st: Looping across observations (forwards and backwards)

[pedromfn <nakashimapedro@gmail.com>]

Dear statalisters

My database looks like:

obs cod pr qt sinalt
1 1 1.4 100 .
2 2 1.5 100 .
3 1 1.5 95 .
4 1 1.4 100 .
5 3 1.5 100 .

and I want to replace observations of sinalt in which cod==3, according to
the following rule:
1) Go across observations looking for observations in which cod=3
2) In the above example, the first observation is observation 5, in which
pr[5]=1.5 and qt[5]=100. Once that observation was found, go backwards
through observations looking for the first observation j in which
pr[j]==pr[5] & qt[j]==qt[5]. In the example, j=2.
3) Replace sinalt[5]=`sinal' , where the macro sinal is defined as:
     if cod[j]==1, store in the local sinal the value 1
     if cod[j]==2, store in the local sinal the value -1
4) Once last replace was done, look for the next observation in which cod==3
and do the same thing.

I wrote the following do-file, but it didn't work:

forvalues i=1/`=_N' {
        if cod[`i']==3{
                local j=`i'-1
                if pr[`j']==pr[`i'] & qt[`j']==qt[`i'] {
                        if cod[j]==1 {
                                local sinal 1
                        }
                        else if cod[`j']==2 {
                                local sinal -1
                        }
                        else {
                                local sinal
                        }
                }
                else {
                        while pr[`j']!=pr[`i'] | qt[`j']!=qt[`i'] {
                                local --j
                        }
                }
        replace sinalt=`sinal' in `i'
        }
}

ERROR:
in not found
r(111);

Does someone know what went wrong?

Thanks very much

Best regards,
Pedro. 

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