letter {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 guaranted to have exactly `length(i)` characters.

### See Also

`subXString`, 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)  # character vector
      BString(x[1])[i]          # BString object

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

[Package Biostrings version 2.8.18 Index]