Everything about String Computer Science totally explained
In
computer programming and some branches of
mathematics, a
string is an ordered
sequence of
symbols. These symbols are chosen from a predetermined
set or
alphabet.
In
computer programming, a string is generally understood as a
data type storing a sequence of data values, usually bytes, in which elements usually stand for characters according to a
character encoding, which differentiates it from the more general
array data type. In this context, the terms
binary string and
byte string are used to suggest strings in which the stored data doesn't (necessarily) represent text.
A
variable declared to have a string data type usually causes storage to be allocated in memory that's capable of holding some predetermined number of symbols.
When a string appears literally in
source code, it's known as a
string literal and has a representation that denotes it as such.
Formal theory
Let Σ be an
alphabet, a
non-empty finite set. Elements of Σ are called
symbols or
characters. A
string (or
word) over Σ is any finite
sequence of characters from Σ. For example, if Σ = and 0 < 1, then the lexicographical ordering of Σ* is ε < 0 < 00 < 000 < … < 011 < 0110 < … < 01111 < … < 1 < 10 < 100 < … < 101 < … < 111 …
String operations
A number of additional operations on strings commonly occur in the formal theory. These are given in the article on
string operations.
Topology
Strings admit the following interpretation as nodes on a graph:
- Fixed length strings can be viewed as nodes on a hypercube;
- Variable length strings (of finite length) can be viewed as nodes on the k-ary tree, where k is the number of symbols in Σ;
- Infinite strings can be viewed as infinite paths on the k-ary tree.
The natural topology on the set of fixed length strings or variable length strings is the discrete topology, but the natural topology on the set of infinite strings is the
limit topology, viewing the set of infinite strings as the
inverse limit of the sets of finite strings. This is the construction used for the
p-adic numbers and some constructions of the
Cantor set, and yields the same topology.
String datatypes
A
string datatype is a
datatype modeled on the idea of a formal string. Strings are such an important and useful datatype that they're implemented in nearly every
programming language. In some languages they're 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.
String length
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 isn't 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 didn't 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 don't 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're in.
Implementations
Some languages like
C++ implement strings as
templates that can be used with any
primitive type, but this is the exception, not the rule.
If an object-oriented language represents strings as objects, they're 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. 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 isn't an allowable character in any string.
The term
bytestring usually indicates to strings of bytes — rather than bits or the wider concept of characters, which may take more space than a byte — that are
not terminated in this way, and in bytes may take any value.
Here is an example of a
null-terminated string stored in a 10-byte
buffer, along with its ASCII representation:
Further Information
Get more info on 'String Computer Science'.
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