sure? Doesn't work for me (3.2) – georg Nov 05 '12 at 22:50
@thg435 Oops, I originally had `lambda x: (type(x), x)` and must have tested just wrapping the value on 2.x thinking it was 3.2 (I have a parallel installation). It turns out that doesn't even work though, current solution is `lambda x: (str(type(x)), x)`, but there must be a better way. – Andrew Clark Nov 05 '12 at 22:53
How about `key=repr` (just kidding)) Otherwise, a good answer to a remarkably poor question. – georg Nov 05 '12 at 23:07

score 0 · Answer 2 · answered Nov 05 '12 at 22:21

0

>>> all(x in b for x in a)

answered Nov 05 '12 at 22:21

yurisich

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This won't work if `a` is a smaller list. For example this will return true when `a = [0, 1]` and `b = [0, 1, 2, 3, 4, 5]` but it should return false. It checks if `a` is contained within `b`, not if they are equal. – Tim Nov 05 '12 at 22:49

Equal elements in two lists

2 Answers2