CODEWITHSUNDEEP
pythonarrayspython-3.x
guoyili
guoyili

Reputation: 93

Assign values to a 2D array using another 2D array as a index

I have a 2D mxn array A and another 2D qxp array B. I want to assign value of 1 to A at each coordinate listed in B.

I know that I could firstly get the length of b at axis=0, and write a loop to do it such like A[B[i,0]][b[i,1]] = 1; but i am working on a big dataset, I want to see if there is any way to do this without a for loop.

Upvotes: 0

Views: 164

Answers (1)

iamchoosinganame
iamchoosinganame

Reputation: 1120

You can directly assign the values by passing the columns of B as indices of A. You don't need to loop through the rows of B.

A[B[:,0],B[:,1]]=1

Here's a more detailed example

A=np.zeros((4,4))

array([[0., 0., 0., 0.],
       [0., 0., 0., 0.],
       [0., 0., 0., 0.],
       [0., 0., 0., 0.]])

B=np.array([[2,1],[2,2],[0,2],[3,0]])

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

# directly pass each of the columns of B as indices of A
A[B[:,0],B[:,1]] = 1

print(A)

array([[0., 0., 1., 0.],
       [0., 0., 0., 0.],
       [0., 1., 1., 0.],
       [1., 0., 0., 0.]])

Upvotes: 1

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