Return a matrix from distinct command

Question

I have a simple question about the distinct command in Stata.

When using with a by prefix, can it return a one dimension matrix of r(N)?

For example:

sysuse auto,clear
bysort foreign: distinct rep78

Can I store a [2,1] matrix, with each row representing the number of distinct values of rep78?

The manual seems to suggest that it only stores the number of distinct values of the last by value.

`distinct` is community-contributed (user-written) so its documentation is its own help file, not the manuals produced by StataCorp. — Nick Cox, Jun 04 '18 at 14:25

score 4 · Accepted Answer · answered Jun 04 '18 at 14:15

4

You can easily create your own wrapper for that:

sysuse auto,clear

sort foreign                
levelsof foreign, local(foreign_levels)
local number_of_foreign_levels : word count `foreign_levels'

matrix distinct_mat = J(`number_of_foreign_levels', 1, 0)

forvalues i = 1 / `number_of_foreign_levels' {
     quietly distinct rep78 if foreign == `i' - 1
     matrix distinct_mat[`i', 1] = r(ndistinct)
}

matrix list distinct_mat

distinct_mat[2,1]
    c1
r1   5
r2   3

Note that the number of distinct observations is stored in r(ndistinct), not r(N).

answered Jun 04 '18 at 14:15

Thanks for the solution. I thought about this and thought there would be an easier solution. – Yan Song Jun 04 '18 at 14:42
2

@YanSong if you have to do this repeatedly for different variables you can include the above into a program.You will then only have to write a single line. – Jun 04 '18 at 14:45

score 2 · Answer 2 · answered Jun 04 '18 at 15:09

Here is another way to get numbers of distinct values into a matrix.

. sysuse auto
(1978 Automobile Data)

. egen tag = tag(foreign rep78)

. tab foreign if tag, matcell(foo)

   Car type |      Freq.     Percent        Cum.
------------+-----------------------------------
   Domestic |          5       62.50       62.50
    Foreign |          3       37.50      100.00
------------+-----------------------------------
      Total |          8      100.00

Return a matrix from distinct command

2 Answers2