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Use a named list as an index for a subset of a vector of strings

I have a long list of strings that need to be converted to numbers according to a given mapping. I put this mapping in a named list and so I can get one element, but I can't figure out how to apply it to a vector

For example:

> X <- c("a", "b", "b", "a", "c")
> M <- list(a = 11, b = 22, c = 33)
> M[["a"]]
[1] 11
> M[[X]]
Error in M[[X]] : recursive indexing failed at level 2
> sapply(X, M)
Error in get(as.character(FUN), mode = "function", envir = envir) : 
  object 'M' of mode 'function' was not found

What's the correct approach?

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2 answers


You only need to make a few small changes to your code:

  • Use a named vector instead of a named list (this is optional - a named list will work as well)
  • But more importantly, use single parentheses [

    , not double [[

    . Individual brackets match a vector, and double brackets match a single element.


Like this:

M <- c(a = 11, b = 22, c = 33)
X <- c("a", "b", "b", "a", "c")

unname(M[X])
[1] 11 22 22 11 33
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Another similar approach:



R> unlist(M[X])
 a  b  b  a  c 
11 22 22 11 33
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