Fastest way to convert lists into plain text?

Question

I need to convert a series of lists into one single line of text separated by a blank space

def Function(lists):
    Numbers= str(len(lists))+ " "
    for item in lists:
        for items in item:
            Numbers = Numbers + str(items) +'  ' 
    print Numbers

Data = [[1,2,3],[10,20,30]]
Function(Data)

this should return "2 1 2 3 10 20 30" However this approach is very slow.

is there a way to do this faster?

score 4 · Accepted Answer · edited May 23 '17 at 11:50

4

You are creating a new string on every + operator, which is horribly slow(when applied a lot). Instead use a generator expression with str.join:

In [11]: from itertools import chain

In [12]: data = [[1,2,3],[10,20,30]]

In [13]: " ".join(str(i) for i in chain(*data))
Out[13]: '1 2 3 10 20 30'

Edit:

And you can append the list length with and method you want, since it doesn't affect performance much:

" ".join((str(len(data)), " ".join(str(i) for i in chain(*data)))

Edit2:

hcwhsa pointed out that generator expression has no advantage with str.join. So you may want to use a list comprehension instead of generator expression.

edited May 23 '17 at 11:50

Community

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answered Oct 20 '13 at 14:56

utdemir

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[A generator expression has no performance/memory advantage when used with `str.join`](http://stackoverflow.com/a/9061024/846892). – Ashwini Chaudhary Oct 20 '13 at 14:58
I thought that is trivial. Added. – utdemir Oct 20 '13 at 15:04
@hcwhsa, I didn't know that `str.join` needs two passes over argument, thank you. – utdemir Oct 20 '13 at 15:05
But as hcwsa mentions in my question, he wants the length of the list as the first element to the string :P – Games Brainiac Oct 20 '13 at 15:21

Games Brainiac · Answer 2 · 2013-10-20T15:20:05.000

2

This was the simplest to me:

>>> data = [[1,2,3],[10,20,30]]
>>> " ".join([str(item) for var in data for item in var])
'1 2 3 10 20 30'

So you can write your function like so:

def fun(data):
    return " ".join([str(item) for var in data for item in var])

I do not know if this was a mistake on your behalf, but if you want to add the length of the list as the first number, then you can do the following:

def fun(data):
    return str(len(data)) + ' ' + " ".join([str(item) for var in data for item in var])

edited Oct 20 '13 at 15:20

answered Oct 20 '13 at 15:06

Games Brainiac

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Expected output is `"2 1 2 3 10 20 30"`. – Ashwini Chaudhary Oct 20 '13 at 15:07
@hcwhsa I think thats a mistake. – Games Brainiac Oct 20 '13 at 15:09
I don't think so, see: `Numbers= str(len(lists))+ " "` – Ashwini Chaudhary Oct 20 '13 at 15:12
@hcwhsa You're right. Still quite weird, don't you think? Thanks for helping me out. – Games Brainiac Oct 20 '13 at 15:20

score 1 · Answer 3 · answered Oct 20 '13 at 14:58

1

please consider to use join method of string

def Function(lists):
    data = " ".join(j for i in lists for j in i))
    return " ".join([len(lists), data])

answered Oct 20 '13 at 14:58

oleg

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Fastest way to convert lists into plain text?

3 Answers3