Finding the first list element for which a condition is true

Question

I was looking for an elegant (short!) way to return the first element of a list that matches a certain criteria without necessarily having to evaluate the criteria for every element of the list. Eventually I came up with:

(e for e in mylist if my_criteria(e)).next()

Is there a better way to do it?

To be more precise: There's built in python functions such as all() and any() - wouldn't it make sense to have something like first() too? For some reason I dislike the call to next() in my solution.

Related : http://stackoverflow.com/questions/2361426/what-is-the-best-way-to-get-the-first-item-from-an-iterable-matching-a-condition — Ashwini Chaudhary, Jun 06 '13 at 13:15
Why must everything be an elegant one liner? Create a function with a meaningful name and be done with it. — Steven Rumbalski, Jun 06 '13 at 13:24
There's no `first` function in python, but it's easy to write: `first = next` ;) — georg, Jun 06 '13 at 13:25

score 15 · Answer 1 · answered Jun 06 '13 at 13:14

15

How about:

next((e for e in mylist if my_criteria(e)), None)

answered Jun 06 '13 at 13:14

iruvar

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score 10 · Accepted Answer · answered Jun 06 '13 at 13:15

10

Nope - looks fine. I would be tempted to re-write possibly as:

from itertools import ifilter
next(ifilter(my_criteria, e))

Or at least break out the computation into a generator, and then use that:

blah = (my_function(e) for e in whatever)
next(blah) # possibly use a default value

Another approach, if you don't like next:

from itertools import islice
val, = islice(blah, 1)

That'll give you a ValueError as an exception if it's "empty"

answered Jun 06 '13 at 13:15

Jon Clements

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For someone who is seeing this for Python 3 , `itertools.ifilter` is now simply `filter` and is available in global namespace. – xssChauhan Jul 20 '17 at 10:45

score 1 · Answer 3 · answered Jun 06 '13 at 13:15

1

I propose to use

next((e for e in mylist if my_criteria(e)), None)

or

next(ifilter(my_criteria, mylist), None)

answered Jun 06 '13 at 13:15

oleg

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score 0 · Answer 4 · answered Jun 06 '13 at 13:29

0

with for loop

lst = [False,'a',9,3.0]
for x in lst:
    if(isinstance(x,float)):
        res = x
        break

print res

answered Jun 06 '13 at 13:29

kiriloff

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Finding the first list element for which a condition is true

4 Answers4

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