Styjun

I have two arrays of different sizes that I want to perform logical functions on in a for loop. For example, if I have:

array1 = [6,7,8]
array2 = [1,2,3,4,5]

I want to do each element from array1 minus each element of array2 to get something like

[6-1,6-2,6-3,6-4,6-5],[7-1,7-2,7-3,7-4,7-5],[8-1,8-2,8-3,8-4,8-5]

which is subtracting each element from array2 from each element of array1

So i tried to do a for loop like:

for i in range(len(array1)):
 ar = array1[i]-array2

and also

for i in range(len(array1)):
 for j in range(len(array2)):
 ar = array1[i]-array2[j]

But neither of these seem to be working

The first way seems to be returning an array of the right shape but certainly not the right values.

I'd like it to be returned in separate arrays because in reality, I have a very large sample size.

We can solve this using itertools.product

from itertools import product
from operator import sub

final = []
for item in array1:
 prods = product([item], array2)
 subs = [sub(*p) for p in prods]
 final.append(subs)

print(final)
# [[5, 4, 3, 2, 1], [6, 5, 4, 3, 2], [7, 6, 5, 4, 3]]

How it works is product returns the cartesian product of the two arrays in the form of tuples, so:

(6, 1), (6, 2), etc....

Then we simply apply the sub function to each tuple using a list-comprehension.

The following solution should work using a list comprehension:

result = []
for value1 in array1:
 result.append([value1 - value2 for value2 in array2])

You could even write this in 1 line using both for loops for the list comprehension:

result = [[value1 - value2 for value2 in array2] for value1 in array1]

Loops solution:

array1 = [6, 7, 8]
array2 = [1, 2, 3, 4, 5]

arr = []
for i in array1:
 tmp = []
 for j in array2:
 tmp.append(i - j)
 arr.append(tmp)

print(arr)

Output:

[[5, 4, 3, 2, 1], [6, 5, 4, 3, 2], [7, 6, 5, 4, 3]]

Your for loop is nearly correct except, you overwrite the value of ar every iteration (and you make it slightly more complicated with range)

You can achieve this through list comprehension

[[i - j for j in array2] for i in array1]
# [[5, 4, 3, 2, 1], [6, 5, 4, 3, 2], [7, 6, 5, 4, 3]]

You were on the right track but you had your arrays switched. You want array1 inside the inner loop to perform the operation you want to perform:

array1 = [6,7,8]
array2 = [1,2,3,4,5]
finalarray=[]

for i in range(len(array2)):
 for j in range(len(array1)):
 ar = array1[j]-array2[i]
 finalarray.append(ar)

print(finalarray)

>>>[5, 6, 7, 4, 5, 6, 3, 4, 5, 2, 3, 4, 1, 2, 3]

ar is not being treated properly in your code, it needs to be an array of arrays (matrix) and you'll need another variable to hold the result per iteration, there's probably a better way to do this using list comprehensions but here is one way:

array1 = [6,7,8]
array2 = [1,2,3,4,5]

ar = []
row = []
for i in range(len(array1)):
 for j in range(len(array2)):
 row.append(array1[i]-array2[j])
 ar.append(row)
 row = []
print ar

There are plenty of good answers here, but another option especially useful for larger arrays is to use numpy, a package designed for moving large arrays of numbers efficiently. One possible answer in numpy would be this:

import numpy as np

array1 = np.arange(6,9) # make arrays 1 and 2
array2 = np.arange(1,6)
output = (array1.repeat(array2.shape[0]) # repeat array1 as many times as there are elements in array2
 .reshape(array1.shape[0], -1) # reshape so we have a row for each element in array1
 ) - array2 # subtract array2 from each row
output

array([[5, 4, 3, 2, 1],
 [6, 5, 4, 3, 2],
 [7, 6, 5, 4, 3]])