RE: st: RE: looping panel data

[Nick Cox <n.j.cox@durham.ac.uk>]

What I think you want is 

local exp
local avgp 

foreach j of local item_code {
	local exp `exp' exp`j'
	local avgp `avgp' avgp`j'
}

What's going on here is explained in [U]. 

Nick 
n.j.cox@durham.ac.uk 

Tomáš Houška

Thank you for the info....

I am trying to run a loop comparable to the one you have written, but
it is not producing the expected output. I have product identifiers:
9401000 9401870 9402500 9402600 9405400 9408160 9410100 9410200
9450850 9451200 9451000 and I would like to use them to group
variables together into macros.

I have adjusted the loop you have sent to:

local item_code 9401000 9401870 9402500 9402600 9405400 9408160
9410100 9410200 9450850 9451200 9451000

foreach j of local item_code {
	local exp = exp`j'
	local avgp = avgp`j'
}

the loop is working in the sence that if I insert a --display--
command there, the values of item_code are displayed. But the local
macro is not created... even if I replace the "local item_code" in the
first line with the actual list of codes "foreach j in ...".

What am I not getting?

Since I am working with many variables and many products, this would
be a real time saver.
Thanks, Tomas

PS: I am getting confused with underscores in the macro names. If I
use command --local item_code .....--, for listing the items I have
refer to it with underscore "_item_code" - should that be in the
--foreach-- command too? (i.e. "foreach j of _item_code")



2011/10/4 Nick Cox <n.j.cox@durham.ac.uk>:
> That said, if your interpretation is really that "missing" should be treated as "zero", then you will have repeated problems working with these data unless you -mvencode-.
>
> Nick
> n.j.cox@durham.ac.uk
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: 04 October 2011 13:10
> To: 'statalist@hsphsun2.harvard.edu'
> Subject: RE: st: RE: looping panel data
>
> I don't know a way to do this all in -egen-. As far as -egen- is concerned the argument supplied is a single expression.
>
> But it sounds like
>
> gen total = 0
> gen work = .
>
> foreach j in 9401000  9401870 9402500 9402600 9405400 9408160 9410100 9410200 9450850 9451200 9451000 {
>        replace work = exp_share`j' * avgp`j'
>      replace total = total + work if !missing(work)
> }
>
> egen P_nt = total(total), by(store week)
>
> Using an intermediate variable -work- is not essential.
>
> Nick
> n.j.cox@durham.ac.uk
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Tomáš Houška
> Sent: 04 October 2011 12:50
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: RE: looping panel data
>
> Hi Nick,
>
> thank you very much for the answer and for pointing out the typo. I
> forgot about --by-- option with --egen-- command.
>
> One more thing I would like overcome is that when any observation of
> any of the variables in the expresion is considered missing, the
> result for that whole row is zero - intuitivelly it should skip
> summing up of missing values and just give the result of the rest of
> summing the expression. But even when only one multiplication contains
> missing values, the overall result for that row is zero. That is the
> result is zero even though there are pairs of prices and exp_shares
> that would produce a real number after multiplication. Is there some
> way to deal with that?
> I have tried replacing all the missing obs in all variables with zeros
> and then I get an actual number in the sum - but I feel it would be
> cleaner to define that in --egen-- commnad rather than to change all
> variables involved in the expression.
>
> Thank you!
> Tomas
>
>
>
>
> 2011/10/4 Nick Cox <n.j.cox@durham.ac.uk>:
>> You are a bit confused about -foreach- syntax, but that is secondary. (I think you meant to use `store' within the loop, not `store_number'.)
>>
>> However, I don't see a need for any loops here. This should be one line of Stata. The basic trick is
>>
>> egen P_nt = total(expression), by(store week)
>>
>> Your expression is
>>
>> exp_share9401000* avgp9401000+
>> exp_share9401870* avgp9401000+ exp_share9402500* avgp9402500+
>> exp_share9402600* avgp9402600+ exp_share9405400* avgp9405400+
>> exp_share9408160* avgp9408160+ exp_share9410100* avgp9410100+
>> exp_share9410200* avgp9410200+ exp_share9450850* avgp9450850+
>> exp_share9451200* avgp9451200+ exp_share9451000* avgp9451000
>>
>> except that it looks as if there is a typo there and it should be
>>
>> exp_share9401000* avgp9401000+
>> exp_share9401870* avgp9401870+ exp_share9402500* avgp9402500+
>> exp_share9402600* avgp9402600+ exp_share9405400* avgp9405400+
>> exp_share9408160* avgp9408160+ exp_share9410100* avgp9410100+
>> exp_share9410200* avgp9410200+ exp_share9450850* avgp9450850+
>> exp_share9451200* avgp9451200+ exp_share9451000* avgp9451000
>>
>> Nick
>> n.j.cox@durham.ac.uk
>>
>> Tomáš Houška
>>
>> I would like to ask you for a help with my looping problem. I have
>> sales panel data with variables week ID, store ID, product ID, price,
>> expenditure_share (share of expenditures for a given product across
>> all products in dataset)
>>
>> I would like to compute Stone price index defined as P_nt=SUM
>> (expenditure_share_int * P_int ). This means that for each combination
>> of store (n) and week (t) I need to compute a multiple of
>> expenditure_share and price for each product (i) and then sum it up
>> over all products sold in the particular store that week. The result
>> is again a panel data variable P_nt which is a price index for each
>> week and store. It is an expenditure system index, so the more you
>> spend on more expensive products (keeping prices constant), the higher
>> the index.
>>
>> And I cannot figure out a way how to calculate P_nt index. I have come
>> up with a loop, which saves the results in a new dataset, but it is
>> not working. I would be very glad for any help on how to calculate
>> this variable.
>>
>> Here is my looping command (j=week number, store_number is a local
>> macro with list of store numbers - they are from 2-131 but with
>> missing values (i.e. not consecutive))
>>
>> postfile index_file store week index using stone_index_res, replace
>> foreach store in `store_number' {
>>    forvalues j=91/317 {
>>         gen index`store_number'`j' = exp_share9401000* avgp9401000+
>> exp_share9401870* avgp9401000+ exp_share9402500* avgp9402500+
>> exp_share9402600* avgp9402600+ exp_share9405400* avgp9405400+
>> exp_share9408160* avgp9408160+ exp_share9410100* avgp9410100+
>> exp_share9410200* avgp9410200+ exp_share9450850* avgp9450850+
>> exp_share9451200* avgp9451200+ exp_share9451000* avgp9451000 if
>> week==`j' & store==`store_number'
>>          post index_file (`week') (`store') (index`store_number'`j')
>>         }
>> }
>> postclose index_file
>>
>

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