Clarification for a loop to create interaction terms in Stata

Question

I found the following question/answer that I think does what I would like to do: https://www.stata.com/statalist/archive/2009-09/msg00449.html

However, I am unclear what is going on in all of it, and would like to understand better. The code for the solution is as follows:

unab vars : var1-var30
local nvar : word count `vars'
forval i = 1/`nvar' {
  forval j = 1/`=`i'-1' {
    local x : word `i' of `vars'
    local y : word `j' of `vars'
    generate `x'X`y' = `x' * `y'
  }
}

I do not understand what is going on in line 4 with the statement: ``=i'-1'.

The i refers to the number in the set {1,...,n}, but I do not understand what the equals or the -1 are doing. My assumption is that the -1 is somehow removing the own observation, but I am unclear. Any explanation would be appreciated.

Answer 1

Suppose you have local macro i that varies over a range and you want its value minus 1. You can always do this

  local j = `i' - 1 

and then refer to j. You can also do this on the fly:

 `= `i' - 1' 

Within

 `=   ' 

Stata will evaluate the expression, here

`i' - 1 

and substitute the result of that expression in a command line.

You can do this with scalars too:

scalar foo = 42 

and then refer to

`= foo' 

However, watch out. Scalar names and variable names occupy the same namespace.

`= scalar(foo)' 

disambiguates and arguably is good style in any case.