Using an "or" operator between variables for a loop in Stata - loops

Question

I have a set of variables that are string variables. For each value in the string, I create a series of binary (0, 1) variables.
Let's say my variables are Engine1 Engine2 Engine3.
The possible values are BHM, BMN, HLC, or missing (coded as ".").
The values of the variables are mutually exclusive, except missing.
In a hypothetical example, to write the new variables, I would write the following code:
egen BHM=1 if Engine1=="BHM"|Engine2=="BHM"|Engine3=="BHM"`
replace BHM=0 if BHM==.
gen BMN=1 if Engine1=="BMN"|Engine2=="BMN"|Engine3=="BMN"`
replace BMN=0 if BMN==.
gen HLC=1 if Engine1=="HLC"|Engine2=="HLC"|Engine3=="HLC"
replace HLC=0 if HLC==.
How could I rewrite this code in a loop? I don't understand how to use the "or" operator | in a loop.

Answer 1

First note that egen is a typo for gen in your first line.
Second, note that
gen BHM=1 if Engine1=="BHM"|Engine2=="BHM"|Engine3=="BHM"
replace BHM=0 if BHM==.
can be rewritten in one line:
gen BHM = Engine1=="BHM"|Engine2=="BHM"|Engine3=="BHM"
Now learn about the handy inlist() function:
gen BHM = inlist("BHM", Engine1, Engine2, Engine3)
If that looks odd, it's because your mathematics education led you to write things like
if x = 1 or y = 1 or z = 1
but only convention stops you writing
if 1 = x or 1 = y or 1 = z
The final trick is to write a loop:
foreach v in BHM BMN HLC {
gen `v' = inlist("`v'", Engine1, Engine2, Engine3)
}
It's not clear what you are finding difficult about |. Your code was fine in that respect.
An bug often seen in learner code is like
gen y = 1 if x == 11|12|13
which is legal Stata but almost never what you want. Stata parses it as
gen y = 1 if (x == 11)|12|13
and uses its rule that non-zero arguments mean true in true-or-false evaluations. Thus y is 1 if
x == 11
or
12 // a non-zero argument, evaluates as true regardless of x
or
13 // same comment
The learner needs
gen y = 1 if (x == 11)|(x == 12)|(x == 13)
where the parentheses can be omitted. That's repetitive, so
gen y = 1 if inlist(x, 11, 12, 13)
can be used instead.
For more on inlist() see
articles here
and
here Section 2.2
and
here.
For more on true and false in Stata, see this FAQ