Portfolio Standard Deviation with Matrix in Stata

I have written the following code to:

(1) make dummy data for 4 stocks with returns ri1 ri2 ri3 ri4

(2) assign each stock equal weight of .25 with names w1 w2 w3 w4 and assign them to matrix W

(3) Find port standard deviation in each year and write the results to file SD.dta in the user current directory

Copy and paste the following code in Stata do editor and run

Note: You need Stata 12.1 or above for using varrets program

//Create Dummy Data

clear

set obs 1000

gen date=_n

format %td date

expand 10

bys date: gen id=_n

gen year = year(date)

gen ri1 = uniform()

gen ri2 = uniform()

gen ri3 = uniform()

gen ri4 = uniform()

gen W1 = .25

gen W2 = .25

gen W3 = .25

gen W4 = .25

//Now Portfolio SD

qui sum year

loc MAX = r(max)

loc MIN = r(min)

forval i = `MIN' / `MAX' {

        //To make variance-covariance matrix from stock returns

varrets ri1 ri2 ri3 ri4 if year==`i'

      //To write covariance matrix to matrix S

mat S = r(cov)

      //To make matrix W from variable W1 W2 W3 W4

mkmat W1 W2 W3 W4 in 1, matrix (W)

      //Write portfolio variance to matrix VAR

mat VAR = W*S*W'

      //Write VAR matrix to variable VAR

svmat double VAR, name(VAR)

preserve

keep year VAR1

drop if VAR1==.

replace year = `i'

if year==`MIN' {

save SD, replace

}

      else {

    append using SD.dta

    save SD.dta, replace

      }

restore

cap drop VAR1

}