How to sum values from a python multidimensional array based on an element in the array?

Question

Say i have an array called my array

my_array= [[1,2],[1,3],[2,3]]

I want to add all the second element from each sub-list starting with 1 so that my output would be 5

Also using NumPy is not an option...

Does anyone know how to do this ?

Answer 1

You can use a conditional list comprehension for this.

my_array=[[1,2],[1,3],[2,3]]
my_sum=sum(v[1] for v in my_array if v[0]==1)

print(my_sum)

Output:

5

Answer 2

Use a list comprehension with a filter to select the items, then sum them:

result = sum([b for a,b in my_array if a == 1])

Answer 3

You can loop over the arrays and check if the first element is 1, then add the second element to a variable:

result = 0
for i in range(0, len(my_array)):
    elem = my_array[i]
    if elem[0] == 1:
        result += elem[1]

Answer 4

my_array= [[1,2],[1,3],[2,3]]
sum = 0
for x in my_array:
    if x[0] == 1:
        sum+=x[1]

print(sum)

Answer 5

OP can't use numpy, but that does not have to be true for future readers of the question. Since we already have the non-numpy solutions covered, here's one with numpy.

>>> import numpy as np
>>> my_array = np.array([[1,2],[1,3],[2,3]])
>>>
>>> np.sum(my_array[:,1][my_array[:,0] == 1])
5

Answer 6

For large data, converting to, or even better, using numpy from the beginning might be faster. Here, to_select is true if the corresponding element in to_sum is to be added:

my_array = numpy.array(my_array)
to select = my_array[:,0] == 1 
to_add = my_array[:,1]
result = numpy.sum(to_add[to_select])