Map fixed length string to an integer

Question

suppose I like to map any combination of string of length k into integers (if count only 26 latters uppers and lowers I will have (26+26) ^k combination. Is there a fast way in r to map, given string of length k into unique integer?

For example: 1. Say for k=1 and c("a","d","z"), then the result will be c(1,4,26). 2. Say for k=2 and c("aa","da","zA"), then the result will be c(53,157,1379).

If you had 16 letters your question would be equivalent to how to transform hexadecimal numbers to decimal numbers. Your base is 52, but the same algorithms can be used. — Roland
– Roland, Commented Nov 13, 2014 at 14:03
I have a string of length k, when k is large (say 10), I need a fast method to map it to an integer. I don't know in advance the string to map, only the fixed length of k. — Alon Hazan
– Alon Hazan, Commented Nov 13, 2014 at 14:09
For a start you should study this: en.wikipedia.org/wiki/Hexavigesimal — Roland
– Roland, Commented Nov 13, 2014 at 14:16
How can I treat a string as a number in say 52 base? Do you know a function that tack string and convert it to number in some base? What about the other symbols like "?",":"," ","!" and so on? — Alon Hazan
– Alon Hazan, Commented Nov 13, 2014 at 14:17

Roland · Accepted Answer · 2014-11-13 18:14:05Z

3

conv <- function(strings, base = letters) {
  s <- strsplit(strings, "") 
  vapply(s, function(x, base) {
    x <- match(x, base)
    as.integer(t(x) %*% (length(base)^(rev(seq_along(x)) - 1L)))
  }, 1L, base=base)
}

#test it
strtoi("5A3", base=16)
#[1] 1443
conv("5A3", base=c(1:9, LETTERS[1:7]))
#[1] 1443

mystring <- c("a", "d", "z", "aa","da","zA")
conv(mystring, base = c(letters, LETTERS))
#[1]    1    4   26   53  209 1379

Other symbols can be included easily in the base parameter.

edited Nov 13, 2014 at 18:14

answered Nov 13, 2014 at 16:35

Roland

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