在Stata中使用变量之间的“或”运算符进行循环

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