IF condition within a loop depending on the iterated value of the loop

[jessica cheng]

I would appreciate any help that you can give on my question below.

For example, I have the following loop:
foreach var of varlist X1 X2 X3 {
…
reg DV `var' control_set1
reg DV `var' control_set2
…
}

I want to add an IF condition depending on the value of var.

For example, for X2 and X3, add the condition “if fyear>2005”. The effect would be like this:

reg DV X1 control_set1
reg DV X1 control_set2

reg DV X2 control_set1 if fyear>2005
reg DV X2 control_set2 if fyear>2005

reg DV X3 control_set1 if fyear>2005
reg DV X3 control_set2 if fyear>2005

Is it possible to do this in a loop?

[Øyvind Snilsberg]

absolutely,

foreach x in x1 x2 x3 {
if `x'==x1 {
reg dv `x'
}
else {
reg dv `x' if fyear>2005
}
}

[Nick Cox]

The advice in #2 is not quite right. The test should be on the names, not the values of the variables. The code in #2 might work by accident as x1 == x1 but if x2 = x1 or x3 = x1 in the first observation (which is all that would be tested) then the code will not work as desired.

Consider rather

foreach x in x1 x2 x3 {
if "`x'"=="x1" {
reg dv `x'
}
else {
reg dv `x' if fyear>2005
}
}

In the example given, it would be even simpler to go

reg dv x1 

foreach x in x2 x3 { 
    reg dv `x' if fyear>2005
}

and indeed I see no great virtue in a loop at all as compared with three plain regress statements. Naturally, your full or real example may be much more complicated.

[jessica cheng]

Thank you so much Øyvind and Nick.

Indeed my code inside the loop is much more complicated than the example I give above and so I wish to use a loop.