How do I merge a 2D array in Python into one string with List Comprehension?

Question

List Comprehension for me seems to be like the opaque block of granite that regular expressions are for me. I need pointers.

Say, I have a 2D list:

l         


        

Answer 1

There's a couple choices. First, you can just create a new list and add the contents of each list to it:

li2 = []
for sublist in li:
    li2.extend(sublist)

Alternately, you can use the itertools module's chain function, which produces an iterable containing all the items in multiple iterables:

import itertools
li2 = list(itertools.chain(*li))

If you take this approach, you can produce the string without creating an intermediate list:

s = ",".join(itertools.chain(*li))

Answer 2

Try that:

li=[[0,1,2],[3,4,5],[6,7,8]]
li2 = [ y for x in li for y in x]

You can read it like this:
Give me the list of every ys.
The ys come from the xs.
The xs come from li.

To map that in a string:

','.join(map(str,li2))

Answer 3

My favorite, and the shortest one, is this:

li2 = sum(li, [])

and

s = ','.join(li2)

EDIT: use sum instead of reduce, (thanks Thomas Wouters!)

Answer 4

There are many ways to do this problem. I like Numpy's tools because it is normally already imported in everything I do. However, if you aren't using Numpy for anything else this probably isn't a good method.

import numpy
li = [[0,1,2],[3,4,5],[6,7,8]]
li2=li[0] #first element of array to merge
i=1 
while i<len(li):
    li2=numpy.concatenate((li2,li[i]))
    i+=1
print li2

This would print [0 1 2 3 4 5 6 7 8] and then you can convert this into your string too.

Answer 5

For the second one, there is a built-in string method to do that :

>>> print ','.join(str(x) for x in li2)
"0,1,2,3,4,5,6,7,8"

For the first one, you can use join within a comprehension list :

>>> print ",".join([",".join(str(x) for x in li])
"0,1,2,3,4,5,6,7,8"

But it's easier to use itertools.flatten :

>>> import itertools
>>> print itertools.flatten(li)
[0,1,2,3,4,5,6,7,8]
>>> print ",".join(str(x) for x in itertools.flatten(li))
"0,1,2,3,4,5,6,7,8"

N.B : itertools is a module that help you to deal with common tasks with iterators such as list, tuples or string... It's handy because it does not store a copy of the structure you're working on but process the items one by one.

EDIT : funny, I am learning plenty of way to do it. Who said that there was only one good way to do it ?

Answer 6

import itertools
itertools.flatten( li )