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<chapter id="lang-expr">
<title>Expressions</title>
<simpara></simpara>
<sect1 id="operators">
<title>Operators</title>
<simpara>
</simpara>
<sect2 id="arithmetic-operators">
<title>Arithmetic Operators</title>
<simpara>
Remember basic arithmetic from school? These work just
like those.
</simpara>
<table>
<title>Arithmetic Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>example</entry>
<entry>name</entry>
<entry>result</entry>
</row>
</thead>
<tbody>
<row>
<entry>$a + $b</entry>
<entry>Addition</entry>
<entry>Sum of $a and $b.</entry>
</row>
<row>
<entry>$a - $b</entry>
<entry>Subtraction</entry>
<entry>Remainder of $b subtracted from $a.</entry>
</row>
<row>
<entry>$a * $b</entry>
<entry>Multiplication</entry>
<entry>Product of $a and $b.</entry>
</row>
<row>
<entry>$a / $b</entry>
<entry>Division</entry>
<entry>Dividend of $a and $b.</entry>
</row>
<row>
<entry>$a % $b</entry>
<entry>Modulus</entry>
<entry>Remainder of $a divided by $b.</entry>
</row>
</tbody>
</tgroup>
</table>
<simpara>
The division operator ("/") returns an integer value (the result
of an integer division) if the two operands are integers (or
strings that get converted to integers). If either operand is a
floating-point value, floating-point division is performed.
</simpara>
<sect2 id="string-operators">
<title>String Operators</title>
<simpara>
There is only really one string operator -- the concatenation
operator (".").
</simpara>
<para>
<informalexample><programlisting>
$a = "Hello ";
$b = $a . "World!"; // now $b = "Hello World!"
</programlisting></informalexample>
</para>
<sect2 id="assignment-operators">
<title>Assignment Operators</title>
<simpara>
The basic assignment operator is "=". Your first inclination might
be to think of this as "equal to". Don't. It really means that
the the left operand gets set to the value of the expression on the
rights (that is, "gets set to").
</simpara>
<para>
The value of an assignment expression is the value assigned. That
is, the value of "$a = 3" is 3. This allows you to do some tricky
things: <informalexample><programlisting>
$a = ($b = 4) + 5; // $a is equal to 9 now, and $b has been set to 4.
</programlisting></informalexample>
</para>
<para>
In addition to the basic assignment operator, there are "combined
operators" for all of the binary arithmetic and string operators
that allow you to use a value in an expression and then set its
value to the result of that expression. For example: <informalexample><programlisting>
$a = 3;
$a += 5; // sets $a to 8, as if we had said: $a = $a + 5;
$b = "Hello ";
$b .= "There!"; // sets $b to "Hello There!", just like $b = $b . "There!";
</programlisting></informalexample>
</para>
<sect2 id="bitwise-operators">
<title>Bitwise Operators</title>
<simpara>
Bitwise operators allow you to turn specific bits within an integer
on or off.
</simpara>
<table>
<title>Bitwise Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>example</entry>
<entry>name</entry>
<entry>result</entry>
</row>
</thead>
<tbody>
<row>
<entry>$a & $b</entry>
<entry>And</entry>
<entry>Bits that are set in both $a and $b are set.</entry>
</row>
<row>
<entry>$a | $b</entry>
<entry>Or</entry>
<entry>Bits that are set in either $a or $b are set.</entry>
</row>
<row>
<entry>~ $a</entry>
<entry>Not</entry>
<entry>Bits that are set in $a are not set, and vice versa.</entry>
</row>
</tbody>
</tgroup>
</table>
<sect2 id="logical-operators">
<title>Logical Operators</title>
<table>
<title>Logical Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>example</entry>
<entry>name</entry>
<entry>result</entry>
</row>
</thead>
<tbody>
<row>
<entry>$a and $b</entry>
<entry>And</entry>
<entry>True of both $a and $b are true.</entry>
</row>
<row>
<entry>$a or $b</entry>
<entry>Or</entry>
<entry>True if either $a or $b is true.</entry>
</row>
<row>
<entry>$a xor $b</entry>
<entry>Or</entry>
<entry>True if either $a or $b is true, but not both.</entry>
</row>
<row>
<entry>! $a</entry>
<entry>Not</entry>
<entry>True if $a is not true.</entry>
</row>
<row>
<entry>$a && $b</entry>
<entry>And</entry>
<entry>True of both $a and $b are true.</entry>
</row>
<row>
<entry>$a || $b</entry>
<entry>Or</entry>
<entry>True if either $a or $b is true.</entry>
</row>
</tbody>
</tgroup>
</table>
<simpara>
The reason for the two different variations of "and" and "or"
operators is that they operate at different precedences. (See below.)
</simpara>
<sect2 id="comparison-operators">
<title>Comparison Operators</title>
<simpara>
Comparison operators, as their name imply, allow you to compare two
values.
</simpara>
<table>
<title>Comparson Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>example</entry>
<entry>name</entry>
<entry>result</entry>
</row>
</thead>
<tbody>
<row>
<entry>$a == $b</entry>
<entry>Equal</entry>
<entry>True if $a is equal to $b.</entry>
</row>
<row>
<entry>$a != $b</entry>
<entry>Not equal</entry>
<entry>True if $a is not equal to $b.</entry>
</row>
<row>
<entry>$a < $b</entry>
<entry>Less than</entry>
<entry>True if $a is strictly less than $b.</entry>
</row>
<row>
<entry>$a > $b</entry>
<entry>Greater than</entry>
<entry>True if $a is strictly greater than $b.</entry>
</row>
<row>
<entry>$a <= $b</entry>
<entry>Less than or equal to </entry>
<entry>True if $a is less than or equal to $b.</entry>
</row>
<row>
<entry>$a >= $b</entry>
<entry>Greater than or equal to </entry>
<entry>True if $a is greater than or equal to $b.</entry>
</row>
</tbody>
</tgroup>
</table>
<sect2 id="precedence">
<title>Precedence</title>
<simpara></simpara>
</chapter>
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