# How to subtract the previous rows column from current column and create a new dimension in the array with this value using numpy?

What I want to do is take my current array and subtract the previous rows column value from the current rows column value. I also want to take the result and add it to the array as a new dimension.

Example Array:

[[1,2,4,7,9,15], [3, 4,3,5,10,2], [5,6,56,7,20,1]]

Lets say I want to do this with the 4th column so I want the output to be an array that looks like this:

[[1,2,4,7,9,15,0], [3, 4,3,5,10,2,-2], [5,6,56,7,20,1,2]]

Thanks

## Answers

You can do this using a combination of np.diff, and concatenate options, like so:

import numpy as np myarray = np.array([[1,2,4,7,9,15], [3, 4,3,5,10,2], [5,6,56,7,20,1]]) #your differences appears to be wraparound, so we repeat the last row at the top: myarray_wrap = np.vstack((myarray[-1],myarray)) #this gets the diffs for all columns: column_diffs = np.diff(myarray_wrap, axis=0) #now we add in only the the column_diff that we want, at the end: print np.hstack((myarray, column_diffs[:,3].reshape(-1,1))) #Output: [[ 1 2 4 7 9 15 0] [ 3 4 3 5 10 2 -2] [ 5 6 56 7 20 1 2]]

This Python code should solve your problem.

previous = None for row in rows: current = row[4] row.append( 0 if previous == None else current - previous ) previous = current