numpy get values in col2 given an array of values matching col1

Question

How can I extract values in Col1 whose Col0 matches any values in a numpy array. I have an np array A, idx. Get me all values in Col1 of array A, whose Col0 values are 1 or 4.

A = np.array([[1, 11], [2, 12], [3, 13], [4,14]])

idx = [1, 4]

I can get for 1 value like this.. but I don't know to get for an array of idx.

vals = A[np.where(A[:,0]==4),1]
vals = A[np.where(A[:,0]==4),4]

a) how can I get the values of Col1 in A where Col0 values are 1 or 4 ( matching idx).

expected result = [11,14]

b) how can I get values of Col1 in A where row indices are 1,4 (matching idx)

expected result = [12, 14]

Pygirl · Accepted Answer · 2021-02-04 18:57:42Z

1

1st part:

idx = [1, 4]
A[np.isin(A[:,0], idx), 1]

array([11, 14])

2nd part:

idx = [1, 3]
A[idx,1]

array([12, 14])

edited Feb 4, 2021 at 18:57

answered Feb 4, 2021 at 18:45

Pygirl

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2 Comments

py_newbie Over a year ago

For the part2 I figured A[idx][:,1] would work. Thanks @Pygirl]

Pygirl Over a year ago

@py_newbie You have given wrong index. for 2nd part.

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