How get column name matrix for especific condition

[Rodrigo Badilla]

Hi all,

I wonder if its possible get column name matrix for a especific condition ie: max value in the matrix.

Until now I only van get the names all matrix:

sysuse auto, clear
tabulate foreign rep78, matcell(x)
matrix list x

x[2,5]
one two three four five
r1 2 8 27 9 2
r2 0 0 3 9 9

matrix colnames x= one two three four five
mat lis x
local ma : coln (x)

My goal its to get the column name for the max value in the matrix= three =3 for rep78

I try with below, but names were removed:

mata : st_matrix("x", max(st_matrix("x")))
matrix list x

    c1
r1  27

Please any comment i will grateful
Thanks in advance
Ro

[Andrew Musau]

The maximum may not be unique, but if you are interested in any maximum, the following will do. The logic in the Mata code is that by sorting \(-X\), the maximum of \(X\) will be in the first row, and you can extract this row vector and look for the column position of the minimum element (which is the maximum in the original matrix).

sysuse auto, clear
qui tabulate foreign rep78, matcell(x)
matrix colnames x= one two three four five
mat lis x
local ma : coln (x)

mata
x= st_matrix("x")
xs = sort(-x,1)
v = xs[(1), .]
maxcol = selectindex(v:==rowmin(v))
st_numscalar("max", maxcol)
end

local maxcol= max 
local maxcolname: word `maxcol' of `ma'
di "`maxcolname'"

This yields

. mat lis x

x[2,5]
      one    two  three   four   five
r1      2      8     27      9      2
r2      0      0      3      9      9


. di "`maxcolname'"
three

[Rodrigo Badilla]

Thanks Andrew for you reply!!

Your code works great!

I did try the make the same for row names, but maybe I am doing something wrong:

sysuse auto, clear
asdoc tabulate foreign rep78, chi matcell(x) replace
matrix list x
local me = x[1,1]
matrix colnames x= one two three four five
matrix rownames x= juan pedro
mat lis x
local ma : coln (x)
local ma2 : rown (x)

mata

*for col names
*your example

x= st_matrix("x")
xs = sort(-x,1)
v = xs[(1), .]
maxcol = selectindex(v:==rowmin(v))
st_numscalar("max", maxcol)

local maxcol= max
local maxcolname: word `maxcol' of `ma'
di "`maxcolname'"

*for row names,
*my effort ;(

xs = sort(1,-x)
v = xs[(1), .]
maxrow = selectindex(v:==colmin(v))
st_numscalar("max", maxrow)
end

local maxrow= max
local maxrowname: word `maxrow' of `ma2'
di "`maxrowname'"

[Andrew Musau]

Rodrigo

xs = sort(-x,1)

This sorts only in terms of the first column. So, disregard the code in #2 because we want to sort in terms of all columns. Here is an alternative code that will pick the column max across all columns. We have the following 5\(\times\) 4 matrix

clear
mat x= (12,15, 25, 2\ 2, 75, 55, 3\  112, 65, 25, 5\ 27, 27, 27, 27\ 3, 2, 1, 55)
matrix colnames x= one two three four
mat list x

. mat list x

x[5,4]
      one    two  three   four
r1     12     15     25      2
r2      2     75     55      3
r3    112     65     25      5
r4     27     27     27     27
r5      3      2      1     55

Code:

clear
mat x= (12,15, 25, 2\ 2, 75, 55, 3\  112, 65, 25, 5\ 27, 27, 27, 27\ 3, 2, 1, 55)
matrix colnames x= one two three four
local ma : coln (x)

mata
x= st_matrix("x")
xs = colmax(x)\colmax(x)\colmax(x)\colmax(x)
v = xs[(1), .]
maxcol = selectindex(v:==rowmax(v))
st_numscalar("max", maxcol)
end

local maxcol= max
local maxcolname: word `maxcol' of `ma'
di "`maxcolname'"

Here, I use the line in red to pick out the maximum elements across each of the 4 columns. If you post in the Mata board, they can suggest a more efficient method of doing this as my Mata skills are not too developed. For rows, use the same code with the transpose of the matrix

clear
mat x= (12,15, 25, 2\ 2, 75, 55, 3\  112, 65, 25, 5\ 27, 27, 27, 27\ 3, 2, 1, 55)
matrix colnames x= one two three four
local ma : coln (x)
local ma2 : rown (x)
mat xp= x'


mata
x= st_matrix("xp")
xs = colmax(x)\colmax(x)\colmax(x)\colmax(x)
v = xs[(1), .]
maxcol = selectindex(v:==rowmax(v))
st_numscalar("max", maxcol)
end


local maxrow= max
local maxrowname: word `maxrow' of `ma2'
di "`maxrowname'"

Remember with multiple maximum values, the row and column names may not refer to the same element.

x[5,4]
      one    two  three   four
r1     12     15     25      2
r2      2     75     55      3
r3    112     65     25      5
r4     27     27     27     27
r5      3      2      1     55


 local maxcol= max
. local maxcolname: word `maxcol' of `ma'
. di "`maxcolname'"
one

 local maxrow= max
. local maxrowname: word `maxrow' of `ma2'
. di "`maxrowname'"
r3

[Rodrigo Badilla]

Thanks Andrew! your code works great!

Was not so easy like I thought!

Regards