Question

I have a list of numbers and I need to split into to corresponding arrays of different sizes, but that make up all the combinations of the array splitting up. For example, if I have an array a=[1,2,3,4,5] and I want to split it to one array of size 3 and the other 2.

So I was thinking of making two arrays to hold each array and since there’s the same number of size 3 and size 2 arrays I could match them up and then perform my tests. (it’s a stats class so if there is a better scipy or numpy implementation I’d love to hear it as I’d like to move use those, in the end I’d like to get the all the differences of means between the different arrays)

But for my code here it is

import itertools


#defines the array of numbers and the two columns
number = [53, 64, 68, 71, 77, 82, 85]
col_one = []
col_two = []

#creates an array that holds the first four
results = itertools.combinations(number,4)

for x in results:
col_one.append(list(x))

print col_one


#attempts to go through and remove those numbers in the first array
#and then add that array to col_two
for i in range(len(col_one)):
holder = number
for j in range(4):
    holder.remove(col_one[i][j])
col_two.append(holder)  

Thanks in advance

EDIT: it seems the spacing of the code it messed up – I assure you the spacing is ok although when I run the code I can’t remove an item from the holder since it’s not there.

Answer 1

I tested your code and I see the problem. In this code,

for i in range(len(col_one)):
    holder = number
    for j in range(4):
        holder.remove(col_one[i][j])
    col_two.append(holder)

the line holder = number doesn’t copy number, it just gives number a second name, holder. Then, when you remove things from holder, they’re also removed from number, so when the loop goes around again, number has four fewer numbers in it. Ad infinitum.

You want to make a copy of number:

for i in range(len(col_one)):
    holder = list(number)
    for j in range(4):
        holder.remove(col_one[i][j])
    col_two.append(holder)

This creates a new list from number called holder. Now only holder is changed.

    holder = number[:]

would also work.

You should also use for‘s full potential by avoiding index variables:

for num_list in col_one:
    holder = list(number)
    for num in num_list:
        holder.remove(num)
    col_two.append(holder)

This does the same thing, is easier to read and probably faster to boot.

Now for the next step, list comprehensions. This is a great way to avoid nested loops.

for c1_list in col_one:
    c2_list = [n for n in number if n not in c1_list]
    col_two.append(c2_list)

This does the same thing as above. You can even make this a one-liner:

col_two = [[n for n in number if n not in c1_list] for c1_list in col_one]

Combining it all together:

number = [53, 64, 68, 71, 77, 82, 85]
col_one = list(itertools.combinations(number, 4))
col_two = [[n for n in number if n not in c1_list] for c1_list in col_one]