Find index of a vector inside a matrix in Python

Question

Given a matrix (numpy array) A and a vector v.

A = np.array([[5,0],[1,-2],[0,2],[-1,3],[1,2]])
v = np.array([0,2])

What is the best way to get the index of the vector v in the matrix A (in this case one should get 2).

If the values are unique (distinct), how about A[A == v][2] ? — ChatterOne
– ChatterOne, Commented Mar 11, 2020 at 12:43

Nicolas Gervais · Accepted Answer · 2020-03-11 12:43:50Z

1

This does it:

np.argwhere((v == A).all(1))

Out[82]: array([[2]], dtype=int64)

answered Mar 11, 2020 at 12:43

Nicolas Gervais

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rlucatoor · Accepted Answer · 2020-03-11 13:43:57Z

1

If by best you mean fastest, after thorough experimentation user hpaulj pointed out that np.flatnonzero is a much faster alternative to np.argwhere. You can use it this way:

np.flatnonzero((v==A).all(1))[0]

Output:

answered Mar 11, 2020 at 13:43

rlucatoor

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