Create index list for np.split from the list that already has number for each section

Question

I have two lists of numbers

list = [1,2,3,4,5,6,7,8,9]
number = [3,2,1,3]

I want to create index for np.split from number

Answer 1

Approach #1

If we want to use np.split resulting in a list of arrays, we need to use np.cumsum on those indexes to give us indices where we need to split the input list -

np.split(list1, np.cumsum(number)[:-1])

Sample run -

In [36]: list1 = [1,2,3,4,5,6,7,8,9]
    ...: number = [3,2,1,3]
    ...: 

In [37]: np.split(list1, np.cumsum(number)[:-1])
Out[37]: [array([1, 2, 3]), array([4, 5]), array([6]), array([7, 8, 9])]

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Approach #2

To have a list of lists, another approach with a loop comprehension again using cumsum -

idx = np.r_[0,np.cumsum(number)]
out = [list1[idx[i]:idx[i+1]] for i in range(len(idx)-1)]

Sample run -

In [45]: idx = np.r_[0,np.cumsum(number)]

In [46]: [list1[idx[i]:idx[i+1]] for i in range(len(idx)-1)]
Out[46]: [[1, 2, 3], [4, 5], [6], [7, 8, 9]]