Equivalent of total() for Product of observations

[Tommaso Bechini]

Dear Statalisters,

This is my first post here.

I am looking for a way to generate the product of the observations within a variable so that I have a separate column of identical values, each being the product of all the observations of the said variable, like egen total() does with the sum.

To make an example, if X is my variable, I would have X= (x1, x2, x3, x4) and I would like to have ProdX = (x1*x2*x3*x4,...)

To the best of my knowledge (that may be very poor, since it's not been long that I encountered this issue) I have not found an answer in a previous post. The next best thing I found was the prod plugin, but it seems to me it's more an analogous to the sum option rather than total.

I sincerely hope I was clear enough.

Thank you for any contribution.

Tommaso

[Nick Cox]

Here are two methods. The second isn't valid if any values are negative. Naturally any zeros annihilate products any way.

sysuse auto, clear

bysort foreign : gen double product = length[1]
by foreign : replace product = product[_n-1] * length if _n > 1
by foreign : replace product = product[_N]
 
by foreign : gen double product2 = sum(log(length))
by foreign : replace product2 = exp(product2[_N])

tabdisp foreign, c(product*)

----------------------------------
 Car type |    product    product2
----------+-----------------------
 Domestic |  1.24e+119   1.24e+119
  Foreign |  9.070e+48   9.070e+48
----------------------------------

[Tommaso Bechini]

Dear Mr. Cox,

Thank you. I shall apply this method to my problem right away.

Thanks for the swift answer.

Tommaso Bechini