Convert lists of lists of string into a list of string efficently

Question

I have a lists of lists of strings which I would like to convert to a list of strings adding a space between each list item. eg.

original_list = [['the', 'cat', 'in', 'the', 'hat'], ['fat', 'cat', 'sat', 'on', 'the', 'mat']]

desired_output = ['the cat in the hat', 'fat cat sat on the mat']

I understand that I can do it using this:

desired_output
for each in original_list:
    desired_output.append(' '.join(each))

but as I am working with large amounts of data am ideally looking for a more efficient way to do this.

It should be ' '.join(each) instead of ''.join(each) in your code — Kaushik NP
– Kaushik NP, Commented Oct 20, 2017 at 15:34

Ajax1234 · Accepted Answer · 2017-10-20 15:36:21Z

4

Use str.join with a full space ' ':

original_list = [['the', 'cat', 'in', 'the', 'hat'], ['fat', 'cat', 'sat', 'on', 'the', 'mat']]
final_list = [' '.join(i) for i in original_list]

Output:

['the cat in the hat', 'fat cat sat on the mat']

edited Oct 20, 2017 at 15:36

answered Oct 20, 2017 at 15:35

Ajax1234

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4 Comments

Kaushik NP Over a year ago

Um, how is this an optimisation? The OP has already done this, just not using list comprehension.

Ajax1234 Over a year ago

@KaushikNP str.join is much faster than string concatenation because strings are immutable and thus cannot be changed in place.

Kaushik NP Over a year ago

But that's not what the user has done. The OP too has used join. But yeah, one thing I overlooked. The user used append and list comprehension has a slight advantage there. So works out. +1

Nick is tired Over a year ago

@KaushikNP it also optimised LOCs, reducing dev time ;)

Rafael Aguilar · Accepted Answer · 2017-10-20 18:47:22Z

1

Another pythonic and simple way, in Python 3, could be using map, says another SO discussion it should be faster, it would go like this:

original_list = [['the', 'cat', 'in', 'the', 'hat'], ['fat', 'cat', 'sat', 'on', 'the', 'mat']]

#              (------where magic happens--------)
desired_list = list(map(' '.join, original_list ))

#print
#output ['the cat in the hat', 'fat cat sat on the mat']

edited Oct 20, 2017 at 18:47

answered Oct 20, 2017 at 15:59

Rafael Aguilar

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