letter              package:Biostrings              R Documentation

Subsetting a string

Description:

     Extract a substring from a string by picking up individual letters
     by their position.

Usage:

       letter(x, i)

Arguments:

       x: A character vector, or an XString, XStringViews or
          MaskedXString object. 

       i: An integer vector with no NAs. 

Details:

     Unlike with the 'substr' or 'substring' functions, 'i' must
     contain valid positions.

Value:

     A character vector of length 1 when 'x' is an XString or
     MaskedXString object (the masks are ignored for the latter).

     A character vector of the same length as 'x' when 'x' is a
     character vector or an XStringViews object.

     Note that, because 'i' must contain valid positions, all non-NA
     elements in the result are guaranteed to have exactly 'length(i)'
     characters.

See Also:

     'subseq', XString-class, XStringViews-class, MaskedXString-class

Examples:

       x <- c("abcd", "ABC")
       i <- c(3, 1, 1, 2, 1)

       ## With a character vector:
       letter(x[1], 3:1)
       letter(x, 3)
       letter(x, i)
       #letter(x, 4)             # Error!

       ## With a BString object:
       letter(BString(x[1]), i)  # returns a character vector
       BString(x[1])[i]          # returns a BString object

       ## With an XStringViews object:
       x2 <- XStringViews(x, "BString")
       letter(x2, i)

