[R] Class attributes

[Allen L]

Dear R forum,
I want to replace all the elements in a data frame (dd) which match the
character "x" with "0".
What's the most elegant way of doing this (there must be an easy way which
I've missed)? I settled on the following loop:

>for(i in 5:12){                # These are the column of dd I am interested
in
>dd[which(dd[,i]=="x"),i]<-0
>}

The problem with this is that the columns which used to contain "x" are
still considered factors and I am unable to coerce them into numeric:

> mean.species.biomass<-colMeans(as.numeric(dd.p[,5:12]))
>Error in inherits(x, "data.frame") : 
  (list) object cannot be coerced to type 'double'

I'm tried unclassing & reclassing, other functions etc. but nothing seems to
work. What is wrong?
Thanks in advance,
Allen
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