Dollar sign when defining local varaible

[Ekaterina Shpanenberg]

Dear all,

I'm trying to run some .do file, and receive an error on a line:

local carry=$carry_months

Error:

invalid syntax
r(198);

As far I googled, a dollar sign might mean global variable or, in some cases, just a dollar sign to be inside a variable? Could you please explain what this line might do and how can I avoid the error?

After defining it, there is a loop:

local names="AS"

foreach k in `names' {
    forval i=1/`carry' {
        sort stock month_id
    
        qui gen `k'_pr_`i'=`k'[_n-`i'] if stock==stock[_n-`i'] & month_id==month_id[_n-`i']+`i'
    }
}

foreach k in `names' {
    forval i=1/`carry' {
        sort stock month_id
    
        qui replace `k'=`k'_pr_`i' if `k'==. & stock==stock[_n-`i'] & month_id==month_id[_n-`i']+`i' 
        qui drop `k'_pr_`i' 
    }
}

Thank you in advance.

[Nick Cox]

That's legal but surprising syntax. The dollar sign there certainly implies a global macro (not global variable).

If the global foo contained say a variable name then

local bar = $foo

would return the value of that variable in the first observation and it seems unlikely that is wanted.

But, backing up, the most likely reason I can think of for your error message is that the global is not even defined. You can check for that directly by inserting previously

di "carry_months is $carry_months"

and if nothing is displayed then that explains the error message.

But without any context -- and without knowing what the global is supposed to contain -- it is really hard to suggest what the code should be.

Conversely, this do-file presumably did what the author(s) wanted at some point, so why not ask them to explain?

(Undocumented do-files can be much more work than undocumented programs!

(As far as I can see your longer code segment casts no light on the error.)

[Clyde Schechter]

In

local carry=$carry_months

Stata interprets $carry_months to be a global macro (not variable) named carry_months. If there is no such global macro defined, then $carry_months is evaluated as an empty string, so that your command reduces to:

local carry =

which is, indeed, a syntax error because you can't have an equals sign with nothing to its right in Stata.

If $carry_months is defined, then whatever it contains is substituted for $carry_months on the right side of that equals sign. If that doesn't happen to be a valid expression (only valid expressions can occur to the right of that equals sign) then this will also be a syntax error.

To find out which of these situations applies to you, run -display `"$carry_months"'-.

Edit: Crossed with Nick's response.

[Ekaterina Shpanenberg]

Dear Nick and Clyde,

you're right, the error is caused by the fact that variable $carry_months does not exist..
Documenting is a nice thing in programming, because I can hardly have an idea of what might be implied.
Thanks for your help.