Strings¶

The following documentation shows functions and methods used to manipulate and process Chapel strings.

Methods Available in Other Modules¶

Besides the functions below, some other modules provide routines that are useful for working with strings. The IO module provides IO.string.format which creates a string that is the result of formatting. It also includes functions for reading and writing strings. The Regexp module also provides some routines for searching within strings.

Casts from String to a Numeric Type¶

This module supports casts from string to numeric types. Such casts will convert the string to the numeric type and throw an error if the string is invalid. For example:

var number = "a":int;

throws an error when it is executed, but

var number = "1":int;

stores the value 1 in number.

To learn more about handling these errors, see the Error Handling technical note.

Unicode Support¶

Chapel strings use the UTF-8 encoding. Note that ASCII strings are a simple subset of UTF-8 strings, because every ASCII character is a UTF-8 character with the same meaning.

UTF-8 strings might not work properly if a UTF-8 environment is not used. See character set environment for more information.

Non-Unicode Data and Chapel Strings¶

For doing string operations on non-Unicode or arbitrary data, consider using bytes instead of string. However, there may be cases where string must be used with non-Unicode data. Examples of this are file system and path operations on systems where UTF-8 file names are not enforced.

In such scenarios, non-UTF-8 data can be escaped and stored in a string in a way that it can be restored when needed. For example:

var myBytes = b"Illegal \xff sequence";  // \xff is non UTF-8
var myEscapedString = myBytes.decode(policy=decodePolicy.escape);

will escape the illegal 0xFF byte and store it in the string. The escaping strategy is similar to Python’s “surrogate escapes” and is as follows.

Each individual byte in an illegal sequence is bitwise-or’ed with 0xDC00 to create a 2-byte codepoint.

Then, this codepoint is encoded in UTF-8 and stored in the string buffer.

This strategy typically results in storing 3 bytes for each byte in the illegal sequence. Similarly escaped strings can also be created with createStringWithNewBuffer using a C buffer.

An escaped data sequence can be reconstructed with encode:

var reconstructedBytes = myEscapedString.encode(policy=encodePolicy.unescape);
writeln(myBytes == reconstructedBytes);  // prints true

Alternatively, escaped sequence can be used as-is without reconstructing the bytes:

var escapedBytes = myEscapedString.encode(policy=encodePolicy.pass);
writeln(myBytes == escapedBytes);  // prints false

Note

Strings that contain escaped sequences cannot be directly used with unformatted I/O functions such as writeln. Formatted I/O can be used to print such strings with binary formatters such as %|s.

Note

The standard FileSystem, Path and IO modules can use strings described above for paths and file names.

Lengths and Offsets in Unicode Strings¶

For Unicode strings, and in particular UTF-8 strings, there are several possible units for offsets or lengths:

bytes

codepoints

graphemes

Most methods on the Chapel string type currently work with codepoint units by default. For example, size returns the length in codepoints and int values passed into this are offsets in codepoint units.

It is possible to indicate byte or codepoint units for indexing in the string methods by using arguments of type byteIndex or codepointIndex respectively.

For speed of indexing with their result values, find() and rfind() return a byteIndex.

Note

Support for grapheme units is not implemented at this time.

record byteIndex¶

A value of type byteIndex can be passed to certain string functions to indicate that the function should operate with units of bytes. See this.

An int can be added to a byteIndex, producing another byteIndex. One byteIndex can be subtracted from another, producing an int distance between them. A byteIndex can also be compared with another byteIndex or with an int .

To create or modify a byteIndex, cast or assign it from an int. For example, the following function returns a string containing only the second byte of the argument:

proc getSecondByte(arg:string) : int {
  var offsetInBytes = 2:byteIndex;
  return arg[offsetInBytes];
}

proc init(i: int)¶

proc init=(other: byteIndex)¶

proc init=(i: int)

proc writeThis(f) throws¶

record codepointIndex¶

A value of type codepointIndex can be passed to certain string functions to indicate that the function should operate with units of codepoints. See this.

An int can be added to a codepointIndex, producing another codepointIndex. One codepointIndex can be subtracted from another, producing an int distance between them. A codepointIndex can also be compared with another codepointIndex or with an int .

To create or modify a codepointIndex, cast or assign it from an int. For example, the following function returns a string containing only the second codepoint of the argument:

proc getSecondCodepoint(arg:string) : int {
  var offsetInCodepoints = 2:codepointIndex;
  return arg[offsetInCodepoints];
}

proc init(i: int)¶

proc init=(i: int)¶

proc init=(cpi: codepointIndex)

proc writeThis(f) throws¶

proc createStringWithBorrowedBuffer(x: string)¶

Creates a new string which borrows the internal buffer of another string. If the buffer is freed before the string returned from this function, accessing it is undefined behavior.

Arguments:	x : string – Object to borrow the buffer from
Returns:	A new string

proc createStringWithBorrowedBuffer(x: c_string, length = x.size) throws

Creates a new string which borrows the internal buffer of a c_string. If the buffer is freed before the string returned from this function, accessing it is undefined behavior.

Arguments:	x : c_string – Object to borrow the buffer from length : int – Length of the string stored in x in bytes, excluding the terminating null byte.
Throws:	DecodeError if x contains non-UTF-8 characters.
Returns:	A new string

proc createStringWithBorrowedBuffer(x: bufferType, length: int, size: int) throws

Creates a new string which borrows the memory allocated for a c_ptr(uint(8)). If the buffer is freed before the string returned from this function, accessing it is undefined behavior.

Arguments:	x : bufferType (i.e. c_ptr(uint(8))) – Object to borrow the buffer from length : int – Length of the string stored in x in bytes, excluding the terminating null byte. size – Size of memory allocated for x in bytes
Throws:	DecodeError if x contains non-UTF-8 characters.
Returns:	A new string

proc createStringWithOwnedBuffer(x: c_string, length = x.size) throws¶

Creates a new string which takes ownership of the internal buffer of a c_string. The buffer will be freed when the string is deinitialized.

Arguments:	x : c_string – Object to take ownership of the buffer from length – Length of the string stored in x in bytes, excluding the terminating null byte.
Returns:	A new string

proc createStringWithOwnedBuffer(x: bufferType, length: int, size: int) throws

Creates a new string which takes ownership of the memory allocated for a c_ptr(uint(8)). The buffer will be freed when the string is deinitialized.

Arguments:	x : bufferType (i.e. c_ptr(uint(8))) – Object to take ownership of the buffer from length : int – Length of the string stored in x in bytes, excluding the terminating null byte. size – Size of memory allocated for x in bytes
Throws:	DecodeError if x contains non-UTF-8 characters.
Returns:	A new string

proc createStringWithNewBuffer(x: string)¶

Creates a new string by creating a copy of the buffer of another string.

Arguments:	x : string – Object to copy the buffer from
Returns:	A new string

proc createStringWithNewBuffer(x: c_string, length = x.size, policy = decodePolicy.strict) throws

Creates a new string by creating a copy of the buffer of a c_string.

Arguments:	x : c_string – Object to copy the buffer from length : int – Length of the string stored in x in bytes, excluding the terminating null byte. policy – decodePolicy.strict raises an error decodePolicy.replace replaces the malformed character with UTF-8 replacement character decodePolicy.drop drops the data silently decodePolicy.escape escapes each illegal byte with private use codepoints
Throws:	DecodeError if decodePolicy.strict is passed to the policy argument and x contains non-UTF-8 characters.
Returns:	A new string

proc createStringWithNewBuffer(x: bufferType, length: int, size = length+1, policy = decodePolicy.strict) throws

Creates a new string by creating a copy of a buffer.

Arguments:	x : bufferType (i.e. c_ptr(uint(8))) – The buffer to copy length : int – Length of the string stored in x in bytes, excluding the terminating null byte. size – Size of memory allocated for x in bytes. This argument is ignored by this function.
Throws:	DecodeError if x contains non-UTF-8 characters.
Returns:	A new string

record string¶

proc init=(s: string)¶

proc init=(cs: c_string)

proc length¶: Deprecated - please use string.size.

proc size¶

Returns:	The number of codepoints in the string.

proc indices¶

Returns:	The indices that can be used to index into the string (i.e., the range `1..this.size`)

proc numBytes¶

Returns:	The number of bytes in the string.

proc numCodepoints¶

Returns:	The number of codepoints in the string, assuming the string is correctly-encoded UTF-8.

proc localize(): string¶

Gets a version of the string that is on the currently executing locale.

Returns:	A shallow copy if the `string` is already on the current locale, otherwise a deep copy is performed.

proc c_str(): c_string¶

Get a c_string from a string.

Warning

This can only be called safely on a string whose home is the current locale. This property can be enforced by calling string.localize() before c_str(). If the string is remote, the program will halt.

For example:

var my_string = "Hello!";
on different_locale {
  printf("%s", my_string.localize().c_str());
}

Returns:	A c_string that points to the underlying buffer used by this `string`. The returned c_string is only valid when used on the same locale as the string.

proc encode(policy = encodePolicy.pass): bytes¶

Returns a bytes from the given string. If the string contains some escaped non-UTF8 bytes, policy argument determines the action.

Arguments:	policy – encodePolicy.pass directly copies the (potentially escaped) data, encodePolicy.unescape recovers the escaped bytes back.
Returns:	`bytes`

iter items(): string¶

Iterates over the string character by character.

For example:

var str = "abcd";
for c in str {
  writeln(c);
}

Output:

a
b
c
d

iter these(): string¶

Iterates over the string character by character, yielding 1-codepoint strings. (A synonym for items)

For example:

var str = "abcd";
for c in str {
  writeln(c);
}

Output:

a
b
c
d

iter chpl_bytes(): byteType¶: Iterates over the string byte by byte.

iter codepoints(): int(32)¶: Iterates over the string Unicode character by Unicode character.

proc toByte(): uint(8)¶

Returns:	The value of a single-byte string as an integer.

proc byte(i: int): uint(8)¶

Returns:	The value of the i th byte as an integer.

proc toCodepoint(): int(32)¶

Returns:	The value of a single-codepoint string as an integer.

proc codepoint(i: int): int(32)¶

Returns:	The value of the i th multibyte character as an integer.

proc this(i: byteIndex): string¶

Return the codepoint starting at the i th byte in the string

Returns:	A string with the complete multibyte character starting at the specified byte index from `1..string.numBytes`

proc this(i: codepointIndex): string

Return the i th codepoint in the string. (A synonym for item)

Returns:	A string with the complete multibyte character starting at the specified codepoint index from `1..string.numCodepoints`

proc this(i: int): string

Return the i th codepoint in the string. (A synonym for item)

Returns:	A string with the complete multibyte character starting at the specified codepoint index from `1..string.numCodepoints`

proc item(i: codepointIndex): string¶

Return the i th codepoint in the string

Returns:	A string with the complete multibyte character starting at the specified codepoint index from `1..string.numCodepoints`

proc item(i: int): string

Return the i th codepoint in the string

Returns:	A string with the complete multibyte character starting at the specified codepoint index from `1..string.numCodepoints`

proc this(r: range(?)): string

proc isEmpty(): bool¶

Returns:	true – when the string is empty false – otherwise

proc startsWith(needles: string ...): bool¶

Arguments:	needles – A varargs list of strings to match against.
Returns:	true – when the string begins with one or more of the needles false – otherwise

proc endsWith(needles: string ...): bool¶

Arguments:	needles – A varargs list of strings to match against.
Returns:	true – when the string ends with one or more of the needles false – otherwise

proc find(needle: string, region: range(?) = 1: byteIndex..): byteIndex¶

proc rfind(needle: string, region: range(?) = 1: byteIndex..): byteIndex¶

Arguments:	needle – the string to search for region – an optional range defining the substring to search within, default is the whole string. Halts if the range is not within `1..string.size`
Returns:	the index of the first occurrence from the right of needle within a string, or 0 if the needle is not in the string.

proc count(needle: string, region: range(?) = 1..): int¶

Arguments:	needle – the string to search for region – an optional range defining the substring to search within, default is the whole string. Halts if the range is not within `1..string.size`
Returns:	the number of times needle occurs in the string

proc replace(needle: string, replacement: string, count: int = -1): string¶

iter split(sep: string, maxsplit: int = -1, ignoreEmpty: bool = false)¶

iter split(maxsplit: int = -1)

proc join(const ref x: string ...): string¶

Returns a new string, which is the concatenation of all of the strings passed in with the receiving string inserted between them.

var x = "|".join("a","10","d");
writeln(x); // prints: "a|10|d"

proc join(const ref x): string

Same as the varargs version, but with a homogeneous tuple of strings.

var x = "|".join("a","10","d");
writeln(x); // prints: "a|10|d"

proc join(const ref S: [] string): string

Same as the varargs version, but with all the strings in an array.

var x = "|".join(["a","10","d"]);
writeln(x); // prints: "a|10|d"

proc strip(chars: string = " trn", leading = true, trailing = true): string¶

Arguments:	chars – A string containing each character to remove. Defaults to ” \t\r\n”. leading – Indicates if leading occurrences should be removed. Defaults to true. trailing – Indicates if trailing occurrences should be removed. Defaults to true.
Returns:	A new string with leading and/or trailing occurrences of characters in chars removed as appropriate.

proc partition(sep: string): 3*(string)¶: Splits the string on sep into a 3*string consisting of the section before sep, sep, and the section after sep. If sep is not found, the tuple will contain the whole string, and then two empty strings.

proc isUpper(): bool¶

Checks if all the characters in the string are either uppercase (A-Z) or uncased (not a letter).

returns:

true – if the string contains at least one uppercase

character and no lowercase characters, ignoring uncased characters.

false – otherwise

proc isLower(): bool¶

Checks if all the characters in the string are either lowercase (a-z) or uncased (not a letter).

returns:

true – when there are no uppercase characters in the string.

false – otherwise

proc isSpace(): bool¶

Checks if all the characters in the string are whitespace (‘ ‘, ‘t’, ‘n’, ‘v’, ‘f’, ‘r’).

returns:

true – when all the characters are whitespace.

false – otherwise

proc isAlpha(): bool¶

Checks if all the characters in the string are alphabetic (a-zA-Z).

returns:

true – when the characters are alphabetic.

false – otherwise

proc isDigit(): bool¶

Checks if all the characters in the string are digits (0-9).

returns:

true – when the characters are digits.

false – otherwise

proc isAlnum(): bool¶

Checks if all the characters in the string are alphanumeric (a-zA-Z0-9).

returns:

true – when the characters are alphanumeric.

false – otherwise

proc isPrintable(): bool¶

Checks if all the characters in the string are printable.

returns:

true – when the characters are printable.

false – otherwise

proc isTitle(): bool¶

Checks if all uppercase characters are preceded by uncased characters, and if all lowercase characters are preceded by cased characters.

Returns:	true – when the condition described above is met. false – otherwise

proc toLower(): string¶

Returns:	A new string with all uppercase characters replaced with their lowercase counterpart.

Note

The case change operation is not currently performed on characters whose cases take different number of bytes to represent in Unicode mapping.

proc toUpper(): string¶

Returns:	A new string with all lowercase characters replaced with their uppercase counterpart.

Note

The case change operation is not currently performed on characters whose cases take different number of bytes to represent in Unicode mapping.

proc toTitle(): string¶

Returns:	A new string with all cased characters following an uncased character converted to uppercase, and all cased characters following another cased character converted to lowercase.

Note

The case change operation is not currently performed on characters whose cases take different number of bytes to represent in Unicode mapping.

proc =(ref lhs: byteIndex, rhs: int)¶: Copies the int rhs into the byteIndex lhs.

proc =(ref lhs: codepointIndex, rhs: int): Copies the int rhs into the codepointIndex lhs.

proc =(ref lhs: string, rhs: string): Copies the string rhs into the string lhs.

proc =(ref lhs: string, rhs_c: c_string)

Copies the c_string rhs_c into the string lhs.

Halts if lhs is a remote string.

proc +(s0: string, s1: string)¶

Returns:	A new string which is the result of concatenating s0 and s1

proc *(s: string, n: integral)¶

Returns:	A new string which is the result of repeating s n times. If n is less than or equal to 0, an empty string is returned.

For example:

writeln("Hello! " * 3);

Results in:

Hello! Hello! Hello!

proc +=(ref lhs: string, const ref rhs: string): void¶: Appends the string rhs to the string lhs.

proc codepointToString(i: int(32))¶

Returns:	A string storing the complete multibyte character sequence that corresponds to the codepoint value i.