If I have, for example, a list of tuples such as
a = [(1,2)] * 4
how would I create a list of the first element of each tuple? That is, [1, 1, 1, 1].
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6 Answers
Use a list comprehension:
>>> a = [(1,2)] * 4
>>> [t[0] for t in a]
[1, 1, 1, 1]
You can also unpack the tuple:
>>> [first for first,second in a]
[1, 1, 1, 1]
If you want to get fancy, combine map and operator.itemgetter. In python 3, you'll have to wrap the construct in list to get a list instead of an iterable:
>>> import operator
>>> map(operator.itemgetter(0), a)
<map object at 0x7f3971029290>
>>> list(map(operator.itemgetter(0), a))
[1, 1, 1, 1]
1 Comment
Fred Foo
The tuple-unpacking option is fastest (on my Python 2.7, using a million-element list with no structure sharing in the tuples). The differences are quite small, though.
Two alternatives to phihag's list comprehension:
[x for x, y in a]
from operator import itemgetter
map(itemgetter(0), a)
Comments
There are several ways:
>>> a = [(1,2)] * 4
>>> # List comprehension
>>> [x for x, y in a]
[1, 1, 1, 1]
>>> # Map and lambda
>>> map(lambda t: t[0], a)
[1, 1, 1, 1]
>>> # Map and itemgetter
>>> import operator
>>> map(operator.itemgetter(0), a)
[1, 1, 1, 1]
The technique of using map fell out of favor when list comprehensions were introduced, but now it is making a comeback due to parallel map/reduce and multiprocessing techniques:
>>> # Multi-threading approach
>>> from multiprocessing.pool import ThreadPool as Pool
>>> Pool(2).map(operator.itemgetter(0), a)
[1, 1, 1, 1]
>>> # Multiple processes approach
>>> from multiprocessing import Pool
>>> def first(t):
return t[0]
>>> Pool(2).map(first, a)
[1, 1, 1, 1]
answered Jun 27, 2012 at 16:48
Raymond Hettinger
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6 Comments
phihag
+1 for the most over-engineered code to work on
[(1,2)] * 4.
Raymond Hettinger
@phihag Never miss an opportunity to use simple examples as way to introduce a full range of Python capabilities :-) I presume the OP will have more interesting datasets to work with ;-)
user1475412
@Raymond Hettinger you are correct. In practice I have a list of over 2 million such tuples.
phihag
@user1475412 If they're indeed tuples, you won't get any performance advantage by using multiple threads or multiple processes, because the cost of calling
operator.itemgetter(0) is minimal. A multithreaded approach is only useful if the operation you're applying on each element is slow.phihag
@RaymondHettinger The last example fails on my system with a rather curious error.
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a = [(1,2)] * 4
first_els = [x[0] for x in a]
1 Comment
Sven Marnach
Don't use
tuple as a variable name. In particular don't use it as the name of a list!Assuming you have a list of tuples:
lta = [(1,2), (2,3), (44,45), (37,38)]
access the first element of each tuple would involve subscripting with [0], and visiting each tuple to extract each first element would involve a a list comprehension, which can be assigned to a variable as shown below:
resultant_list = [element[0] for element in lta]
>>> resultant_list
[1, 2, 44, 37]
Comments
I recently found out about Python's zip() function. Another way to do what I want to do here is:
list( zip( *a )[0] )
tup_list = zip( list1, list2 ) interleaves two lists into a list of 2-tuples, but zip( *tup_list ) does the opposite, resulting in a list of a tuple of list1 and a tuple of list2.