Python numpy 2D array indexing -

i quite new python , numpy. can 1 pls me understand how can indexing of arrays used indices. have following 6 2d arrays this-

array([[2, 0],    [3, 0],    [3, 1],    [5, 0],    [5, 1],    [5, 2]]) 

i want use these arrays indices , put value 10 in corresponding indices of new empty matrix. output should this-

array([[ 0,  0,  0],    [ 0,  0,  0],    [10,  0,  0],    [10, 10,  0],    [ 0,  0,  0],    [10, 10, 10]]) 

so far have tried this-

    numpy import*     = array([[2,0],[3,0],[3,1],[5,0],[5,1],[5,2]])     b = zeros((6,3),dtype ='int32')     b[a] = 10 

but gives me wrong output. pls.

in [1]: import numpy np in [2]: = np.array([[2,0],[3,0],[3,1],[5,0],[5,1],[5,2]]) in [3]: b = np.zeros((6,3), dtype='int32')  in [4]: b[a[:,0], a[:,1]] = 10  in [5]: b out[5]:  array([[ 0,  0,  0],        [ 0,  0,  0],        [10,  0,  0],        [10, 10,  0],        [ 0,  0,  0],        [10, 10, 10]]) 

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why works:

if index b two numpy arrays in assignment,

b[x, y] = z 

then think of numpy moving simultaneously on each element of x , each element of y , each element of z (let's call them xval, yval , zval), , assigning b[xval, yval] value zval. when z constant, "moving on z returns same value each time.

that's want, x being first column of a , y being second column of a. thus, choose x = a[:, 0], , y = a[:, 1].

b[a[:,0], a[:,1]] = 10 

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why b[a] = 10 not work

when write b[a], think of numpy creating new array moving on each element of a, (let's call each 1 idx) , placing in new array value of b[idx] @ location of idx in a.

idx value in a. int32. b of shape (6,3), b[idx] row of b of shape (3,). example, when idx is

in [37]: a[1,1] out[37]: 0 

b[a[1,1]]

in [38]: b[a[1,1]] out[38]: array([0, 0, 0]) 

so

in [33]: b[a].shape out[33]: (6, 2, 3) 

so let's repeat: numpy creating new array moving on each element of a , placing in new array value of b[idx] @ location of idx in a. idx moves on a, array of shape (6,2) created. since b[idx] of shape (3,), @ each location in (6,2)-shaped array, (3,)-shaped value being placed. result array of shape (6,2,3).

now, when make assignment like

b[a] = 10 

a temporary array of shape (6,2,3) values b[a] created, assignment performed. since 10 constant, assignment places value 10 @ each location in (6,2,3)-shaped array. values temporary array reassigned b. see reference docs. values in (6,2,3)-shaped array copied (6,3)-shaped b array. values overwrite each other. main point not obtain assignments desire.