R help - Mar 2014 - function "Stack"

Dears R Users,

I have another question on function "stack".

Given a data frame like:
df=data.frame(a=c(3,5),b=c(2,8),a=c(9,1),b=c(6,4),check.names=F)
   a b a b
   3 2 9 6
   5 8 1 4
 
I would like to form a new data frame like: 
    values ind
1      3   a
2      5   a
3      2   b
4      8   b
5      9   a
6      1   a
7      6   b
8      4   b

I arrived with the script: 
    stack = function (x, ...) 
{
    x = as.list(x)
    keep = unlist(lapply(x, is.vector))
    if (!sum(keep)) 
        stop("at least one vector element is required")
    if (!all(keep)) 
        warning("non-vector elements will be ignored")
    x = x[keep]
    data.frame(values = unlist(unname(x)), ind = factor(rep.int(names(x), 
        lapply(x, length))), stringsAsFactors = FALSE, ...)
}
     stack(df, check.names = FALSE)

Do you have any other simple manner to that?

Thanks in advance for your helps!

Tham




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