replace a variable with elements from a matrix

[Nishan Lamichhane]

Hello Statisticians,

I have a variable x=.

From a cross tab if I want to pick up a frequency from the 3x3 table and replace x with this frequency. How can it be done? I also produced a matrix using tab var1 var2, matcell(M)

mat li M gives

c1 c2 c3
r1 45598 116 8
r2 22797 55 1
r3 22801 55 8

Now I want to replace x with 116. How do I do it?

[Nick Cox]

This is fairly easy to answer, but mainly I am wondering why you want to do that, as knowing the context might lead to quite different suggestions.

. sysuse auto, clear
(1978 automobile data)

. tab foreign rep78, matcell(M)

           |                   Repair record 1978
Car origin |         1          2          3          4          5 |     Total
-----------+-------------------------------------------------------+----------
  Domestic |         2          8         27          9          2 |        48 
   Foreign |         0          0          3          9          9 |        21 
-----------+-------------------------------------------------------+----------
     Total |         2          8         30         18         11 |        69 

. mat li M

M[2,5]
    c1  c2  c3  c4  c5
r1   2   8  27   9   2
r2   0   0   3   9   9

. gen foo = M[1, 3]

. l foo in 1

     +-----+
     | foo |
     |-----|
  1. |  27 |
     +-----+

[Nishan Lamichhane]

Thank you NIck. I was actually running cross tabs within loop. Then wanted to pick up frequency from those 100s of cross-tabs and replace and empty variable with those frequencies. I simply produced matrix and then replace my empty variable : replace x =M[2,1] in `i' then it put value for each iteration. You have been of great help here. Thanks again.

[Nick Cox]

It might be as easy or easier to go

count if x == 2 & y == 3

and use the saved result r(N). A problem with referring to a particular row and column of that matrix is that it can change in size depending on whether particular values are absent in the data.