st: Mata Question

[Danielle H Ferry <DFerry@bear.com>]

Dear Statlisters,
I'm hoping someone can help me with this. I know how to do it in Stata, 
but not Mata.

Suppose in my pure time series dataset I have a variable, Y, and want to 
create a new variable, Z, where Z = . if t=1, Z = Y if t = 2, and Z = 
Z[_n-1] if t>2. After I create a data view, this is simple to implement 
in Mata (assuming my data is already sorted):

drop _all
set obs 5
ge Y = 1
ge t = _n
scalar N1 = 2

mata
st_view(y=., ., "Y", .)
n1 = st_numscalar("N1")
z = J(rows(y), 1, .)
z[n1,1] = y[n1,1]
for(j=3; j<=rows(z); j++) {
	i = j - 1
	z[j, .] = z[i, .] + 1
}
end
list

Suppose, now, that I have a panel. I want the same definition for Z to 
apply WITHIN EACH PANEL. It is clearly this part that needs to be 
amended somehow, but I'm not sure how to do it best:

z[n1,1] = y[n1,1]
for(j=3; j<=rows(z); j++) {
	i = j - 1
	z[j, .] = z[i, .] + 1
}

The brute force way would be:

drop _all
set obs 10
ge p = 1 in 1/5
replace p = 2 in 6/l
ge Y = 1
bys p: ge t = _n
scalar N1 = 2
scalar numpanels = 2
scalar numonpanel = 5

mata
st_view(y=., ., "Y", .)
n1 = st_numscalar("N1")
nump = st_numscalar("numpanels")
numonp = st_numscalar("numonpanel")
z = J(rows(y), 1, .)
z[n1,1] = y[n1,1]
for(j=3; j<=rows(z)/nump; j++) {
	i = j - 1
	z[j, .] = z[i, .] + 1
}
z[n1+numonp,1] = y[n1+numonp,1]
for(j=3+numonp; j<=rows(z); j++) {
	i = j - 1
	z[j, .] = z[i, .] + 1
}
idx = st_addvar("float", "Z")
st_store(., idx, z)
end
list

But this requires me to know a lot about the data a priori. Is there a 
clever way to do this? In real life I have several different data 
samples and N1, numpanels, & numonpanels will vary across samples.

Many thanks,
Danielle

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email;internet:dferry@bear.com
tel;work:212/272-3415
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