Python - split list in to sublists based on another list

Question

I have two lists: l1 = [0, 0.002, 0.3, 0.5, 0.6, 0.9, 1.3, 1.9] and l2 = [0.5, 1.0, 1.5, 2.0]. I want to split l1 in to sublists that are defined as the elements between two indexes of l2. So for example l1 would be equal to [[0,0.002, 0.3], [0.5, 0.6, 0.9], [1.3], [1.9]].

Here is my solution:

l3 = []
b=0
for i in l2:
    temp = []
    for p in l1:
        if b <= p < i:
        temp.append(p)
    l3.append(temp)
    b+=0.5

This solution is a huge bottleneck in my code. Is there a faster way to do this?

Answer 1

Your lists are sorted, so there is no need to do a double loop here.

The following generates the sublists based on the two lists as inputs:

def partition(values, indices):
    idx = 0
    for index in indices:
        sublist = []
        while idx < len(values) and values[idx] < index:
            sublist.append(values[idx])
            idx += 1
        if sublist:
            yield sublist

You can then iterate over partition(l1, l2) to get individual sublists, or call list() to produce the whole list-of-lists in one go:

>>> l1 = [0, 0.002, 0.3, 0.5, 0.6, 0.9, 1.3, 1.9] 
>>> l2 = [0.5, 1.0, 1.5, 2.0]
>>> list(partition(l1, l2))
[[0, 0.002, 0.3], [0.5, 0.6, 0.9], [1.3], [1.9]]