Although formal strings can have an arbitrary (but finite) length, the length of strings in real languages is often constrained to an artificial maximum. In general, there are two types of string datatypes: fixed length strings which have a fixed maximum length and which use the same amount of memory whether this maximum is reached or not, and variable length strings whose length is not arbitrarily fixed and which use varying amounts of memory depending on their actual size. Most strings in modern programming languages are variable length strings. Despite the name, even variable length strings are limited in length; although, generally, the limit depends only on the amount of memory available.
Character encoding
Historically, string datatypes allocated one byte per character, and although the exact character set varied by region, character encodings were similar enough that programmers could generally get away with ignoring this — groups of character sets used by the same system in different regions usually either had a character in the same place, or did not have it at all. These character sets were typically based on ASCII or EBCDIC.
Logographic languages such as Chinese, Japanese, and Korean (known collectively as CJK) need far more than 256 characters (the limit of a one 8-bit byte per-character encoding) for reasonable representation. The normal solutions involved keeping single-byte representations for ASCII and using two-byte representations for CJK ideographs. Use of these with existing code led to problems with matching and cutting of strings, the severity of which depended on how the character encoding was designed. Some encodings such as the EUC family guarantee that a byte value in the ASCII range will only represent that ASCII character, making the encoding safe for systems that use those characters as field separators. Other encodings such as ISO-2022 and Shift-JIS do not make such guarantees, making matching on byte codes unsafe. Another issue is that if the beginning of a string is deleted, important instructions for the decoder or information on position in a multibyte sequence may be lost. Another is that if strings are joined together (especially after having their ends truncated by code not aware of the encoding), the first string may not leave the encoder in a state suitable for dealing with the second string.
Unicode has simplified the picture somewhat. Most languages have a datatype for Unicode strings (usually UTF-16 as it was usually added before Unicode supplemental planes were introduced). Converting between Unicode and local encodings requires an understanding of the local encoding, which may be problematic for existing systems where strings of various encodings are being transmitted together with no real marking as to what encoding they are in.
Implementations
Some languages like C++ implement strings as templates that can be used with any datatype, but this is the exception, not the rule.
If an object-oriented language represents strings as objects, they are called mutable if the value can change at runtime and immutable if the value is frozen after creation. For example, Ruby has mutable strings, while Python's strings are immutable.
Other languages, most notably Prolog and Erlang, avoid implementing a string datatype, instead adopting the convention of representing strings as lists of character codes.
Representations
Representations of strings depend heavily on the choice of character repertoire and the method of character encoding. Older string implementations were designed to work with repertoire and encoding defined by ASCII, or more recent extensions like the ISO 8859 series. Modern implementations often use the extensive repertoire defined by Unicode along with a variety of complex encodings such as UTF-8 and UTF-16.
Most string implementations are very similar to variable-length arrays with the entries storing the character codes of corresponding characters. The principal difference is that, with certain encodings, a single logical character may take up more than one entry in the array. This happens for example with UTF-8, where single characters can take anywhere from one to four bytes. In these cases, the logical length of the string differs from the logical length of the array.
The length of a string can be stored implicitly by using a special terminating character; often this is the null character having value zero, a convention used and perpetuated by the popular C programming language[1]. Hence, this representation is commonly referred to as C string. The length of a string can also be stored explicitly, for example by prefixing the string with the length as a byte value — a convention used in Pascal; consequently some people call it a P-string.
In terminated strings, the terminating code is not an allowable character in any string.
The term bytestring usually indicates a general-purpose string of bytes — rather than strings of only (readable) characters, strings of bits, or such. Byte strings often imply that bytes can take any value and any data can be stored as-is, meaning that there should be no value interpreted as a termination value.
Here is an example of a null-terminated string stored in a 10-byte buffer, along with its ASCII representation:
| F | R | A | N | K | NUL | k | e | f | w |
| 46 | 52 | 41 | 4E | 4B | 00 | 6B | 66 | 66 | 77 |
The length of a string in the above example is 5 characters, but it occupies 6 bytes. Characters after the terminator do not form part of the representation; they may be either part of another string or just garbage. (Strings of this form are sometimes called ASCIZ strings, after the original assembly language directive used to declare them.)
Here is the equivalent (old style) Pascal string stored in a 10-byte buffer, along with its ASCII representation:
| length | F | R | A | N | K | k | e | f | w |
| 05 | 46 | 52 | 41 | 4E | 4B | 6B | 66 | 66 | 77 |
Both character termination and length codes limit strings: for example, C character arrays that contain Nul characters cannot be handled directly by C string library functions: strings using a length code are limited to the maximum value of the length code.
Both of these limitations can be overcome by clever programming, of course, but such workarounds are by definition not standard.
Historically, rough equivalents of the C termination method appear in both hardware and software. For example "data processing" machines like the IBM 1401 used a special word mark bit to delimit strings at the left, where the operation would start at the right. This meant that while the IBM 1401 had a seven-bit word in "reality", almost no-one ever thought to use this as a feature, and override the assignment of the seventh bit to (for example) handle ASCII codes.
It is possible to create data structures and functions that manipulate them that do not have the problems associated with character termination and can in principle overcome length code bounds. It is also possible to optimize the string represented using techniques from run length encoding (replacing repeated characters by the character value and a length) and Hamming encoding.
While these representations are common, others are possible. Using ropes makes certain string operations, such as insertions, deletions, and concatenations more efficient.
Vectors
While character strings are very common uses of strings, a string in computer science may refer generically to any vector of homogenously typed data. A string of bits or bytes, for example, may be used to represent data retrieved from a communications medium. This data may or may not be represented by a string-specific datatype, depending on the needs of the application, the desire of the programmer, and the capabilities of the programming language being used.
String processing algorithms
There are many algorithms for processing strings, each with various trade-offs. Some categories of algorithms include
- string searching algorithms for finding a given substring or pattern;
- string manipulation algorithms;
- sorting algorithms;
- regular expression algorithms; and
- parsing a string.
Advanced string algorithms often employ complex mechanisms and data structures, among them suffix trees and finite state machines.
Character string oriented languages and utilities
Character strings are such a useful datatype that several languages have been designed in order to make string processing applications easy to write. Examples include the following languages:
Many UNIX utilities perform simple string manipulations and can be used to easily program some powerful string processing algorithms. Files and finite streams may be viewed as strings.
Some Application Programming Interfaces like Multimedia Control Interface, embedded SQL or printf use strings to hold commands that will be interpreted.
Recent scripting programming languages, including Perl, Python, Ruby, and Tcl employ regular expressions to facilitate text operations.
Some languages such as Perl and Ruby support string interpolation, which permits arbitrary expressions to be evaluated and included in string literals.
Character string functions
String functions are used to manipulate a string or change or edit the contents of a string. They also are used to query information about a string. They are usually used within the context of a computer programming language.
The most basic example of a string function is the length(string) function, which returns the length of a string (not counting any terminator characters or any of the string's internal structural information) and does not modify the string. For example, length("hello world") returns 11.
There are many string functions which exist in other languages with similar or exactly the same syntax or parameters. For example in many languages the length function is usually represented as len(string). Even though string functions are very useful to a computer programmer, a computer programmer using these functions should be mindful that a string function in one language could in another language behave differently or have a similar or completely different function name, parameters, syntax, and results.
Notes
- ^ Bryant, Randal E.; David, O'Hallaron (2003), Computer Systems: A Programmer's Perspective (2003 ed.), Upper Saddle River, NJ: Pearson Education, p. 40, ISBN 0-13-034074-X, http://csapp.cs.cmu.edu/
See also
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