“Less-than sign”的意思、由来-开放百科全书

词条 Less-than sign

释义

Computing
Programming language Double less-than sign Triple less-than sign Less-than sign plus equals sign Less-than sign plus Hyphen-minus Shell scripts Spaceship operator HTML
Mathematics
See also

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The less-than sign is a mathematical symbol that denotes an inequality between two values. The widely adopted form of two equal-length strokes connecting in an acute angle at the left, <, has been found in documents dated as far back as the 1560s. In typical mathematical usage, the less-than sign is typically placed between the two values being compared and signals that the first number is less than the second number. Examples of typical usage include ½ < 1 and −2 < 1. Since the development of computer programming languages, the less-than sign and the greater-than sign have been repurposed for a range of uses and operations.

Computing

The less-than sign (<) is an original ASCII character (hex 3C, decimal 60).

The less-than sign is used for an approximation of the opening angle bracket (⟨). ASCII does not have angle brackets.

Programming language

In BASIC, Lisp-family languages, and C-family languages (including Java and C++), operator < means "less than".

In Coldfusion, operator .lt. means "less than".

In Fortran, operator .LT. means "less than"; later versions allow <.

In Bourne shell, operator -lt means "less than".

Double less-than sign

The double less-than sign (<<) is used for an approximation of the much-less-than sign (≪) or of the opening guillemet («). ASCII does not have much-less-than sign.

In Bash, Perl, and Ruby, operator <<EOF (where "EOF" is an arbitrary string, but commonly "EOF" denoting "end of file") is used to denote the beginning of a here document.

In C and C++, operator << represents a binary left shift.

In the C++ Standard Library, operator <<, when applied on an output stream, acts as insertion operator and performs an output operation on the stream.

In Ruby, operator << acts as append operator when used between an array and the value to be appended.

Triple less-than sign

In PHP, operator <<<OUTPUT is used to denote the beginning of a heredoc statement (where OUTPUT is an arbitrary named variable.)

In Bash, <<<word is used as a "here string", where word is expanded and supplied to the command on its standard input, similar to a heredoc.

Less-than sign plus equals sign

The less-than sign plus the equals sign (<=) is used for an approximation of the less-than-or-equal-to sign (≤). ASCII does not have a less-than-or-equal-to sign, but Unicode defines it at code point U+2264.

In BASIC, Lisp-family languages, and C-family languages (including Java and C++), operator <= means "less than or equal to". In Sinclair BASIC it is encoded as a single-byte code point token.

In Fortran, operator .LE. means "less than or equal to".

In Bourne shell and Windows PowerShell, the operator -le means "less than or equal to".

Less-than sign plus Hyphen-minus

In the R programming language, the less-than sign is used in conjunction with a hyphen-minus to create an arrow (<-), this can be used as the left assignment operator.

Shell scripts

In Bourne shell (and many other shells), less-than sign is used to redirect input from a file. Less-than plus ampersand (<&) is used to redirect from a file descriptor.

Spaceship operator

Less-than sign is used in the spaceship operator.

HTML

In HTML (and SGML and XML), the less-than sign is used at the beginning of tags. The less-than sign may be included with <. The less-than-or-equal-to sign may be included with ≤.

Mathematics

In an inequality, the less-than sign always "points" to the smaller number. Put another way, the "jaws" (the wider section of the symbol) always direct to the larger number.