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?

• roganjosh: 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?
• Priyank: Also, your appends should look like arr1.append([x,y]) to be consistent with your original list
• roganjosh: 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.
• Rohan: Do you just need to split based on index where first difference greater than 10?
• agrom: 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.

Answer 1

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

Answer 2

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

• agrom: 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: 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.

Answer 3

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])

Answer 4

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.

Answer 5

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.

Answer 6

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

• Dmitry: Use a comment, not answer
• Donald Duck is with Ukraine: 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.

Answer 7

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.

• Rohan: This is not an answer.
• agrom: 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: This should find the first place in the array where the x values are not continous. Thanks! @Rohan Sadale