From Wikipedia, the free encyclopedia Empty string
Empty string
In computer science and formal language theory, the form any operation on it, but an empty string is less
empty string (or null string [1] is the unique string of
string) likely to do so. The empty string is a legitimate string,
length zero. upon which most string operations should work. Some
languages treat some or all of the following in similar
Formal Theory ways, which can lessen the danger: empty strings, null
references, the integer 0, the floating point number 0,
Formally, a string is a finite sequence of symbols such the boolean value false, the ascii character NUL, or other
as letters or digits. The empty string is the extreme case such values.
where the sequence has length zero, so there are no sym- The empty string is usually represented similarly to
bols in the string. There is only one empty string, because other strings. In implementations with string terminat-
two strings are only different if they have different ing character (null-terminated strings or plain text
lengths or a different sequence of symbols. In formal lines), the empty string is indicated by the immediate use
treatments, the empty string is denoted with λ or some- of this terminating character.
times Λ or ε.
The empty string should not be confused with the Examples of Empty Strings
empty language ?, which is a formal language (i.e. a set The empty string is a syntactically valid representation
of strings) that contains no strings, not even the empty of zero in positional notation (in any base), which does
string. not contain leading zeros. Since the empty string does
The empty string has several properties: not have a standard visual representation outside of for-
• . The string length is zero. mal language theory, the number zero is traditionally
• . The empty string is the represented by a single decimal digit 0 instead.
identity element of the concatenation operation Zero-filled memory area, interpreted as a null-termi-
(which forms a free monoid on the alphabet Σ). nated string, is an empty string.
Empty lines of text show the empty string. This can
• . Reversal of the empty string produces
occur from two consecutive EOLs), as often occur in text
the empty string.
files, and this is sometimes used in text processing to sep-
• The empty string precedes any other string under
arate paragraphs, e.g. in MediaWiki.
lexicographical order, because it is the shortest of all
strings.[2]
See also
Use In Programming Lan- • Empty set
• Null-terminated string
guages
In most programming languages, strings are a data type. References
Individual strings are typically stored in consecutive
memory locations. This means that the same string (for [1] Kernighan and Ritchie, C, p. 38
example the empty string) could be stored in two differ- [2] CSE1002 Lecture Notes - Lexicographic
ent places in memory. (Note that even a string of length
zero can require memory to store it, depending on the
format being used.) In this way there could be multiple
empty strings in memory, in contrast with the formal
theory definition, for which there is only one possible
empty string. However, a string comparison function
would indicate that all of these empty strings are equal to
each other.
The empty string is distinct from a null reference
(or null pointer) because a null reference does not point
to any string at all, not even the empty string. A null
reference is likely to cause an error if one tries to per-
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From Wikipedia, the free encyclopedia Empty string
λ representation Programming languages
"" C, C++, Perl, Python, C#, Go, PHP, Visual Basic .NET, Java, Turing, JavaScript, Haskell, Objective
Caml, Scala, Tcl
’’ Perl, PHP, Python, JavaScript, Delphi, Pascal
{’\0’} C, C++
std::string() C++
@"" Objective-C
qw() Perl
"""""" Python
str()
string.Empty C#
String.Empty Visual Basic .NET
String.make 0 Objective Caml
’-’
{} Tcl
Retrieved from "http://en.wikipedia.org/w/index.php?title=Empty_string&oldid=467963315"
Categories:
• Formal languages
• String (computer science)
• Zero
• Formal methods stubs
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