Question

I tried to use acast from reshape2 within a self written function, but had the problem that acast did not find the data I send to it.

Here is my data:

library("reshape2")
x <- data.frame(1:3, rnorm(3), rnorm(3), rnorm(3))    
colnames(x) <- c("id", "var1", "var2", "var3")
y <-melt(x, id = "id", measure = c("var1", "var2", "var3"))

y then looks like this:

  id variable      value
1  1     var1  0.1560812
2  2     var1  1.0343844
3  3     var1 -1.4157728
4  1     var2  0.8808935
5  2     var2  0.1719239
6  3     var2  0.6723758
7  1     var3 -0.7589631
8  2     var3  1.1325995
9  3     var3 -1.5744876

now I can cast it back via acast:

> acast(y,y[,1] ~ y[,2])
        var1      var2       var3
1  0.1560812 0.8808935 -0.7589631
2  1.0343844 0.1719239  1.1325995
3 -1.4157728 0.6723758 -1.5744876

However, when writing a small wrapper for acast that should do the same, i get a stupid error messages:

wrap.acast <- function(dat, v1 = 1, v2 = 2) {
    out <- acast(dat, dat[,v1] ~ dat[,v2])
    return(out)
}

wrap.acast(y)

Error in eval(expr, envir, enclos) : object 'dat' not found

The problem is obviously related to something like environments and global/local variables. As it gives other error messages after declaring dat in the global environment (i.e., v1 and v2 not found as long as they aren’t global).

I would like to use resahpe (especially acast) within a function without the necessity of declaring the variables outside the function. What is the trick?

Thanks.

Answer 1

Instead of using the formula specification, use the character specification:

acast(y, list(names(y)[1], names(y)[2]))