[R] replace a for loop with lapply or relative

[David Freedman]

Dear helpers.
I often need to make dichotomous variables out of continuous ones (yes, I
realize the problems with throwing away much of the information), but I then
like to check the min and max of each category.  I have the following simple
code to do this that cuts each variable (x1,x2,x3) at the 90th percentile,
and then prints the min and max of each category:

d=data.frame(x1=rnorm(100),x2=runif(100)); d=transform(d,x3=x1-x2)
d[,4:6]=data.frame(sapply(d,function(v)as.numeric(v>=quantile(v,0.9))));
names(d)[4:6]=c('x1high','x2high','x3high')
head(d)
for (i in
1:3){print(do.call(rbind,by(d[,i],d[,i+3],function(x)(c(min(x),max(x))))))}

Is there a way to replace the ugly for loop in the last line with some type
of apply function that would know that my continuous and indicator variable
are 3 variables apart in the dataframe?

Thanks very much
David Freedman
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