Complicated reshaping

Question

I want to reshape my dataframe from long to wide format and I loose some data that I\'d like to keep. For the following example:

df <- data.frame(Par1 =          


        

Answer 1

Trying to wrap different aggregation expressions into a self-contained function (expressions should yield atomic values)...

multi.by <- function(X, INDEX,...) {
    expressions <- substitute(...())
    duplicates <- duplicated(INDEX)
    res <- do.call(rbind,sapply(split(X,cumsum(!duplicates),drop=T), function(part) 
        sapply(expressions,eval,part,simplify=F),simplify=F))
    if (is.data.frame(INDEX)) res <- cbind(INDEX[!duplicates,],res)
    else rownames(res) <- INDEX[!duplicates]
    res
}

multi.by(df,df[,1:2],
    pre=mean(Val[Type=="pre"]), 
    post=mean(Val[Type=="post"]),
    Num.pre=sum(Type=="pre"),
    Num.post=sum(Type=="post"),
    ParD=paste(ParD, collapse="_"))