Summarizing a variable in Stata and extracting standard deviation - stata

Question

I am trying to create a variable for each year in my data based on mathematical expressions of other variables (I have annual data and used "..." to avoid writing each year). I am using the summarize command in Stata to extract the standard deviation but Stata does not recognize the frac variable. I have tried to use egen but that results in an unknown function error. Using gen results in an already defined variable. I would appreciate anyone helping with the following code or pointing me to a link where this issue has been discussed.
foreach yr of numlist 1995...2012 {
local row = `yr' - 1994
local numerator = 100*(income - L1.income)
local denominator = ((abs(income) + abs(L1.income)) / 2)
local frac = (`numerator' / `denominator')
summarize frac
local sdfrac = r(sd)
matrix C[`row', 1] = `numerator'
matrix C[`row', 2] = `denominator'
matrix C[`row', 3] = `sdfrac'
}

Answer 1

If I am understanding your question right, maybe you don't need to use a loop until the end and then you can post the results to a postfile:
This is just a thought:
tempname memhold
tempfile filename
postfile `memhold' year sdfrac using `filename'
gen row=year-1994
gen numerator=100*(income-L1.income)
gen denominator=((abs(income)+abs(L1.income))/2)
gen frac=numerator/denominator
foreach yr of numlist 1995...2012 {
summarize frac if year=`yr'
local sdfrac=r(sd)
post `memhold' (year) (`sdfrac')
}
postclose `memhold'
clear all
use `filename'
*View Results
list
This code should get you a data set with the name of the year and the standard deviation of the frac variable as variables.

Answer 2

In a comment, OP added a question about code similar to this (but ignored the request to post it in a more civilised form). Note that backticks or left quotation marks in Stata clash with SO mark-up codes in comments. Presumably some
tempname memhold
definition preceded this.
postfile `memhold' year sdfrac sex race using myresults
levels of sex, local (s)
levelsof race, local (r)
foreach a of local s {
foreach b of local r {
forval yr = 1995/2012 {
summarize frac if year == `yr' & sex == `a' & race == `b'
post `memhold' (`yr') (`r(sd)') (`sex') (`race')
}
}
}
Let's focus on what the problem is. You want the standard deviations of frac for all combinations of sex, race and year in a separate file. That's one line
collapse (sd) frac, by(year sex race)
If you want to see a table alongside the data, consider
egen group = group(sex race year), label
and then
tab group, su(frac)
or
tabstat frac, by(group) stat(sd)

Answer 3

This code modifies that by #Pcarlitz, mostly by simplifying it. I can't check with your data, which I don't have.
It's too long to fit into a comment.
I would not use a temporary file as you want to save these results, it seems.
tempname memhold
postfile `memhold' year sdfrac using myresults
gen frac = (100*(income - L1.income))/((abs(income) + abs(L1.income))/2)
forval yr = 1995/2012 {
summarize frac if year==`yr'
post `memhold' (`yr') (`r(sd)')
}
postclose `memhold'
use myresults
list
UPDATE As in a later answer, consider collapse as a much simpler direct alternative here.