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Re: st: Programming Problem: How to prevent macro substitution


From   "Eva Poen" <eva.poen@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Programming Problem: How to prevent macro substitution
Date   Wed, 16 Jul 2008 17:59:13 +0100

Nick, thank you.

I was looking for a solution that can be done independently of the
creation of the temporary variables, since this is how Christoph
started the thread. What I didn't realise is that my solution will
contain the interior double quotes, which a simple -display- will not
reveal:

. local test "hello " "hello"

. di "`test'"
hello hello

. di `"`test'"'
hello " "hello

It is therefore clearly not suitable, and plainly wrong. Thanks for
pointing it out to me. Austin gave a perfect, working solution.

I also agree that escaping macro substitution is often not the way
forward. At least I have managed to avoid it completely in my work.

Eva


2008/7/16 Nick Cox <n.j.cox@durham.ac.uk>:
> I see. Well, there will be an almost equally simple way of doing that,
> or an equivalent, and one which certainly won't entail escaping macro
> substitution.
>
> For example, suppose you are in a loop
>
> Within some loop {
>        ...
>        tempvar foo
>
>        local xlist `xlist' i.land|`foo'
>        ...
> }
>
> Otherwise put, I don't think there is ever a problem for which lines
> like
>
>        local ldep`u' "ldep`u'"
>
> are part of the solution.
>
> (We had a similar thread a while back in which someone asserted that it
> was sometimes necessary to delay substitution, and I kept disputing
> that. It's a myth that seems to grow out of trying to translate other
> programming styles to Stata macro programming.)
>
> Nick
> n.j.cox@durham.ac.uk
>
> Eva Poen
>
> Thanks, Nick.
>
> However, your suggestion does not produce the desired result:
>
> . di "`xlist'"
> i.land|ldep0 i.land|ldep1 i.land|ldep2
>
> While Christoph was after:
>
> . di "`xlist'"
>  i.land|`ldep0' i.land|`ldep1' i.land|`ldep2'
>
> Unless I am missing something here.
>
> Eva
>
> 2008/7/16 Nick Cox <n.j.cox@durham.ac.uk>:
>
>> I agree. In fact, I'd go one step further:
>>
>> foreach u of numlist 0/2{
>>  local xlist `xlist'  i.land|ldep`u'
>> }
>>
>> Sometimes the quotes just make things more complicated.
>
> Eva Poen
>
>> take it in steps and it works:
>>
>> foreach u of numlist 0/2{
>>  local xlist "`xlist' " "i.land|" "`" "ldep`u'" "'"
>> }
>>
>>
>> -di "`xlist'"- returns
>>
>> i.land|`ldep0' i.land|`ldep1' i.land|`ldep2'
>>
>> on my machine.
>>
>> 2008/7/16 Christoph Birkel <christoph.birkel@soziologie.uni-halle.de>:
>>
>>> I want to program a loop which produces a macro `xlist' containing a
>> list of
>>> dummy variable interaction expansion expressions with temporary
>> variable
>>> names `ldep0' , `ldep1' etc. as a string . For this purpose I wrote:
>>> foreach u of numlist 0/`y2'{
>>>  local ldep`u' "ldep`u'"
>>> local xlist "`xlist' i.land|``ldep`u'''"
>>> }
>>> The resulting macro should contain an expression like "
> i.land|`ldep0'
>>>  i.land|`ldep1'  i.land|`ldep2' "  (when  `y2'  contains 2)  which
>> can be
>>>  used as argument in -xi: reg-, as in: xi: reg yvar i.land `xlist' .
>> (The
>>> temporary variables `ldep0' and so on, corresponding to the macros
> are
>>> generated later in  a separate loop.)
>>> What actually happens, is that, due to the single quotation marks,
> the
>>> macros containing the names of the temporary variables are subsituted
>> by
>>> their content (which should not happen), so the content of the
>> resulting
>>> macro is " i.land|ldep0  i.land|ldep1  i.land|ldep2 ", which cannot
> be
>> used
>>> to refer to temporary variables. For the same reason, it is not
>> possible to
>>> define a local macro with a string as content which starts and ends
>> with
>>> quotation marks. I found no way to avoid the unintended macro
>> substitution
>>> using "\" or compound quotes. Any recommendation is highly
>> appreciated.
>
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