R - selection of the kth element of a column

Question

I have performed statistical matching in R. For each case "VAR2002", I have found one or more statistical twin(s) "VAR2004". In R, I have a data frame "TwinWeight" like this:

VAR2002     VAR2004    Weight
1           2955        1.00000000
2           3961        1.00000000
3           2913        0.33333333
3           3430        0.33333333
3           3554        0.33333333
4           2996        0.20000000
4           4618        0.20000000

. . .

Additionally, I have a dataset D with many variables D1, D2,..., D55.

With this knowledge, I want to make my calculations for all D variables in the following way: For each VAR2002: Weight*D[Statistical Twin 1]

This means for VAR2002=1 and D1:

D$D1[1]<-TwinWeight[1,3]*D$D1[TwinWeight[1,2]]

Unfortunately, this command is not valid in R, as D$D1[] needs a numeric value in []. I cannot name a numeric value, but rather want this number to be taken from the TwinWeight table.

Does anybody know how to solve this problem?

Thanks so much for your help!

Answer 1

You can index by name and / or position. Try

D[1,"D1"] <- TwinWeight[1,3] * D[ TwinWeight[1,2],"D1"]

which assigns to row 1 of column D1 the product of the two scalars selected on the right.