`Stdlib.String`

Strings.

A string `s`

of length `n`

is an indexable and immutable sequence of `n`

bytes. For historical reasons these bytes are referred to as characters.

The semantics of string functions is defined in terms of indices and positions. These are depicted and described as follows.

positions 0 1 2 3 4 n-1 n +---+---+---+---+ +-----+ indices | 0 | 1 | 2 | 3 | ... | n-1 | +---+---+---+---+ +-----+

- An
*index*`i`

of`s`

is an integer in the range [`0`

;`n-1`

]. It represents the`i`

th byte (character) of`s`

which can be accessed using the constant time string indexing operator`s.[i]`

. - A
*position*`i`

of`s`

is an integer in the range [`0`

;`n`

]. It represents either the point at the beginning of the string, or the point between two indices, or the point at the end of the string. The`i`

th byte index is between position`i`

and`i+1`

.

Two integers `start`

and `len`

are said to define a *valid substring* of `s`

if `len >= 0`

and `start`

, `start+len`

are positions of `s`

.

**Unicode text.** Strings being arbitrary sequences of bytes, they can hold any kind of textual encoding. However the recommended encoding for storing Unicode text in OCaml strings is UTF-8. This is the encoding used by Unicode escapes in string literals. For example the string `"\u{1F42B}"`

is the UTF-8 encoding of the Unicode character U+1F42B.

**Past mutability.** Before OCaml 4.02, strings used to be modifiable in place like `Bytes.t`

mutable sequences of bytes. OCaml 4 had various compiler flags and configuration options to support the transition period from mutable to immutable strings. Those options are no longer available, and strings are now always immutable.

The labeled version of this module can be used as described in the `StdLabels`

module.

`make n c`

is a string of length `n`

with each index holding the character `c`

.

`init n f`

is a string of length `n`

with index `i`

holding the character `f i`

(called in increasing index order).

Return a new string that contains the same bytes as the given byte sequence.

Return a new byte sequence that contains the same bytes as the given string.

`get s i`

is the character at index `i`

in `s`

. This is the same as writing `s.[i]`

.

**Note.** The `Stdlib.(^)`

binary operator concatenates two strings.

`concat sep ss`

concatenates the list of strings `ss`

, inserting the separator string `sep`

between each.

`compare s0 s1`

sorts `s0`

and `s1`

in lexicographical order. `compare`

behaves like `Stdlib.compare`

on strings but may be more efficient.

`starts_with `

`~prefix s`

is `true`

if and only if `s`

starts with `prefix`

.

`ends_with `

`~suffix s`

is `true`

if and only if `s`

ends with `suffix`

.

`contains_from s start c`

is `true`

if and only if `c`

appears in `s`

after position `start`

.

`rcontains_from s stop c`

is `true`

if and only if `c`

appears in `s`

before position `stop+1`

.

`contains s c`

is `String.contains_from`

` s 0 c`

.

`sub s pos len`

is a string of length `len`

, containing the substring of `s`

that starts at position `pos`

and has length `len`

.

`split_on_char sep s`

is the list of all (possibly empty) substrings of `s`

that are delimited by the character `sep`

.

The function's result is specified by the following invariants:

- The list is not empty.
- Concatenating its elements using
`sep`

as a separator returns a string equal to the input (`concat (make 1 sep) (split_on_char sep s) = s`

). - No string in the result contains the
`sep`

character.

`map f s`

is the string resulting from applying `f`

to all the characters of `s`

in increasing order.

`mapi f s`

is like `map`

but the index of the character is also passed to `f`

.

`fold_left f x s`

computes `f (... (f (f x s.[0]) s.[1]) ...) s.[n-1]`

, where `n`

is the length of the string `s`

.

`fold_right f s x`

computes `f s.[0] (f s.[1] ( ... (f s.[n-1] x) ...))`

, where `n`

is the length of the string `s`

.

`for_all p s`

checks if all characters in `s`

satisfy the predicate `p`

.

`exists p s`

checks if at least one character of `s`

satisfies the predicate `p`

.

`trim s`

is `s`

without leading and trailing whitespace. Whitespace characters are: `' '`

, `'\x0C'`

(form feed), `'\n'`

, `'\r'`

, and `'\t'`

.

`escaped s`

is `s`

with special characters represented by escape sequences, following the lexical conventions of OCaml.

All characters outside the US-ASCII printable range [0x20;0x7E] are escaped, as well as backslash (0x2F) and double-quote (0x22).

The function `Scanf.unescaped`

is a left inverse of `escaped`

, i.e. `Scanf.unescaped (escaped s) = s`

for any string `s`

(unless `escaped s`

fails).

`uppercase_ascii s`

is `s`

with all lowercase letters translated to uppercase, using the US-ASCII character set.

`lowercase_ascii s`

is `s`

with all uppercase letters translated to lowercase, using the US-ASCII character set.

`capitalize_ascii s`

is `s`

with the first character set to uppercase, using the US-ASCII character set.

`uncapitalize_ascii s`

is `s`

with the first character set to lowercase, using the US-ASCII character set.

`iter f s`

applies function `f`

in turn to all the characters of `s`

. It is equivalent to `f s.[0]; f s.[1]; ...; f s.[length s - 1]; ()`

.

`iteri`

is like `iter`

, but the function is also given the corresponding character index.

`index_from s i c`

is the index of the first occurrence of `c`

in `s`

after position `i`

.

`index_from_opt s i c`

is the index of the first occurrence of `c`

in `s`

after position `i`

(if any).

`rindex_from s i c`

is the index of the last occurrence of `c`

in `s`

before position `i+1`

.

`rindex_from_opt s i c`

is the index of the last occurrence of `c`

in `s`

before position `i+1`

(if any).

`index s c`

is `String.index_from`

` s 0 c`

.

`index_opt s c`

is `String.index_from_opt`

` s 0 c`

.

`rindex s c`

is `String.rindex_from`

` s (length s - 1) c`

.

`rindex_opt s c`

is `String.rindex_from_opt`

` s (length s - 1) c`

.

`to_seq s`

is a sequence made of the string's characters in increasing order. In `"unsafe-string"`

mode, modifications of the string during iteration will be reflected in the sequence.

`to_seqi s`

is like `to_seq`

but also tuples the corresponding index.

`blit src src_pos dst dst_pos len`

copies `len`

bytes from the string `src`

, starting at index `src_pos`

, to byte sequence `dst`

, starting at character number `dst_pos`

.

The functions in this section binary decode integers from strings.

All following functions raise `Invalid_argument`

if the characters needed at index `i`

to decode the integer are not available.

Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are stored first. Big-endian is also known as network byte order. Native-endian encoding is either little-endian or big-endian depending on `Sys.big_endian`

.

32-bit and 64-bit integers are represented by the `int32`

and `int64`

types, which can be interpreted either as signed or unsigned numbers.

8-bit and 16-bit integers are represented by the `int`

type, which has more bits than the binary encoding. These extra bits are sign-extended (or zero-extended) for functions which decode 8-bit or 16-bit integers and represented them with `int`

values.

`let hash: t => int;`

An unseeded hash function for strings, with the same output value as `Hashtbl.hash`

. This function allows this module to be passed as argument to the functor `Hashtbl.Make`

.

`let seeded_hash: int => t => int;`

A seeded hash function for strings, with the same output value as `Hashtbl.seeded_hash`

. This function allows this module to be passed as argument to the functor `Hashtbl.MakeSeeded`

.