File Name: string and list processing in snobol 4 .zip
In computer programming , a string is traditionally a sequence of characters , either as a literal constant or as some kind of variable. The latter may allow its elements to be mutated and the length changed, or it may be fixed after creation. A string is generally considered as a data type and is often implemented as an array data structure of bytes or words that stores a sequence of elements, typically characters, using some character encoding. String may also denote more general arrays or other sequence or list data types and structures.
Depending on the programming language and precise data type used, a variable declared to be a string may either cause storage in memory to be statically allocated for a predetermined maximum length or employ dynamic allocation to allow it to hold a variable number of elements. When a string appears literally in source code , it is known as a string literal or an anonymous string.
In formal languages , which are used in mathematical logic and theoretical computer science , a string is a finite sequence of symbols that are chosen from a set called an alphabet.
A string datatype is a datatype modeled on the idea of a formal string. Strings are such an important and useful datatype that they are implemented in nearly every programming language. In some languages they are available as primitive types and in others as composite types. The syntax of most high-level programming languages allows for a string, usually quoted in some way, to represent an instance of a string datatype; such a meta-string is called a literal or string literal.
Although formal strings can have an arbitrary 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 to be determined at compile time and which use the same amount of memory whether this maximum is needed or not, and variable-length strings , whose length is not arbitrarily fixed and which can use varying amounts of memory depending on the actual requirements at run time see Memory management.
Most strings in modern programming languages are variable-length strings. Of course, even variable-length strings are limited in length — by the size of available computer memory. The string length can be stored as a separate integer which may put another artificial limit on the length or implicitly through a termination character, usually a character value with all bits zero such as in C programming language.
See also " Null-terminated " below. String datatypes have historically allocated one byte per character, and, although the exact character set varied by region, character encodings were similar enough that programmers could often get away with ignoring this, since characters a program treated specially such as period and space and comma were in the same place in all the encodings a program would encounter.
If text in one encoding was displayed on a system using a different encoding, text was often mangled , though often somewhat readable and some computer users learned to read the mangled text. Logographic languages such as Chinese , Japanese , and Korean known collectively as CJK need far more than characters the limit of a one 8-bit byte per-character encoding for reasonable representation.
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. These encodings also were not "self-synchronizing", so that locating character boundaries required backing up to the start of a string, and pasting two strings together could result in corruption of the second string. Unicode has simplified the picture somewhat.
Most programming languages now have a datatype for Unicode strings. Unicode's preferred byte stream format UTF-8 is designed not to have the problems described above for older multibyte encodings.
UTF-8, UTF and UTF require the programmer to know that the fixed-size code units are different than the "characters", the main difficulty currently is incorrectly designed APIs that attempt to hide this difference UTF does make code points fixed-sized, but these are not "characters" due to composing codes. In other languages, such as Java and Python , the value is fixed and a new string must be created if any alteration is to be made; these are termed immutable strings some of these languages also provide another type that is mutable, such as Java and.
Strings are typically implemented as arrays of bytes, characters, or code units, in order to allow fast access to individual units or substrings—including characters when they have a fixed length. A few languages such as Haskell implement them as linked lists instead.
Some languages, such as Prolog and Erlang , avoid implementing a dedicated string datatype at all, instead adopting the convention of representing strings as lists of character codes. Representations of strings depend heavily on the choice of character repertoire and the method of character encoding. Modern implementations often use the extensive repertoire defined by Unicode along with a variety of complex encodings such as UTF-8 and UTF The term byte string 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. 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 codes UCS code points can take anywhere from one to four bytes, and single characters can take an arbitrary number of codes.
In these cases, the logical length of the string number of characters differs from the physical length of the array number of bytes in use. UTF avoids the first part of the problem. The length of a string can be stored implicitly by using a special terminating character; often this is the null character NUL , which has all bits zero, a convention used and perpetuated by the popular C programming language.
In terminated strings, the terminating code is not an allowable character in any string. Strings with length field do not have this limitation and can also store arbitrary binary data.
An example of a null-terminated string stored in a byte buffer , along with its ASCII or more modern UTF-8 representation as 8-bit hexadecimal numbers is:.
Characters after the terminator do not form part of the representation; they may be either part of other data or just garbage. Strings of this form are sometimes called ASCIZ strings , after the original assembly language directive used to declare them.
Using a special byte other than null for terminating strings has historically appeared in both hardware and software, though sometimes with a value that was also a printing character. Somewhat similar, "data processing" machines like the IBM used a special word mark bit to delimit strings at the left, where the operation would start at the right. This bit had to be clear in all other parts of the string.
This meant that, while the IBM had a seven-bit word, 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. Early microcomputer software relied upon the fact that ASCII codes do not use the high-order bit, and set it to indicate the end of a string.
It must be reset to 0 prior to output. The length of a string can also be stored explicitly, for example by prefixing the string with the length as a byte value. This convention is used in many Pascal dialects; as a consequence, some people call such a string a Pascal string or P-string.
Storing the string length as byte limits the maximum string length to To avoid such limitations, improved implementations of P-strings use , , or bit words to store the string length. When the length field covers the address space , strings are limited only by the available memory. In the latter case, the length-prefix field itself doesn't have fixed length, therefore the actual string data needs to be moved when the string grows such that the length field needs to be increased.
Many languages, including object-oriented ones, implement strings as records with an internal structure like:. However, since the implementation is usually hidden , the string must be accessed and modified through member functions.
Both character termination and length codes limit strings: For example, C character arrays that contain null 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. 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 [ clarification needed ].
While these representations are common, others are possible. Using ropes makes certain string operations, such as insertions, deletions, and concatenations more efficient.
The core data structure in a text editor is the one that manages the string sequence of characters that represents the current state of the file being edited. While that state could be stored in a single long consecutive array of characters, a typical text editor instead uses an alternative representation as its sequence data structure—a gap buffer , a linked list of lines, a piece table , or a rope —which makes certain string operations, such as insertions, deletions, and undoing previous edits, more efficient.
The differing memory layout and storage requirements of strings can affect the security of the program accessing the string data. String representations requiring a terminating character are commonly susceptible to buffer overflow problems if the terminating character is not present, caused by a coding error or an attacker deliberately altering the data. String representations adopting a separate length field are also susceptible if the length can be manipulated.
In such cases, program code accessing the string data requires bounds checking to ensure that it does not inadvertently access or change data outside of the string memory limits.
String data is frequently obtained from user input to a program. As such, it is the responsibility of the program to validate the string to ensure that it represents the expected format. Performing limited or no validation of user input can cause a program to be vulnerable to code injection attacks. Sometimes, strings need to be embedded inside a text file that is both human-readable and intended for consumption by a machine.
This is needed in, for example, source code of programming languages, or in configuration files. In this case, the NUL character doesn't work well as a terminator since it is normally invisible non-printable and is difficult to input via a keyboard. Storing the string length would also be inconvenient as manual computation and tracking of the length is tedious and error-prone.
While character strings are very common uses of strings, a string in computer science may refer generically to any sequence of homogeneously typed data. A bit string or byte string , for example, may be used to represent non-textual binary 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.
If the programming language's string implementation is not 8-bit clean , data corruption may ensue. C programmers draw a sharp distinction between a "string", aka a "string of characters", which by definition is always null terminated, vs. Using C string handling functions on such a "byte string" often seems to work, but later leads to security problems.
There are many algorithms for processing strings, each with various trade-offs. Competing algorithms can be analyzed with respect to run time, storage requirements, and so forth. Advanced string algorithms often employ complex mechanisms and data structures, among them suffix trees and finite-state machines. The name stringology was coined in by computer scientist Zvi Galil for the issue of algorithms and data structures used for string processing.
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. Recent scripting programming languages , including Perl, Python , Ruby, and Tcl employ regular expressions to facilitate text operations.
Perl is particularly noted for its regular expression use,  and many other languages and applications implement Perl compatible regular expressions. Some languages such as Perl and Ruby support string interpolation , which permits arbitrary expressions to be evaluated and included in string literals. String functions are used to create strings or change the contents of a mutable string.
They also are used to query information about a string. The set of functions and their names varies depending on the computer programming language. The most basic example of a string function is the string length function — the function that 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.
This function is often named length or len.
Thousands of programming languages were invented in the first 50 years of the age of computing. Many of them were similar, and many followed a traditional, evolutionary path from their predecessors. But some revolutionary languages had a slant that differentiated them from their more general-purpose brethren. LISP was for list processing. And APL was for mathematics, with an emphasis on array processing. Iverson in as a mathematical notation, not as a computer programming language.
Farber , Ralph E. Griswold and Ivan P. SNOBOL4 stands apart from most programming languages of its era by having patterns as a first-class data type i.
Written in English. Strings and Pattern Matching 2 String Searching a paragraph, a book, etc. Strings are defined as an array of characters.
There are two fundamental facts about programming languages: there are lots of them; all but a handful are never used beyond the immediate circle of friends of the inventor. Sammet's latest annual survey lists languages currently in use in the United States, and this can only be a minor fraction of those that have been constructed at one time or another. Report bugs here.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. Clinton L. Jeffery, Chair String scanning and pattern matching is used in diverse applications such as bioinformatics, natural language processing and web applications. However most mainstream languages do not provide facilities for string processing other than regular expressions. Save to Library. Create Alert.
The language and its implementation are described in and . External functions can be statically linked poor man's loading into thesnobol4 executableon ALL platforms. Listings are directed to standard output or file specified by the -l command line option.
On anything after , these all refer to the same final version of the language. There were also a handful of extensions and implementations. Snocone is a language preprocessor that provides syntactic sugar to the language, making it easier to use. Because of this, it has a relatively unique feature: patterns are considered first-class data types. This allows patterns themselves to be manipulated, just like any other data structure. Additionally, strings can be treated as code and evaluated. This allows for recursive use of patterns and highly complex string processing and analysis.
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