Fcollapse generating incorrect results

[davidmuhlestein]

I've found an odd bug with fcollapse that I can't figure out. I've reduced it to this example file.

Here's the code to test:

clear
input int p byte c float p2
237 0 0
 41 0 5
 31 1 6
end
sort p2

preserve
fcollapse (sum) c, by(p)
rename c fc
tempfile t
save `t'
restore

collapse (sum) c, by(p)
merge 1:1 p using `t'

quietly count if c!=fc
disp "Mismatches: `r(N)'"

However, if you add any of the following after the input statement and "sort p2", the problem goes away:

• drop p2
• sort p
• sort c
• gen n = _n; sort n
• rename p2 x

It has something to do with p2 starting with the same letters as p and p2 being sorted, but I couldn't figure out the problem.

Let me know if you need any more information to help figure it out.