55

I have a set in Python from which I am removing elements one by one based on a condition. When the set is left with just 1 element, I need to return that element. How do I access this element from the set?

A simplified example:

S = set(range(5))
for i in range(4):
    S = S - {i}
# now S has only 1 element: 4
return ? # how should I access this element
# a lame way is the following
# for e in S:
#    return S
martineau
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Nik
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    If you want to remove an item from a set, `S.remove(i)` is faster. (Note that `S.remove(i)` modifies `S`, while `S = S - {i}` replaces it with a new set.) – user2357112 Dec 17 '13 at 03:52

6 Answers6

57

Use set.pop:

>>> {1}.pop()
1
>>>

In your case, it would be:

return S.pop()

Note however that this will remove the item from the set. If this is undesirable, you can use min|max:

return min(S) # 'max' would also work here

Demo:

>>> S = {1}
>>> min(S)
1
>>> S
set([1])
>>> max(S)
1
>>> S
set([1])
>>>
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    +1, didn't know about `set.pop` although it's not too surprising I suppose. And here I would have done `next(iter(S))` – mgilson Dec 17 '13 at 03:50
  • Only if you're okay with removing the item. Otherwise, a `for` loop or `next(iter(S))` works better. – user2357112 Dec 17 '13 at 03:50
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    Off the record, I tend to use `min(S)`, although I Officially Support(tm) `next(iter(S))`. – DSM Dec 17 '13 at 03:53
  • @DSM - Actually, I like that better than the other method. It is shorter. –  Dec 17 '13 at 03:57
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    From a code documentation perspective, `max()` or `min()` are just.. wrong. Too surprising to the unsuspecting reader of the code. – Martijn Pieters Mar 13 '15 at 19:24
43

I would use:

e = next(iter(S))

This is non-destructive and works even when there is more than one element in the set. Even better, it has an option to supply a default value e = next(iter(S), default).

You could also use unpacking:

[e] = S

The unpacking technique is likely to be the fastest way and it includes error checking to make sure the set has only one member. The downside is that it looks weird.

Raymond Hettinger
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    I timeit'd and the unpacking seems to be 2.5x faster than the iter approach. And min/max are about 2x faster than iter too. – bgusach Apr 11 '14 at 11:58
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    The unpacking assignment approach is nice too. – Martijn Pieters Mar 13 '15 at 19:23
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    Or if you are not fond of the trailing comma, square brackets will work too: `[e] = S`. –  Mar 13 '15 at 19:54
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    Unpacking does look a bit weird, but of the options seems the least bad. – Laizer Oct 23 '17 at 03:52
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    +1 for readability and conveying the intention. This is my pick unless performance is proven to be critical. – Julian Jan 30 '20 at 11:04
  • Unpacking looks slightly less weird if you frame the left-hand side as a tuple rather than a list, i.e. `e, = S`. (I agree with this route being nice as it checks the element count as well.) – Ed Bennett Jun 03 '23 at 10:31
4

Sorry, late to the party. To access an element from a set you can always cast the set into a list and then you can use indexing to return the value you want.

In the case of your example:

return list(S)[0]
J4y
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    As pointed out in this SO question, using `list` is significantly slowly than using `iter`. https://stackoverflow.com/questions/59825/how-to-retrieve-an-element-from-a-set-without-removing-it – Pretzel Dec 06 '17 at 21:24
2

One way is: value=min(set(['a','b','c')) produces 'a'. Obviously you can use max instead.

Jon Nicholson
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1

You can assign the last element to a variable, using star operation.

>>> a={"fatih"}
>>> b=str(*a)
>>> b
'fatih'

Now the varible b have a string object.

If the sole elemant is a number, you could use int():

>>> a={1}
>>> c=int(*a)
>>> c
1
Fatih1923
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0

  1. A set is an unordered collection with no duplicate elements.

  2. Basic uses include membership testing and eliminating duplicate entries.

  3. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

  4. to create an empty set you have to use set(), not {}; the latter creates an empty dictionary, a data structure that we discuss in the next section.

  5. sort a set in acceding/Descending order -> Returns a List of sorted elements, doesn't change the set

    sorted(basket) # In Ascending Order sorted(basket, reverse=True) # In descending order

To get x element of set or from sorted set

list(basket)[x] or sorted(basket)[x]

Note: set is unordered set, so you can use the below code to get x element from a sorted set

Arpan Saini
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