Split an array dependent on the array values in Python

Question

I have an array of coordinates like this:

array = [[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]]

I want to split the array between 6. and 7. coordinate ([5,7],[18,6]) because there is a gap in the X value there. I want to get two separate arrays, arr1 and arr2, where arr1 is the values before the split and arr2 is the values after.

I want to say that if the next X value is larger than a difference of 10, it will append to arr2, else arr1, something like this:

arr1 = []
arr2 = []
for [x,y] in array: 
    if next(x) > 10:
        arr2.append(x,y)
    else:
        arr1.append(x,y)

Can someone please help me with this problem?

The > 10: is a hard-coded attempt then, the number 10 isn't relevant, only the split in continuity? What about [5,6],[5,7], this should be ignored too? — roganjosh
– roganjosh, Commented Mar 7, 2017 at 14:02
Also, your appends should look like arr1.append([x,y]) to be consistent with your original list — Priyank
– Priyank, Commented Mar 7, 2017 at 14:06
Actually it's not clear to me at all why this would only result in two arrays from the example input, the split seems arbitrary. — roganjosh
– roganjosh, Commented Mar 7, 2017 at 14:06
Do you just need to split based on index where first difference greater than 10? — Rohan
– Rohan, Commented Mar 7, 2017 at 14:09
The 10 is a hard-coded attempt, if there is a better way please let me know ;) The array size may change, but there will always be one lagre "jump" in the x value somewhere, and this is where I want to split the array. — agrom
– agrom, Commented Mar 7, 2017 at 14:14

GPhilo · Accepted Answer · 2017-03-07 14:19:20Z

3

If I got your question correctly, you're trying to split the array at the first point where the difference between two subsequent x-values is greater than 10. You can do that using numpy:

import numpy as np
THRESH = 10
array = [[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]]
array = np.asarray(array)
deltas_x = np.abs(array[1:,0] - array[:-1,0])
split_idx = np.where(deltas_x > THRESH)[0][0] + 1
arr1 = array[:split_idx,:]
arr2 = array[split_idx:,:]

Note that we need to add 1 to the result of np.where to account for the fact that the deltas_x array is 1 value shorter than array

answered Mar 7, 2017 at 14:19

GPhilo

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Comments

Rohan · Accepted Answer · 2017-03-07 14:28:33Z

3

This might be what you are looking for

array = [[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]]
# Declare two array variables
arr1 = None
arr2 = None
n = len(array)
for i in range(n-1): 
    if abs(array[i][0] - array[i+1][0]) >= 10:
       arr1 = array[:i+1]
       arr2 = array[i+1:]
       break

print arr1
print arr2

edited Mar 7, 2017 at 14:28

answered Mar 7, 2017 at 14:14

Rohan

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2 Comments

agrom Over a year ago

How would one do it with respect to the Y value? changing if abs(array[i][0] - array[i+1][0]) >= 10: to if abs(array[0][i] - array[0][i+1]) >= 10: just returns an IndexError: list index out of range

Rohan Over a year ago

That should return out of range error since the range of for loop depends on the number of elements in array. If you want to deal with Y values, you can write an inner for loop.

Cleb · Accepted Answer · 2017-03-07 14:50:08Z

2

You can do the following:

ar = np.array([[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]])

# get differences of x values
dif = ar[1:, 0] - ar[:-1, 0]

# get the index where you first observe a jump
fi = np.where(abs(dif) > 10)[0][0]

ar1 = ar[:fi+1]
ar2 = ar[fi+1:]

Then dif would be:

array([ 1,  1,  1,  1,  0, 13,  1, -2, -7])

fi would be 5 and ar1 and ar2 would be:

array([[ 1,  6],
       [ 2,  6],
       [ 3,  8],
       [ 4, 10],
       [ 5,  6],
       [ 5,  7]])

and

array([[18,  6],
       [19,  5],
       [17,  9],
       [10,  5]]),

respectively.

That would also allow you to get all jumps in your data (you would just have to change fi = np.where(abs(dif) > 10)[0][0] to fi = np.where(abs(dif) > 10)[0])

edited Mar 7, 2017 at 14:50

answered Mar 7, 2017 at 14:24

Cleb

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Comments

MSeifert · Accepted Answer · 2017-03-07 14:27:05Z

2

Probably easiest to iterate over successive pairs and split as soon as a gap is found:

array = [[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]]

for idx, (cur, nxt) in enumerate(zip(array, array[1:])):  # successive pairs and index
    if abs(cur[0] - nxt[0]) > 10:  # compare difference of first items
        arr1, arr2 = array[:idx+1], array[idx+1:]  # split
        break  # no further splits, end the loop now
else:  # no break, keep the original array
    arr1, arr2 = array, []

Which gives:

>>> arr1
[[1, 6], [2, 6], [3, 8], [4, 10], [5, 6], [5, 7]]
>>> arr2
[[18, 6], [19, 5], [17, 9], [10, 5]]

It would be a bit more difficult if you wanted to split several times but this should well in your case.

edited Mar 7, 2017 at 14:27

answered Mar 7, 2017 at 14:22

MSeifert

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Comments

hectorcanto · Accepted Answer · 2017-03-20 10:59:52Z

2

When comparing consecutive elements I usually use enumerate:

array = [[1,6],[2,6],[3,8],[4,10],[5,6],[5,7],[18,6],[19,5],[17,9],[10,5]]
arr1 = list()
arr2 = list()
gap = 10

for index, value in enumerate(array[:-1]): # [:-1] prevents out of range
    if abs(value[0]-array[index+1][0]) >= gap:
        arr1.append(value)
    else:
        arr2.append(value)

arr2.append(array[-1])  # Take into account that the last element needs to be added to one of the arrays.

edited Mar 20, 2017 at 10:59

answered Mar 7, 2017 at 14:37

hectorcanto

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Comments

hankym · Accepted Answer · 2017-03-07 14:32:48Z

1

arry1 = []
arry2 = []
for i in arry:
    if (i[0] - i[1]) > 10:
        arry1.append(i)
    else:
        arry2.append(i)

edited Mar 7, 2017 at 14:32

answered Mar 7, 2017 at 14:17

hankym

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2 Comments

Dmitry Over a year ago

Use a comment, not answer

Donald Duck is with Ukraine Over a year ago

While this code may answer the question, providing additional context regarding how and/or why it solves the problem would improve the answer's long-term value.

Yablargo · Accepted Answer · 2023-06-06 16:50:20Z

0

Try this:

prev = array[0][0] 
pos = -1 
for i in range (1, len(array)): 
  if array[i][0] - prev >1: 
    break  
  else:  
    prev = array[i][0]  
if pos != -1:  
  arr1 = array[:pos]  
  arr2 = array[pos:]

This should split array the way you want. Note that lists are indexed from 0.

edited Jun 6, 2023 at 16:50

Yablargo

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answered Mar 7, 2017 at 14:04

Priyank

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3 Comments

Rohan Over a year ago

This is not an answer.

agrom Over a year ago

Thanks for the response! I should have mentioned that the size of the array may change, but there will always be a gap in the X value somewhere. That is why I want a method to split dependent of the X value.

Priyank Over a year ago

This should find the first place in the array where the x values are not continous. Thanks! @Rohan Sadale

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