XString-class {Biostrings} | R Documentation

## BString objects

### Description

The BString class is a general container for storing a big string (a long sequence of characters) and for making its manipulation easy and efficient.

The DNAString, RNAString and AAString classes are similar containers but with the more biology-oriented purpose of storing a DNA sequence (DNAString), an RNA sequence (RNAString), or a sequence of amino acids (AAString).

All those containers derive directly (and with no additional slots) from the XString virtual class.

### Details

The 2 main differences between an XString object and a standard character vector are: (1) the data stored in an XString object are not copied on object duplication and (2) an XString object can only store a single string (see the XStringSet container for an efficient way to store a big collection of strings in a single object).

Unlike the DNAString, RNAString and AAString containers that accept only a predefined set of letters (the alphabet), a BString object can be used for storing any single string based on a single-byte character set.

### Constructor-like functions and generics

In the code snippet below, `x` can be a single string (character vector of length 1) or an XString object.

  `BString(x="", start=1, nchar=NA)`: Tries to convert `x` into a BString object by reading `nchar` letters starting at position `start` in `x`.

### Accessor methods

In the code snippets below, `x` is an XString object.

  `alphabet(x)`: `NULL` for a `BString` object. See the corresponding man pages when `x` is a DNAString, RNAString or AAString object.

  `length(x)` or `nchar(x)`: Get the length of an XString object, i.e., its number of letters.

### Coercion

In the code snippets below, `x` is an XString object.

  `as.character(x)`: Converts `x` to a character string.

  `toString(x)`: Equivalent to `as.character(x)`.

### Subsetting

In the code snippets below, `x` is an XString object.

  `x[i]`: Return a new XString object made of the selected letters (subscript `i` must be an NA-free numeric vector specifying the positions of the letters to select). The returned object belongs to the same class as `x`.

  Note that, unlike `subseq`, `x[i]` does copy the sequence data and therefore will be very inefficient for extracting a big number of letters (e.g. when `i` contains millions of positions).

### Equality

In the code snippets below, `e1` and `e2` are XString objects.

  `e1 == e2`: `TRUE` if `e1` is equal to `e2`. `FALSE` otherwise.

  Comparison between two XString objects of different base types (e.g. a BString object and a DNAString object) is not supported with one exception: a DNAString object and an RNAString object can be compared (see RNAString-class for more details about this).

  Comparison between a BString object and a character string is also supported (see examples below).

  `e1 != e2`: Equivalent to `!(e1 == e2)`.

### Author(s)

H. Pagès

### See Also

`subseq`, `letter`, DNAString-class, RNAString-class, AAString-class, XStringSet-class, XStringViews-class, `reverseComplement`, `compact`, XVector-class

### Examples

    b <- BString("I am a BString object")
    b
    length(b)

    ## Extracting a linear subsequence:
    subseq(b)
    subseq(b, start=3)
    subseq(b, start=-3)
    subseq(b, end=-3)
    subseq(b, end=-3, width=5)

    ## Subsetting:
    b2 <- b[length(b):1]       # better done with reverse(b)

    as.character(b2)

    b2 == b                    # FALSE
    b2 == as.character(b2)     # TRUE

    ## b[1:length(b)] is equal but not identical to b!
    b == b[1:length(b)]        # TRUE
    identical(b, 1:length(b))  # FALSE
    ## This is because subsetting an XString object with [ makes a copy
    ## of part or all its sequence data. Hence, for the resulting object,
    ## the internal slot containing the memory address of the sequence
    ## data differs from the original. This is enough for identical() to
    ## see the 2 objects as different.

    ## Compacting. As a particular type of XVector objects, XString
    ## objects can optionally be compacted. Compacting is done typically
    ## before serialization. See ?compact for more information.

[Package Biostrings version 2.56.0 Index]