[Numpy-discussion] How to remove any row or column of a numpy matrix whose sum is 3?

[Chris Barker]

On Mon, Jun 4, 2012 at 9:21 AM, bob tnur <bobtnur78 at gmail.com> wrote:
> Hello every body. I am new to python.
> How to remove any row or column of a numpy matrix whose sum is 3.
> To obtain and save new matrix P with (sum(anyrow)!=3 and sum(anycolumn)!=3
> elements.

well, one question is -- do you want to remove the particular rows
first, then remove the particular columns, or compute the sums of both
with all in place, then remove rows and columns -- but some ideas:

In [357]: m
Out[357]:
array([[ 1.,  1.,  1.,  1.,  0.],
       [ 1.,  0.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  0.,  0.],
       [ 1.,  1.,  1.,  1.,  1.]])

# which rows, columns sum to 3?

In [363]: rows_3 = np.argwhere(m.sum(axis=1) == 3)

In [364]: rows_3
Out[364]: array([[2]])

In [365]: cols_3 = np.argwhere(m.sum(axis=0) == 3)

In [366]: cols_3
Out[366]:
array([[1],
       [3]])

# but it's probably easier to know which do not sum to 3:

In [367]: rows_3 = np.argwhere(m.sum(axis=1) != 3)

In [368]: rows_3
Out[368]:
array([[0],
       [1],
       [3]])

In [371]: cols_3 = np.argwhere(m.sum(axis=0) != 3)

In [372]: cols_3
Out[372]:
array([[0],
       [2],
       [4]])


now build the new array:

m2 = In [415]: m2 = m[rows_3[:,0]][:, cols_3[:,0]]

In [416]: m2
Out[416]:
array([[ 1.,  1.,  0.],
       [ 1.,  1.,  1.],
       [ 1.,  1.,  1.]])


some trickery there:

the [:,2] index is because argwhere() returns a 2-d (nXm)array of
indexes -- where m is the rank of the input array -- in this case one,
so we want the single column (it's done this way so you can do:

arr[ argwhere(arr == something) ]

for n-dim arrays

I also found I need to pull out the rows first, then the columns, because:

arr[a_1, a_2]

is interpreted as two arrays of individual indexes, not as indexes to
the roes an columsn, i.e.:

In [428]: arr
Out[428]:
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11]])

In [430]: arr[(1,2), (2,3)]
Out[430]: array([ 6, 11])

you got the elements at: (1,2) and (2,3)

There may be a way to do that a bit cleaner -- it escapes me at the moment.

-Chris



-- 

Christopher Barker, Ph.D.
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