Re: st: syntax error in forvalues range definition using a tempvar??

[Richard Williams <Richard.A.Williams.5@ND.edu>]

At 05:00 PM 7/16/2005 -0700, Brian R. Sala wrote:
> Can someone enlighten me on this syntax error, please.
>
> why does this fail:
>
> . tempvar maxn
>
> . egen byte `maxn' = max(assignment_count)
>
> . tab `maxn'
>
>    __000001 |      Freq.     Percent        Cum.
> ------------+-----------------------------------
>           4 |     20,299      100.00      100.00
> ------------+-----------------------------------
>       Total |     20,299      100.00
>
> . forvalues i = 1/`maxn' {
>   2. di `i'
>   3. }
> invalid syntax
> r(198);
>
>
> clearly, `maxn' has a value. Superficially, this looks equivalent to the 
> syntax shown in the STATA 8 programming manual (pp. 206-207).
Actually, `maxn' is a variable and hence every case has a value for 
it.  All cases may have the same value on the variable, but it is still a 
variable.  forvalues doesn't like you specifying a variable here, it wants 
a single number.


> yet when I do this:
>
> . tempvar max_assignments
> . egen byte `max_assignments' = max(assignment_count)
> . local maxn = `max_assignments'
> . forvalues i = 1/`maxn' {
>   2. display `i'
>   3. }
> 1
> 2
> 3
> 4
Here, `maxn' is a constant and not a variable; what the local statement has 
done is set it equal to the value the first case has on 
`max_assignments'.  Since every case has the same value on 
`max_assignments' you are fine, but if `max_assignments' had different 
values you could be in trouble if the first case didn't have the value you 
want.

To further see this:

. webuse auto
(1978 Automobile Data)

. local maxn = price

. di `maxn'
4099

4099 is the value of the first case for price.

. sort length

. local maxn = price

. di `maxn'
3895

After sorting, we now have a different value for the first case and hence 
`maxn' now has a different value.

Perhaps a more common way to get what you want is to do something like this:

. sum price

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       price |        74    6165.257    2949.496       3291      15906

. return list

scalars:
                 r(N) =  74
             r(sum_w) =  74
              r(mean) =  6165.256756756757
               r(Var) =  8699525.974268789
                r(sd) =  2949.495884768919
               r(min) =  3291
               r(max) =  15906
               r(sum) =  456229

. local maxn = r(max)

. di `maxn'
15906



-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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