Creating multiple arrays from one array, based on values in another array - python

Question

The situation is the following:

Lets say I have two arrays, x and y: Input:

x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
y = [2,6,9,13]

Expected output:

arr1 = [2,3,4,5]
arr2 = [6,7,8]
arr3 = [9,10,11,12]

I would like to create a python script that can let me split up array x into multiple arrays based on the values of array y as the end points.

So x will split up between 2 and 6, then between 6 and 9, then between 9 and 13 in this example.

I am not sure how to get started on this, I am a beginner. I would appreciate the help and I would love to know how you broke down the problem to solve it? Thank you!

In arr1, 2 is the starting point whereas in arr2 and arr3, 9 and 13 are ending points of y — Vaishali
– Vaishali, Commented May 27, 2020 at 20:46
shouldn't arr1 be equal to [2,3,4,5,6] then, as I understand your values in y should determine end/start values of the output lists — vnikonov_63
– vnikonov_63, Commented May 27, 2020 at 20:46

Vaishali · Accepted Answer · 2020-05-27 21:04:41Z

1

Find the index of x based on value in y and then use indexing

arr = []
for i in range(len(y)-1):
    arr.append(x[x.index(y[i]):x.index(y[i+1])])

arr

[[2, 3, 4, 5], [6, 7, 8], [9, 10, 11, 12]]

This works if there are duplicates in the array,

x = [2,3,4,5,6,7,7,8,9,9,10,11,11,12,13,14,15]
y = [2,6,9,13]

arr = []
for i in range(len(y)-1):
    arr.append(x[x.index(y[i]):x.index(y[i+1])])

[[2, 3, 4, 5], [6, 7, 7, 8], [9, 9, 10, 11, 11, 12]]

edited May 27, 2020 at 21:04

answered May 27, 2020 at 20:59

Vaishali

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Comments

yatu · Accepted Answer · 2020-05-27 21:08:11Z

1

For x sorted, we can use np.searchsorted with np.split to split x using the indices where y is contained in x:

import numpy as np

i = np.searchsorted(x, y)
np.split(x,i+1)[1:-1]
# [array([3, 4, 5, 6]), array([7, 8, 9]), array([10, 11, 12, 13])]

edited May 27, 2020 at 21:08

answered May 27, 2020 at 20:47

yatu

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3 Comments

rnm1996 Over a year ago

My bad! Yeah I fixed the response

Vaishali Over a year ago

This code doesn't look at values in y at all, try the same code for x = [2,3,4,5,6,7,8,9,10,11,12,13,14,15]

yatu Over a year ago

Updated. Same idea but finding the indices through searchsorted @vaishal

HackLab · Accepted Answer · 2020-05-27 20:49:54Z

0

You are somewhat inconsistent in your expected output, or I don`t get the pattern behind it, but this should get you close to what you want

x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
y = [2,6,9,13]

m = []

for i in range(len(y) - 1):
    m += [x[y[i] - 1:y[i+1] - 1]]

print(m)

Output

[[2, 3, 4, 5], [6, 7, 8], [9, 10, 11, 12]]

answered May 27, 2020 at 20:49

HackLab

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2 Comments

Vaishali Over a year ago

This code doesn't look at values in y at all, try the same code for x = [2,3,4,5,6,7,8,9,10,11,12,13,14,15]

HackLab Over a year ago

It does but interprets them as indices, the example was kind of vague on what was expected.

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