[R] Create matrix with column names wiht the same prefix xxxx and that end in 1, 2

[Sorkin, John]

I am trying to create an array, myvalues, having 2 rows and 4 columns, where the column names are j,k,xxx1,xxx2. The code below fails, with the following error, "Error in dimnames(myvalues) <- list(NULL, zzz) : 
  length of 'dimnames' [2] not equal to array extent" 

Please help me get the code to work.

Thank you,
John

# create variable names xxx1 and xxx2.
string=""
for (j in 1:2){
  name <- paste("xxx",j,sep="")
  string <- paste(string,name)
  print(string)
}
# Creation of xxx1 and xxx2 works
string

# Create matrix
myvalues <- matrix(nrow=2,ncol=4)
head(myvalues,1)
# Add "j" and "k" to the string of column names
zzz <- paste("j","k",string)
zzz
# assign column names, j, k, xxx1, xxx2 to the matrix
# create column names, j, k, xxx1, xxx2.
dimnames(myvalues)<-list(NULL,zzz)