Anatomical Position Multiple Choice Questions

Unique string of length nada

In formal language theory, the empty string, or empty word, is the unique string of length zero.

Formal theory [edit]

Formally, a string is a finite, ordered sequence of characters such as letters, digits or spaces. The empty string is the special instance where the sequence has length zero, so in that location are no symbols in the cord. In that location is only one empty string, because two strings are but unlike if they have different lengths or a different sequence of symbols. In formal treatments,^[1] the empty cord is denoted with ε or sometimes Λ or λ.

The empty cord should not be dislocated with the empty linguistic communication ∅, which is a formal language (i.eastward. a set of strings) that contains no strings, not even the empty string.

The empty string has several properties:

|ε| = 0. Its string length is zero.
ε ⋅ s = s ⋅ ε = s. The empty string is the identity element of the concatenation operation. The gear up of all strings forms a free monoid with respect to ⋅ and ε.
ε^R = ε. Reversal of the empty cord produces the empty cord.
The empty cord precedes any other string under lexicographical order, because it is the shortest of all strings.^[2]

In context-free grammars, a production rule that allows a symbol to produce the empty cord is known equally an ε-production, and the symbol is said to be "nullable".

Apply in programming languages [edit]

In most programming languages, strings are a data blazon. Strings are typically stored at distinct memory addresses (locations). Thus, the aforementioned string (for example, the empty string) may be stored in two or more places in memory.

In this way, in that location could be multiple empty strings in memory, in contrast with the formal theory definition, for which at that place is only one possible empty cord. However, a string comparing office would betoken that all of these empty strings are equal to each other.

Even a string of length zero can crave memory to store information technology, depending on the format being used. In about programming languages, the empty string is singled-out from a zero reference (or goose egg pointer) because a zilch reference points to no string at all, not even the empty string. The empty string is a legitimate cord, upon which most string operations should work. Some languages treat some or all of the following in similar ways: empty strings, null references, the integer 0, the floating point number 0, the Boolean value imitation, the ASCII character NUL, or other such values.

The empty string is ordinarily represented similarly to other strings. In implementations with string terminating character (null-terminated strings or obviously text lines), the empty string is indicated by the immediate employ of this terminating graphic symbol.

λ representation	Programming languages
`""`	C, C#, C++, Become, Haskell, Java, JavaScript, Julia, Lua, M, Objective-C (equally a C string), OCaml, Perl, PHP, Python, Ruby, Scala, Standard ML, Swift, Tcl, Visual Basic .NET
`''`	APL, Delphi, JavaScript, Lua, MATLAB, Pascal, Perl, PHP, Python, R, Ruddy, Smalltalk, SQL
`character(0)`	R^[3]
`{'\0'}`	C, C++, Objective-C (every bit a C string)
`std::string()`	C++
`""southward`	C++ (since the 2014 standard)
`@""`	Objective-C (as a constant `NSString` object)
`[NSString string]`	Objective-C (as a new `NSString` object)
`q(), qq()`	Perl
`str()`	Python
`%{}` `%()`	Ruby
`string.Empty`	C#, Visual Basic .NET
`String.make 0 '-'`	OCaml
`{}`	Tcl
`[[]]`	Lua

Examples of empty strings [edit]

This section needs expansion. You can help by adding to it. (March 2010)

The empty string is a syntactically valid representation of zero in positional notation (in whatever base), which does non contain leading zeros. Since the empty cord does not have a standard visual representation exterior of formal language theory, the number nada is traditionally represented by a unmarried decimal digit 0 instead.

Zero-filled memory expanse, interpreted as a nil-terminated string, is an empty string.

Empty lines of text prove the empty string. This tin occur from two sequent EOLs, as ofttimes occur in text files, and this is sometimes used in text processing to separate paragraphs, e.grand. in MediaWiki.

See likewise [edit]

Empty set
Zip-terminated string
Concatenation theory

References [edit]

^ Corcoran, John; Frank, William; Maloney, Michael (1974). "String theory". Journal of Symbolic Logic. 39 (4): 625–637. doi:10.2307/2272846. JSTOR 2272846.
^ CSE1002 Lecture Notes – Lexicographic
^ At that place are 2 means to create "empty strings" in R; the other is listed here as "". character(0) creates empty character vectors, which volition output 0 when counted.