Generating interaction variables in R dataframes

Question

Is there a way - other than a for loop - to generate new variables in an R dataframe, which will be all the possible 2-way interactions between the existing ones? i.e. supposing a dataframe with three numeric variables V1, V2, V3, I would like to generate the following new variables:

Inter.V1V2 (= V1 * V2)  Inter.V1V3 (= V1 * V3) Inter.V2V3 (= V2 * V3) 

Example using for loop :

x <- read.table(textConnection('    V1 V2 V3 V4 1  9   25   18 2  5   20   10 3  4   30   12 4  4   34   16' ), header=TRUE)  dim.init <- dim(x)[2] for (i in 1: (dim.init - 1) ) {         for (j in (i + 1) : (dim.init) ) {                 x[dim(x)[2] + 1]    <- x[i] * x[j]                 names(x)[dim(x)[2]] <- paste("Inter.V",i,"V",j,sep="")          } } 

Answer 1

Here is a one liner for you that also works if you have factors:

> model.matrix(~(V1+V2+V3+V4)^2,x)   (Intercept) V1 V2 V3 V4 V1:V2 V1:V3 V1:V4 V2:V3 V2:V4 V3:V4 1           1  1  9 25 18     9    25    18   225   162   450 2           1  2  5 20 10    10    40    20   100    50   200 3           1  3  4 30 12    12    90    36   120    48   360 4           1  4  4 34 16    16   136    64   136    64   544 attr(,"assign")  [1]  0  1  2  3  4  5  6  7  8  9 10