r - The ways to construct columns associated with interaction terms in data frame -

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i have data frame 6 columns, 

dat<-data.frame(x1,x2,x3,x4,x5,x6)

right now, need build 2 columns associated interaction terms, x1*x2 , x3*x4*x5 how in r. there special consideration when of them, such x2 categorical?

i guess function model.matrix want.

for instance, can fit linear model including variables , interaction terms you're interested in , extract model matrix fitted object

model.matrix(lm(drat ~ mpg * cyl + disp * hp * wt, data = mtcars))

factors need explicitly coded factors, find example below

mtcars$cyl <- factor(mtcars$cyl) model.matrix(lm(drat ~ mpg * cyl + disp * hp * wt, data = mtcars))

the default kind of contrasts used factors treatment coding. can change sum coding (or other codings: ?contr.sum) using command below

contrasts(mtcars$cyl) <- contr.sum

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