How to find the index of an array where summation is greater than a target value?

Question

Suppose I have a 1D array sorted in descending order, like:

arr = np.array([10, 10, 8, 5, 4, 4, 3, 2, 2, 2])

I want the index value, where the summation of this array starting from 0 to that index is greater than or equal to a specified target value. For example, let the target value be 40:

index=0 (0) => sum=10 (10)

index=1 (0,1) => sum=20 (10+10)

index=2 (0,1,2) => sum=28 (10+10+8)

index=3 (0,1,2,3) => sum=33 (10+10+8+5)

index=4 (0,1,2,3,4) => sum=37 (10+10+8+5+4)

index=5 (0,1,2,3,4,5) => sum=41 (10+10+8+5+4+4)

and finally I want to get the index value 5, since the sum 41 is greater than the target value 40. How can I do this in most Pythonic and appropriate way, so it can work with large numbers and large sized arrays.

Answer 1

Find the cumulative sum of the array with np.cumsum() . Find the index of the first element of the cumsum that is greater than the target value with np.where() :

cumsum = arr.cumsum()
np.where(cumsum > 40)[0][0]
# 5