Reverse Index Arrays

Question

I'm extracting a sub matrix from a quadratic matrix using a list of indexes.
The index list applies to both rows and columns.
The sub matrix is updated.
Finally the sub matrix is merged back in the original matrix.
The extract function is short and fast.
The merge function is longer and slower, as explicit loops are killing performance

Is there a better way of writing merge?

import numpy as np

def extract(a,indexes): 
    return a[indexes].T[indexes].T

def merge(a,indexes,b): 
    for i,ix in enumerate(indexes):
        for j,jx in enumerate(indexes):
            a[ix,jx] = b[i,j]
    return a

N = 10
a = np.array(np.arange(N*N)).reshape(N,N)

indexes = [0,2,4,6,8]
b = extract(a,indexes)

print(a)
print(indexes)
print(b)

# make some changes in b
b=-b

res = merge(a,indexes,b)

https://docs.scipy.org/doc/numpy-1.10.0/user/basics.indexing.html#index-arrays

a:
[[ 0  1  2  3  4  5  6  7  8  9]
 [10 11 12 13 14 15 16 17 18 19]
 [20 21 22 23 24 25 26 27 28 29]
 [30 31 32 33 34 35 36 37 38 39]
 [40 41 42 43 44 45 46 47 48 49]
 [50 51 52 53 54 55 56 57 58 59]
 [60 61 62 63 64 65 66 67 68 69]
 [70 71 72 73 74 75 76 77 78 79]
 [80 81 82 83 84 85 86 87 88 89]
 [90 91 92 93 94 95 96 97 98 99]]

indexes: [0, 2, 4, 6, 8]

b: 
[[ 0  2  4  6  8]
 [20 22 24 26 28]
 [40 42 44 46 48]
 [60 62 64 66 68]
 [80 82 84 86 88]]

res:  [[  0,   1,  -2,   3,  -4,   5,  -6,   7,  -8,   9],
       [ 10,  11,  12,  13,  14,  15,  16,  17,  18,  19],
       [-20,  21, -22,  23, -24,  25, -26,  27, -28,  29],
       [ 30,  31,  32,  33,  34,  35,  36,  37,  38,  39],
       [-40,  41, -42,  43, -44,  45, -46,  47, -48,  49],
       [ 50,  51,  52,  53,  54,  55,  56,  57,  58,  59],
       [-60,  61, -62,  63, -64,  65, -66,  67, -68,  69],
       [ 70,  71,  72,  73,  74,  75,  76,  77,  78,  79],
       [-80,  81, -82,  83, -84,  85, -86,  87, -88,  89],
       [ 90,  91,  92,  93,  94,  95,  96,  97,  98,  99]])

Divakar · Accepted Answer · 2017-07-18 11:06:15Z

1

You can use np.ix_ to form such a grid spread across rows and columns. So, simply index into input array with it and assign the negated b values, like so -

idx_grid = np.ix_(indexes, indexes)
a[idx_grid] = -b

answered Jul 18, 2017 at 11:06

Divakar

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