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 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 <- as(BStringSet(x), "Views")
    letter(x2, i)

[Package Biostrings version 2.60.2 Index]