st: forvalues loop doesn't work?

[econqian222 <maguixianstatalist@gmail.com>]

Hi, statalist
    I have tried to record percentile 90 of quantity for each value of
index, unfortunately, my result still only record the percentile 90 of
quantity for all the values of index, so what's the problem of my program?
    My program goes like the following:

     forvalues i=1/4760{
      if index==`i' {
         sum quantity, detail
         replace q_cutoff90=r(p90)
       }
     }

My data goes as this:
index	quantity	q_cutoff90
1	4	.
1	67	.
2	89	.
2	57	.
2	91	.
3	98	.
3	95	.
3	27	.
.	.	.
.	.	.
4760	88	.
4760	56	.

Many thanks
Qian

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