[R] list multiplied by a factor / mapply

[David Winsemius]

On Feb 23, 2011, at 4:11 AM, Patrick Hausmann wrote:

> Dear list,
>
> this works fine:
>
> x <- split(iris, iris$Species)
> x1 <- lapply(x, function(L) transform(L, g = L[,1:4] * 3))
>
> but I would like to multiply each Species with another factor:

You will probably confuse yourself by not adopting R terminology. A  
"factor" on R is an object with discrete categories and is not  
something that is subject to multiplication.

> setosa by 2, versicolor by 3 and virginica by 4. I've tried mapply  
> but without success.

Furthermore in this case you must mean something less general than  
"multiply"-ing every vector in each of those levels of "x" since one  
of the vectors is an R "factor".

This is not a clean solution since there are in each of the levels  
that Species factor which when multiplied returns "NA", but the rest  
of the vectors survive the process with the intended ratio to the  
original:

res <- sapply(2:4, function(z) mapply("*", x[z-1] , MoreArgs=list(z)))

You could make this a bit clean by not including Species in the levels  
of the split:

x <- split(iris[1:4], iris$Species)

Then the above would be free of those NA values, but it would be  
stripped of the identifying labels. You could also use mapply and get  
an index-able matrix each of whose values in the new "x" is a vector:

res <- mapply("*", x, 2:4)

 > res["Sepal.Length", "setosa"]
[[1]]
  [1] 10.2  9.8  9.4  9.2 10.0 10.8  9.2 10.0  8.8  9.8 10.8  9.6   
9.6  8.6 11.6 11.4 10.8 10.2 11.4
[20] 10.2 10.8 10.2  9.2 10.2  9.6 10.0 10.0 10.4 10.4  9.4  9.6 10.8  
10.4 11.0  9.8 10.0 11.0  9.8
[39]  8.8 10.2 10.0  9.0  8.8 10.0 10.2  9.6 10.2  9.2 10.6 10.0


>
> Any thoughts? Thanks for any idea!
> Patrick
>
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David Winsemius, MD
West Hartford, CT