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From |
Tomáš Houška <xbender@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: looping / refering to multiple variables |

Date |
Thu, 6 Oct 2011 10:44:06 +0200 |

Hi, I am facing a problem that intuitively probably has an easy solution, but I cannot find it. I have panel data describing sales of products (i) in time (t) and stores (n). I woud like to compute a sum P_nt = SUM (p_int * exp_share_int) where p_int is a price of a given product in a time _t and store _n, and exp_share is an expenditure share for product _i from total expenditure in that week and store for all products. As was adviced to be here earlier (see http://www.stata.com/statalist/archive/2011-10/msg00082.html ), my situation could be resolved by this: egen P_nt = total(expression), by(store week) where expression = 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 number at the end of the variable is a code for the product the variable refers to. My problem is that I am using different lists of products and I would like to be able to change my do file easily without having to rewrite all of the expressions like the one here that I am using. with a macro containing list of product codes (i.e. global item_code "9401000 9401870...."), I was able to come up with this: foreach i of global item_code { local multiple (l_avgp`i'*exp_share`i') + `multiple' } the problem is that at the first instance, the "+" sign is created with the first "multiple" and then the plus sign gets transfered to the whole expression - I end up if expression with "+" at the end. The problem is that I need "i" cycles but only "i-1" plus signs in the expression. But I feel there has to be a simple way how to do that. I am not very happy with haveing to create an expression that is just a string and then having to insert it into the actual calculation --egen-- command. Is something like this possible? foreach i of global code { egen P_nt = sum (l_avgp`i' * exp_share`i') } the problem is that this would create i variables P_nt, one for each i.... Thank you for any tips! Tomas * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**st: RE: looping / refering to multiple variables***From:*Nick Cox <n.j.cox@durham.ac.uk>

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