Numpy: how to roll 1 "row" in an array of arrays

You can do this by selecting the row you want to operate on and using numpy.roll.

Let's say we want to roll the first row one place to the right:

import numpy as npgrid = np.array([['A', 'D', 'G'],                 ['B', 'E', 'H'],                 ['C', 'F', 'I']])grid[0] = np.roll(grid[0], 1)print grid

This yields:

[['G' 'A' 'D'] ['B' 'E' 'H'] ['C' 'F' 'I']]

Notice that we're modifying the original array.

You should decide whether you want to operate on the array in-place (modifying the original) or if you want to make a copy each time. Repeated calls will have different effects depending on what you decide:

import numpy as npdef independent_roll_inplace(arr, ind, amount):    arr[ind] = np.roll(arr[ind], amount)def independent_roll_copy(arr, ind, amount):    arr = arr.copy()    arr[ind] = np.roll(arr[ind], amount)    return arrgrid = np.array([['A', 'D', 'G'],                 ['B', 'E', 'H'],                 ['C', 'F', 'I']])

As an example of the difference, if we make a copy each time, we start "fresh" with the original grid. Repeated calls have no effect on the original:

print 'Roll the second row one place'print independent_roll_copy(grid, 1, 1)print 'Roll the second row two places'print independent_roll_copy(grid, 1, 2)print 'Roll the second row three places'print independent_roll_copy(grid, 1, 3)

This yields:

Roll the second row one place[['A' 'D' 'G'] ['H' 'B' 'E'] ['C' 'F' 'I']]Roll the second row two places[['A' 'D' 'G'] ['E' 'H' 'B'] ['C' 'F' 'I']]Roll the second row three places[['A' 'D' 'G'] ['B' 'E' 'H'] ['C' 'F' 'I']]

However, if we're modifying the original each time, we'd get the same result by rolling one place multiple times:

for _ in range(3):    print 'Roll the first row one place, modifying the original'    independent_roll_inplace(grid, 0, 1)    print grid

Yielding:

Roll the second row one place, modifying the original[['A' 'D' 'G'] ['H' 'B' 'E'] ['C' 'F' 'I']]Roll the second row one place, modifying the original[['A' 'D' 'G'] ['E' 'H' 'B'] ['C' 'F' 'I']]Roll the second row one place, modifying the original[['A' 'D' 'G'] ['B' 'E' 'H'] ['C' 'F' 'I']]

This code might solve your problem.

REV_GRID is the rolled over version of GRID.

COL01 = ["A", "D", "G"]COL02 = ["B", "E", "H"]COL03 = ["C", "F", "I"]GRID = []GRID.append(COL01)GRID.append(COL02)GRID.append(COL03)REV_GRID = []for col in GRID:    REV_GRID.append(col[::-1])print GRIDprint REV_GRID

Please let me know if it does not solve your problem. We will work on that.