How to do median splits within factor levels in R?

Question

Here I make a new column to indicate whether myData is above or below its median

### MedianSplits based on Whole Data
#create some test data
myDataFrame=data.frame(myData=runif(15),myFactor=rep(c("A","B","C"),5)) 

#create column showing median split
myBreaks= quantile(myDataFrame$myData,c(0,.5,1))
myDataFrame$MedianSplitWholeData = cut(
    myDataFrame$myData,
    breaks=myBreaks, 
    include.lowest=TRUE,
    labels=c("Below","Above"))

#Check if it's correct
myDataFrame$AboveWholeMedian = myDataFrame$myData > median(myDataFrame$myData)
myDataFrame

Works fine. Now I want to do the same thing, but compute the median splits within each level of myFactor.

I've come up with this:

#Median splits within factor levels
byOutput=by(myDataFrame$myData,myDataFrame$myFactor, function (x) {
     myBreaks= quantile(x,c(0,.5,1))
     MedianSplitByGroup=cut(x,
       breaks=myBreaks, 
       include.lowest=TRUE,
       labels=c("Below","Above"))
     MedianSplitByGroup
     })

byOutput contains what I want. It categorizes each element of factors A, B, and C correctly. However I'd like to create a new column, myDataFrame$FactorLevelMedianSplit, that shows the newly-computed median split.

How do you convert the output of the "by" command into a useful data-frame column?

I think perhaps the "by" command is not R-like way to do this ...

Update:

With Thierry's example of how to use factor() cleverly, and upon discovering the "ave" function in Spector's book, I've found this solution, which requires no additional packages.

myDataFrame$MediansByFactor=ave(
    myDataFrame$myData,
    myDataFrame$myFactor,
    FUN=median)

myDataFrame$FactorLevelMedianSplit = factor(
    myDataFrame$myData>myDataFrame$MediansByFactor, 
    levels = c(TRUE, FALSE), 
    labels = c("Above", "Below"))

Answer 1

Here is a solution using the plyr package.

myDataFrame <- data.frame(myData=runif(15),myFactor=rep(c("A","B","C"),5))
library(plyr)
ddply(myDataFrame, "myFactor", function(x){
    x$Median <- median(x$myData)
    x$FactorLevelMedianSplit <- factor(x$myData <= x$Median, levels = c(TRUE, FALSE), labels = c("Below", "Above"))
    x
})