Empty string

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In computer science and formal language theory, the empty string is the unique string of length zero. It is denoted with ϵ or e.

Properties

When present in a formal language, empty strings have several properties:

• $|\; epsilon\; ,\; |\; =\; 0$. The string length is zero.
• $epsilon\; ,\; +\; s\; =\; s\; +\; epsilon\; ,\; =\; s$. Under concatenation, the empty string is the identity element of the free monoid on the alphabet Σ.
• $\{\; epsilon\; ,\; \}^R\; =\; epsilon\; ,$. Reversal of the empty string produces the empty string.

These properties may hold in some programming languages, but this is left up to the particular implementation.

Representations

Programming language	ϵ representation
C, C++	{''}
Perl	"" qw()
Python	"" str()

See also

• Empty set
• Null

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Last updated on Wednesday March 12, 2008 at 17:44:56 PDT (GMT -0700)
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