Drop a matrix row

[Charlie Joyez]

Dear Mata users,

I'm trying to drop a matrix row (or column, the matrix being symmetric), or at least not counting it in the row() function.
E.g :

Consider the following matrix

mata
M=(0,4,0,0\ 4,0,2,0\ 0,2,0,0\ 0,0,0,0)
r=rows(M)
r

The row function counts 4 rows. However the last one is empty.
I'd like in this case to return "3", i.e. the number of non empty rows.

How could I do that?
Best,
Charlie

[Szabolcs Lorincz]

Hi Charlie,

I would use the select() and rowsum() functions. See an attempt below.

mata:
M=(0,4,0,0\ 4,0,2,0\ 0,2,0,0\ 0,0,0,0)
// colvector counting the number of zeros in each row of M
row_n_zeros=rowsum((M:==J(1,cols(M),0)))
// select those rows of M which contain at least one non-zero element
M_select=select(M,row_n_zeros:<cols(M))
// number of rows of selected matrix
r=rows(M_select)
r
end

In a more compact form:

mata:
M=(0,4,0,0\ 4,0,2,0\ 0,2,0,0\ 0,0,0,0)
r=rows(select(M,rowsum((M:==J(1,cols(M),0))):<cols (M)))
r
end

[Charlie Joyez]

Thank you very much Szabolcs,
This is exactly what I was trying to do.

Thanks for the stepwise description, it makes it clearer for me.

Charlie,

[daniel klein]

This can be simplified

M=(0,4,0,0\ 4,0,2,0\ 0,2,0,0\ 0,0,0,0)
r = colsum(rowsum(M):!=0)
r

Edit. To select the non-zero rows

non_zero = select(M, rowsum(abs(M)))

Best
Daniel

[Charlie Joyez]

Thanks Daniel, very useful and elegant.

Edit

I even went a step further :

mata 
 M=(0,4,0,0\ 4,0,2,0\ 0,2,0,0\ 0,0,0,0)     
 non_zero= select(select(M, rowsum(abs(M))),colsum(abs(M)))  
     
 non_zero   
     
 end

Because only the first line kept an empty column, and make my matrix no longer symetric.

Best,
Charlie