mask a 2D numpy array based on values in one column

Question

Suppose I have the following numpy array:

a = [[1, 5, 6],
     [2, 4, 1],
     [3, 1, 5]]

I want to mask all the rows which have 1

Answer 1

You can create the desired mask by

mask = numpy.repeat(a[:,0]==1, a.shape[1])

and the masked array by

masked_a = numpy.ma.array(a, mask=numpy.repeat(a[:,0]==1, a.shape[1]))