Dropping Columns in Matricies

[Daniel Jensen Warwick]

Hi hello everyone!

I am coming to you with a question about matrices. I have an NxN matrix. I want to delete the row "i" so that the matrix would have (N-1)x(N-1) dimensions. Does anyone know how to do that?

Thanks for help!
Daniel

[Clyde Schechter]

If you delete the i'th row, the matrix will have (N-1)xN dimensions. Do you mean you want to delete both the i'th row and the i'th column?

[Andrew Musau]

One way is through submatrix extraction, excluding the to be deleted rows/columns. As Clyde states, deleting a column in an \(N \times N\) matrix results in a \(N \times N-1\) matrix.

help matrix_extraction

sysuse auto, clear
keep in 1/7
mkmat price mpg rep78 headroom trunk weight length, mat(auto)
mat l auto
*SUPPOSE THAT i=3
mat auto= auto[1...,1..2], auto[1..., 4...]
mat l auto

Res.:

. mat l auto

auto[7,7]
       price       mpg     rep78  headroom     trunk    weight    length
r1      4099        22         3       2.5        11      2930       186
r2      4749        17         3         3        11      3350       173
r3      3799        22         .         3        12      2640       168
r4      4816        20         3       4.5        16      3250       196
r5      7827        15         4         4        20      4080       222
r6      5788        18         3         4        21      3670       218
r7      4453        26         .         3        10      2230       170

. *SUPPOSE THAT i=3

. mat auto= auto[1...,1..2], auto[1..., 4...]

. mat l auto

auto[7,6]
       price       mpg  headroom     trunk    weight    length
r1      4099        22       2.5        11      2930       186
r2      4749        17         3        11      3350       173
r3      3799        22         3        12      2640       168
r4      4816        20       4.5        16      3250       196
r5      7827        15         4        20      4080       222
r6      5788        18         4        21      3670       218
r7      4453        26         3        10      2230       170

.

ADDED IN EDIT: #1 asks about deleting a row, in which case you have:

sysuse auto, clear
keep in 1/7
mkmat price mpg rep78 headroom trunk weight length, mat(auto)
mat l auto
*SUPPOSE THAT i=3
mat auto= auto[1..2, 1...]\auto[4..., 1...]
mat l auto

Res.:

. mat l auto

auto[7,7]
       price       mpg     rep78  headroom     trunk    weight    length
r1      4099        22         3       2.5        11      2930       186
r2      4749        17         3         3        11      3350       173
r3      3799        22         .         3        12      2640       168
r4      4816        20         3       4.5        16      3250       196
r5      7827        15         4         4        20      4080       222
r6      5788        18         3         4        21      3670       218
r7      4453        26         .         3        10      2230       170

. *SUPPOSE THAT i=3

. mat auto= auto[1..2, 1...]\auto[4..., 1...]

. mat l auto

auto[6,7]
       price       mpg     rep78  headroom     trunk    weight    length
r1      4099        22         3       2.5        11      2930       186
r2      4749        17         3         3        11      3350       173
r4      4816        20         3       4.5        16      3250       196
r5      7827        15         4         4        20      4080       222
r6      5788        18         3         4        21      3670       218
r7      4453        26         .         3        10      2230       170

.

[Daniel Jensen Warwick]

Thanks a lot! It worked!