Python: Anyway to use map to get first element of a tuple

Question

I have a tuple of tuples and I want to put the first value in each of the tuples into a set. I thought using map() would be a good way of doing this the only thing is I can't find an easy way to access the first element in the tuple. So for example I have the tuple ((1,), (3,)). I'd like to do something like set(map([0], ((1,), (3,)))) (where [0] is accessing the zeroth element) to get a set with 1 and 3 in it. The only way I can figure to do it is to define a function: def first(t): return t[0]. Is there anyway of doing this in one line without having to declare the function?

Answer 1

Use a list comprehension:

data = ((1,), (3,))
print [x[0] for x in data]

Answer 2

from operator import itemgetter
map(itemgetter(0), ((1,), (3,)))

While the list comprehensions are generally more readable, itemgetter is closest to what he asked for.

Timing information:

>>> from timeit import Timer
>>> mapped = Timer(setup='from operator import itemgetter\nlst=( ("a",), ("b",), (1,), (2,))', stmt='map(itemgetter(0), lst)')
>>> comprehended = Timer(setup='lst=( ("a",), ("b",), (1,), (2,))', stmt='[i[0] for i in lst]')
>>> comprehended.repeat()
[0.5402599483924249, 0.47599876684973275, 0.48340872102501464]
>>> mapped.repeat()
[0.4333492937609478, 0.31100689245737456, 0.3106918944053909]

Answer 3

mySet = set(x[0] for x in TUPLES)

or in python3:

mySet = {x[0] for x in TUPLES}

Answer 4

You can use a set comprehension in Python 2.7 and 3.x:

>>> t = ((1,), (3,))
>>> s = {x[0] for x in t}
>>> s
set([1, 3])

or in Python < 2.7:

>>> s = set([x[0] for x in t])
>>> s
set([1, 3])

Answer 5

Just another way to get it:

set(x for x, in data)

Answer 6

data = ((1,), (3,))
s = set(zip(*data)[0])

If there are more items in your tuples you might save some memory and time using izip and islice.

Answer 7

Python supports the creation of anonymous function using the lambda keyword. This allows you to use a function without formally defining it. Given your example, you'd use the lambda like this:

data = ((1,), (3,))
set(map(lambda x: x[0], data))

This is equivalent to:

def f(x):
    return x[0]

set(map(f, data))

But as other people have said, list comprehensions are preferred over the use of map.

Answer 8

Go with @Winston. List comprehensions are great. If you really want to use map, use a lambda as previously suggested, or the logically equivalent...

from operator import itemgetter
data = ((1,), (3,))
map(itemgetter(0), data)

This is just for info; You should use the list comp