Indexing a nested list in python

Question

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?

Answer 1

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)

Answer 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.