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st: RE: Macro questions.


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Macro questions.
Date   Tue, 10 Aug 2004 23:51:26 +0100

adiallo5@worldbank.org
 
> I have some questions about macros.
> 
> 1/ I wonder how to count the number of variables in a macro.
> I try the macro extended function (word count) but it doesn't work.
> For example, if I have:
> local myvars "var1 var2 var3 var4"
> then `myvars' contains 4 variables.
> But I do not know how to create a macro that contains the value 4.

I go 

. local myvars "var1 var2 var3 var4"

. local nv : word count `myvars'

. di `nv'
4

and Stata tells me 4. (The macro contains not four 
variables, but the names of four variables.) 
What are you doing wrong? I have to guess. 
Perhaps you are doing something like 

. local nv : word count "`myvars'"

. di `nv'
1

Here you must remember Stata's definition of 
a word. Stata by default parses on white space meaning
that 

Amadou loves Stata macros 

would be considered by Stata to be 4 words. So far, 
so good. But Stata has another rule: double quotes 
bind tighter than white space separates, so 

"Amadou loves Stata macros" 

is just _one_ word. This is just Stata's sense of 
a word; forget here what you know about the grammar of 
any ordinary language. Now, when you defined 

. local myvars "var1 var2 var3 var4"

the " " were just ornamental; Stata took 
them as delimiters, not as part of your 
definition of the local myvars (and you
could have omitted them). But if you go 

. local nv : word count "`myvars'"

the " " are not ornamental at all; they 
are working hard to bind what follows 
-: word count- into one word. 
 

> 2/ Suppose several datasets containing the same variable floor indexed
> 1, 2, 3, etc.... Floor* number varies with datasets.
> I want to create the macro myfloor
> local myfloor "floor*"
> but then, I do not know how to create the macro containing 
> the number of floor
> as in question 1.

This question was asked earlier today. As said then, 
the number of variables is not often an issue. Nevertheless, 

unab floor : floor* 
local nfloor : word count `floor' 

> 
> 3/ How to work "inside" macros?
>    suppose you have the following: local "var1 var2 var3 .....varN"
>    suppose later in your dofile you want to do some tables on 
> only - say 20-
> variables of  the macro.
>   Is it possible to just choose them (perhaps by specifying 
> their position IN
> the macro)
>   without creating other (sub)macros?

The term submacro is yours; I don't know what you mean by it. 
There are several ways of doing this, but they usually 
involve creating other macros. 
> 
> 4/ Finally my last question is how to merge/append several datasets?
>     Suppose you have :
>    local mydata "data1 data2 data3 data4 data5"
>    foreach if i of local mydata {
>        foreach j of local mydata {
>          append `i' using `j'
>       }
>    }
> 
> How to avoid data1 being appended twice?

Various ways. Here's one. 

Start with the second word in the macro. 

local mydata "data1 data2 data3 data4 data5" 
local nsets : word count `mydata' 
tokenize `mydata' 

use `1' 
forval i = 2/`nsets' { 
	append using ``i'' 
} 

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