unintended behavior of -ereplace- when combined with an if statement

[Andrew Benson]

I was using -ereplace- today with if statements. (See also its introduction on Statalist here.)

I noticed that erplace was not only replacing values for observations that meet the conditions of the if statement (what I wanted); it was also changing all values of the variable in question to missing for all observations that did not meet the conditions of the if statement (not what I wanted!). This seems like unintended behavior, as -replace- doesn't do this and -ereplace- is intended to be identical to -replace- except that it works for egen commands.

Here is a minimum working example loosely based on my dataset:

clear
input float var1 str1 var2 float var3
1 "A" 0
2 "A" 0
3 "A" 0
4 "B" 0
5 "B" 0
6 "B" 0
7 "C" 1
8 "C" 1
9 "C" 1
10 "D" 1
11 "D" 1
12 "D" 1
end

Suppose I want to generate a new variable that follows one rule for those observations which take the value 0 on var3 and different rule for those observations that take the value 1.

egen var4=concat(var1 var2) if var3==0, punct("-")
(6 missing values generated)

My data now look like:

var1	var2	var3	var4
1	A	0	1-A
2	A	0	2-A
3	A	0	3-A
4	B	0	4-A
5	B	0	5-A
6	B	0	6-A
7	C	1
8	C	1
9	C	1
10	D	1
11	D	1
12	D	1

Now to replace the missing values of var4 with what I want for those observations which take on the value 1 for var3:

ssc install ereplace
ereplace var4=concat(var1 var2) if var3==1, punct("-")
(6 missing values generated)
(12 real changes made)

My data now look like:

var1	var2	var3	var4
1	A	0
2	A	0
3	A	0
4	B	0
5	B	0
6	B	0
7	C	1	7-C-1
8	C	1	8-C-1
9	C	1	9-C-1
10	D	1	10-D-1
11	D	1	11-D-1
12	D	1	12-D-1

Why did -ereplace- modify var4 for obs. 1 through 6? Is this intended behavior of -ereplace- or a bug?

I am assuming only the authors will be able to answer this question, but if anyone knows if I'm using -ereplace- wrong, your advice is appreciated.

(I have already found an alternate approach that solves my problem, so no need to suggest alternate solutions.)