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rindexingletters

How to get the index of a letter from a list A-Z?

I want to get the index of a particular letter, e.g. 

>  match(LETTERS,"G")
 [1] NA NA NA NA NA NA  1 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

Gives me that the letter exists, but I want it to return 6 in this case for the 6th element of the list.

Solution

  • Or which:

    which(LETTERS=="G")

    The which function is designed specifically for this purpose:

    Give the 'TRUE' indices of a logical object, allowing for array indices.

    The which function can also return the index for logical TRUE values in a matrix by setting the arr.ind argument to TRUE (this is very useful).

    > which(matrix(LETTERS, nrow=5)=="G")
[1] 7
> which(matrix(LETTERS, nrow=5)=="G", arr.ind=TRUE)
     row col
[1,]   2   2

    You may also want to read this recent blog post from Seth Falcon where he talks about optimizing it in C.