How do you write one element of multiple lists to a file before moving on to the next element?

Question

I have multiple lists, for example,

x = [1,2,3,4]
y = [5,6,7,8]
z = [9,10,11,12]

And I want to write one element of each list to a file.

The output I'm trying to get is 1 5 9 2 6 10 3 7 11 4 8 12. So all the first elements, then all the second, and so forth.

What have you tried? Can you show your code please? Also, are you assuming all lists are the same size? — idjaw, Mar 11 '16 at 02:26
`zip(x, y, z)`? Have you tried search your question before asking? — Remi Guan, Mar 11 '16 at 02:26
I've tried nested for loops, but that doesn't work since it repeats it way too many times. Like `for i in range(len(x)): and for j in range(len(y)) and k in range(len(x))` Also, no the lists are not the same size (not necessarily) — 79t97g, Mar 11 '16 at 02:26
`write_file.write(" ".join(x for tup in zip(x, y, z) for x in tup))` — zondo, Mar 11 '16 at 02:28

score 1 · Accepted Answer · edited May 23 '17 at 12:15

1

Use Zip and convert each integer element to string so that you can join them:

x = [1,2,3,4]
y = [5,6,7,8]
z = [9,10,11,12]

g = " ".join(str(x) for t in zip(x, y, z) for x in t)

Write g to your file.

Excerpt on how Zip works:

When you zip() together M lists containing N elements each, the result has N elements. Each element is a M-tuple.

In other words, zip(x,y,z) changes your:

[1,2,3,4]
[5,6,7,8]
[9,10,11,12]

to

[(1,5,9),(2,6,10),(3,7,11),(4,8,12)]

edited May 23 '17 at 12:15

Community

1
1

answered Mar 11 '16 at 02:31

Ian

30,182
19
69
107

Thanks, this worked. I forgot I had to convert it to a string before joining it. – 79t97g Mar 11 '16 at 02:33

score 0 · Answer 2 · answered Mar 11 '16 at 02:37

0

This should give you the ability to have three lists that might have different number of items in each one.

number = max(x.len(), y.len(),z.len())
count = 0
total = {}
while count < number:
     try:
           total.append(x[count])
     except:
           pass
     try:
           total.append(y[count])
     except:
           pass
     try:
           total.append(z[count])
     except:
           pass
     count += 1
print(' '.join(total))

answered Mar 11 '16 at 02:37

arcee123

101
9
41
118

Why not use `itertools.zip_longest()`? – zondo Mar 11 '16 at 02:39
because I'm not that educated. I'll have to go dig it about. – arcee123 Mar 11 '16 at 02:40
Don't worry. I didn't know about until this month :) – zondo Mar 11 '16 at 02:41
It is good that your solution can handle multiple lists with different number of items. However, if you just remove the try-catch and doing pre-check before adding the element, the solution would be a lot better – Ian Mar 11 '16 at 02:42
is it faster to run a If vs a try? I always thought a try was faster. – arcee123 Mar 11 '16 at 02:42
`try` is OK, but `except` is not. If your lists contain different number of items, then it is expected that it will have many `excepts` – Ian Mar 11 '16 at 02:44

score 0 · Answer 3 · answered Mar 11 '16 at 02:50

In case you don't have lists of equal lengths, you can use zip_longest (or izip_longest in python 2):

from itertools import zip_longest

x = [1, 2, 3, 4, 1]  
y = [5, 6, 7, 8, 99]
z = [9, 10, 11, 12]  # short list

reordered = []
for elements in zip_longest(x, y, z):
    reordered.extend(elements)

Results in a list of:

[1, 5, 9, 2, 6, 10, 3, 7, 11, 4, 8, 12, 1, 99, None]

How do you write one element of multiple lists to a file before moving on to the next element?

3 Answers3