Adding all the numbers from a list together(not summing). Python 3.3.2

Question

say that I have this list:

alist = [2, 0, 1]

How would I make a string named "hahaha" for example equal to 201 ? Not 5 (sum). Would I need to something like "

for number in alist .....

thanks !

Just for printing it as `201` you can do: `print (*alist, sep='')` — Ashwini Chaudhary, Sep 15 '13 at 19:59

score 4 · Answer 1 · answered Sep 15 '13 at 19:53

4

You can use ''.join() in conjunction with map():

>>> alist = [2, 0, 1]
>>> ''.join(map(str, alist))
'201'

answered Sep 15 '13 at 19:53

arshajii

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```imap``` doesn't create intermediate list :P – Maciej Gol Sep 15 '13 at 20:32

Michael0x2a · Answer 2 · 2013-09-15T19:59:58.210

Is this what you're looking for?

hahaha = ''.join([str(num) for num in alist])
print hahaha  # '201'

To explain what's going on:

The portion [str(num) for num in alist] is called a "list comprehension" -- it's like a very condensed for-loop that transforms one list into another list. In this case, it turns [2, 0, 1] into ['2', '0', '1'].

(We could, of course, write this as a for-loop instead of as a list comprehension, but it take more lines of code, and would be a little tedious to type)

The section portion ''.join(list) takes a list of strings, and sticks them all together with the empty string as the separating character. If I were to do '+'.join([str(num) for num in alist]), the output would be '2+0+1' instead.

So, the line can be thought of as a series of transformations that slowly converts the initial list into a string.

[2, 0, 1] -> ['2', '0', '1'] -> '201'

We can also wrap this whole process into a function, like so:

def make_str(list):
    return ''.join([str(num) for num in list])

print make_str([4, 3, 2, 9])  # '4329'
print make_str([9, 0, 0, 9])  # '9009'

score 1 · Answer 3 · edited Sep 15 '13 at 20:07

1

What about:

>>> alist = [2, 0, 1]
>>> "".join(str(i) for i in alist)
'201'

The code inside join() method is a generator expression.

edited Sep 15 '13 at 20:07

wim

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answered Sep 15 '13 at 19:54

Roman Susi

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[For `str.join` a genexp is both slower and less efficient than LC.](http://stackoverflow.com/a/9061024/846892) – Ashwini Chaudhary Sep 15 '13 at 20:01
@AshwiniChaudhary this is useful information, but do you have some links explaining? I can test it but it would be nice to understand why is it so – Roman Pekar Sep 15 '13 at 20:03
@RomanPekar I've updated my comment with a link. – Ashwini Chaudhary Sep 15 '13 at 20:04
@AshwiniChaudhary thanks, reading... – Roman Pekar Sep 15 '13 at 20:04
@Ashwini Chaudhary You are right. It's even in the docs http://docs.python.org/2/library/timeit.html#basic-examples , and I checked it for both Python2.7 and 3.2, however, generator expressions may be more efficient in other settings. – Roman Susi Sep 15 '13 at 20:08
@wim - thanks for editing. – Roman Susi Sep 15 '13 at 20:09

score 1 · Answer 4 · answered Sep 15 '13 at 19:55

1

The string join function needs a list of strings.

''.join([str(i) for i in alist])

answered Sep 15 '13 at 19:55

Graeme Stuart

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Adding all the numbers from a list together(not summing). Python 3.3.2

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