Given data as
data = [ [0, 1], [2,3] ]
I want to index all first elements in the lists inside the list of lists. i.e. I need to index 0 and 2.
I have tried
print data[:][0]
but it output the complete first list .i.e.
[0,1]
Even
print data[0][:]
produces the same result.
My question is specifically how to accomplish what I have mentioned. And more generally, how is python handling double/nested lists?
Using list comprehension:
>>> data = [[0, 1], [2,3]]
>>> [lst[0] for lst in data]
[0, 2]
>>> [first for first, second in data]
[0, 2]
Using map:
>>> map(lambda lst: lst[0], data)
[0, 2]
Using map with operator.itemgetter:
>>> import operator
>>> map(operator.itemgetter(0), data)
[0, 2]
Using zip:
>>> zip(*data)[0]
(0, 2)
With this sort of thing, I generally recommend numpy:
>>> data = np.array([ [0, 1], [2,3] ])
>>> data[:,0]
array([0, 2])
As far as how python is handling it in your case:
data[:][0]
Makes a copy of the entire list and then takes the first element (which is the first sublist).
data[0][:]
takes the first sublist and then copies it.
The list indexing or nesting in general (be it dict, list or any other iterable) works Left to Right. Thus,
data[:][0]
would work out as
(data[:]) [0] == ([[0, 1], [2,3]]) [0]
which ultimately gives you
[0, 1]
As for possible workaronds or proper methods, falsetru & mgilson have done a good job in that regards.
try this:
print [x for x, y in data[:]]
data[:]? It could just be data. (And, doesn't look like OP needs a new list for manipulation)