<chapter title="Data types">

<p>In this chapter we will discuss all the different ways to store data
in Pike in detail. We have seen examples of many of these, but we haven't
really gone into how they work. In this chapter we will also see which
operators and functions work with the different types.
There are two categories of data types in Pike: <b>basic types</b>, and
<b>pointer types</b>. The difference is that basic types are copied when
assigned to a variable. With pointer types, merely the pointer is copied,
that way you get two variables pointing to the same thing.</p>

<section title="Basic types">

<p>The basic types are <tt>int</tt>, <tt>float</tt> and <tt>string</tt>.
For you who are accustomed to C or C++, it may seem odd that a string
is a basic type as opposed to an array of char, but it is surprisingly
easy to get used to.</p>

<subsection title="int">

<p><tt>Int</tt> is short for integer, or integer number. They are
normally 32 bit integers, which means that they are in the range
-2147483648 to 2147483647. (Note that on some machines an <tt>int</tt>
might be larger than 32 bits.) If Pike is compiled with bignum support
the 32 bit limitation does not apply and thus the integers can be of
arbitrary size.  Since they are integers, no decimals are allowed. An
integer constant can be written in several ways:</p>

<matrix>
<r><c><b>Pattern</b></c><c><b>Example</b></c><c><b>Description</b></c></r>
<r><c>-?[1-9][0-9]*</c><c>78</c><c>Decimal number</c></r>
<r><c>-?0[0-9]*</c><c>0116</c><c>Octal number</c></r>
<r><c>-?0[xX][0-9a-fA-F]+</c><c>0x4e</c><c>Hexadecimal number</c></r>
<r><c>-?0[bB][01]+</c><c>0b1001110</c><c>Binary number</c></r>
<r><c>-?'\\?.'</c><c>'N'</c><c>ASCII character</c></r>
</matrix>

<p>All of the above represent the number 78. Octal notation means that
each digit is worth 8 times as much as the one after. Hexadecimal notation
means that each digit is worth 16 times as much as the one after.
Hexadecimal notation uses the letters a, b, c, d, e and f to represent the
numbers 10, 11, 12, 13, 14 and 15. In binary notation every digit is worth
twice the value of the succeding digit, but only 1:s and 0:s are used. The
ASCII notation gives the ASCII value of the character between the single
quotes. In this case the character is <tt>N</tt> which just happens to be
78 in ASCII. Some characters, like special characters as newlines, can not
be placed within single quotes. The special generation sequence for those
characters, listed under strings, must be used instead. Specifically this
applies to the single quote character itself, which has to be written as
<expr>'\''</expr>.</p>

<p>When pike is compiled with bignum support integers in never
overflow or underflow when they reach the system-defined
maxint/minint. Instead they are silently converted into bignums.
Integers are usually implemented as 2-complement 32-bits integers, and
thus are limited within -2147483648 and 2147483647. This may however
vary between platforms, especially 64-bit platforms. <fixme>Conversion
back to normal integer?</fixme></p>

<p>All the arithmetic, bitwise and comparison operators can be used on
integers. Also note these functions:</p>

<dl>
<dt><tt>int <ref>intp</ref>(mixed <i>x</i>)</tt></dt>
<dd>This function returns 1 if <i>x</i> is an int, 0 otherwise.</dd>
<dt><tt>int <ref>random</ref>(int <i>x</i>)</tt></dt>
<dd>This function returns a random number greater or equal to zero and smaller than <i>x</i>.</dd>
<dt><tt>int <ref>reverse</ref>(int <i>x</i>)</tt></dt>
<dd>This function reverses the order of the bits in <i>x</i> and returns the new number. It is not very useful.</dd>
<dt><tt>int <ref>sqrt</ref>(int <i>x</i>)</tt></dt>
<dd>This computes the square root of <i>x</i>. The value is always rounded down.</dd>
</dl>
</subsection>

<subsection title="float">
<p>Although most programs only use integers, they are unpractical when doing
trigonometric calculations, transformations or anything else where you
need decimals. For this purpose you use <expr>float</expr>. Floats are
normally 32 bit floating point numbers, which means that they can represent
very large and very small numbers, but only with 9 accurate digits. To write
a floating point constant, you just put in the decimals or write it in the
exponential form:</p>

<matrix>
<r><c><b>Pattern</b></c><c><b>Example</b></c><c><b>Equals</b></c></r>
<r><c>-?[0-9]*\.[0-9]+</c><c>3.1415926</c><c>3.1415926</c></r>
<r><c>-?[0-9]+e-?[0-9]+</c><c>-5e3</c><c>-5000.0</c></r>
<r><c>-?[0-9]*\.[0-9]+e-?[0-9]+</c><c>.22e-2</c><c>0.0022</c></r>
</matrix>

<p>Of course you can have any number of decimals to increase the accuracy.
Usually digits after the ninth digit are ignored, but on some architectures
<expr>float</expr> might have higher accuracy than that. In the exponential
form, <expr>e</expr> means "times 10 to the power of", so <expr>1.0e9</expr>
is equal to "1.0 times 10 to the power of 9". <fixme>float and int is not
compatible and no implicit cast like in C++</fixme></p>

<p>All the arithmetic and comparison operators can be used on floats.
Also, these functions operates on floats:</p>

<dl>
<dt>trigonometric functions</dt>
<dd> The trigonometric functions are: <ref>sin</ref>, <ref>asin</ref>,
     <ref>cos</ref>, <ref>acos</ref>, <ref>tan</ref> and <ref>atan</ref>.
     If you do not know what these functions do you probably don't
     need them. Asin, acos and atan are of course short for
     arc sine, arc cosine and arc tangent. On a calculator they
     are often known as inverse sine, inverse cosine and
     inverse tangent.</dd>

<dt><tt>float <ref>log</ref>(float <i>x</i>)</tt></dt>
<dd>This function computes the natural logarithm of <i>x</i>,</dd>

<dt><tt>float <ref>exp</ref>(float <i>x</i>)</tt></dt>
<dd>This function computes <b>e</b> raised to the power of <i>x</i>.</dd>

<dt><tt>float <ref>pow</ref>(float|int <i>x</i>, float|int <i>y</i>)</tt></dt>
<dd>This function computes <i>x</i> raised to the power of <i>y</i>.</dd>

<dt><tt>float <ref>sqrt</ref>(float <i>x</i>)</tt></dt>
<dd>This computes the square root of <i>x</i>.</dd>

<dt><tt>float <ref>floor</ref>(float <i>x</i>)</tt></dt>
<dd>This function computes the largest integer value less than or equal
    to <i>x</i>. Note that the value is returned as a <tt>float</tt>,
    not an <tt>int</tt>.</dd>

<dt><tt>float <ref>ceil</ref>(float <i>x</i>)</tt></dt>
<dd>This function computes the smallest integer value greater than or
    equal to <i>x</i> and returns it as a <tt>float</tt>.</dd>

<dt><tt>float <ref>round</ref>(float <i>x</i>)</tt></dt>
<dd>This function computes the closest integer value to <i>x</i> 
    and returns it as a <tt>float</tt>.</dd>
</dl>
</subsection>

<subsection title="string">

<p>A <tt>string</tt> can be seen as an array of values from 0 to 2��-1.
Usually a string contains text such as a word, a sentence, a page or
even a whole book. But it can also contain parts of a binary file,
compressed data or other binary data. Strings in Pike are <b>shared</b>,
which means that identical strings share the same memory space. This
reduces memory usage very much for most applications and also speeds
up string comparisons. We have already seen how to write a constant
string:</p>

<example>
"hello world" // hello world
"he" "llo"    // hello
"\116"        // N (116 is the octal ASCII value for N)
"\t"          // A tab character
"\n"          // A newline character
"\r"          // A carriage return character
"\b"          // A backspace character
"\0"          // A null character
"\""          // A double quote character
"\\"          // A singe backslash
"\x4e"        // N (4e is the hexadecimal ASCII value for N)
"\d78"        // N (78 is the decimal ACII value for N)
"hello world\116\t\n\r\b\0\"\\" // All of the above
"\xff"        // the character 255
"\xffff"      // the character 65536
"\xffffff"    // the character 16777215
"\116""3"     // 'N' followed by a '3'
</example>

<matrix>
<r><c><b>Pattern</b></c><c><b>Example</b></c></r>
<r><c>.</c><c>N</c></r>
<r><c>\\[0-7]+</c><c>\116</c></r>
<r><c>\\x[0-9a-fA-F]+</c><c>\x4e</c></r>
<r><c>\\d[0-9]+</c><c>\d78</c></r>
<r><c>\\u[0-9a-fA-F]+ (4)</c><c>\u004E</c></r>
<r><c>\\U[0-9a-fA-F]+ (8)</c><c>\U0000004e</c></r>
</matrix>

<matrix>
<r><c><b>Sequence</b></c><c><b>ASCII code</b></c><c><b>Charcter</b></c></r>
<r><c>\a</c><c>7</c><c>An acknowledge character</c></r>
<r><c>\b</c><c>8</c><c>A backspace character</c></r>
<r><c>\t</c><c>9</c><c>A tab character</c></r>
<r><c>\n</c><c>10</c><c>A newline character</c></r>
<r><c>\v</c><c>11</c><c>A vertical tab character</c></r>
<r><c>\f</c><c>12</c><c>A form feed character</c></r>
<r><c>\r</c><c>13</c><c>A carriage return character</c></r>
<r><c>\"</c><c>34</c><c>A double quote character</c></r>
<r><c>\\</c><c>92</c><c>A backslash character</c></r>
</matrix>

<p>As you can see, any sequence of characters within double quotes is a string.
The backslash character is used to escape characters that are not allowed or
impossible to type. As you can see, <tt>\t</tt> is the sequence to produce
a tab character, <tt>\\</tt> is used when you want one backslash and
<tt>\"</tt> is used when you want a double quote (<tt>"</tt>) to be a part
of the string instead of ending it.
Also, <tt>\<i>XXX</i></tt> where <i>XXX</i> is an
octal number from 0 to 37777777777 or <tt>\x<i>XX</i></tt> where <i>XX</i>
is 0 to ffffffff lets you write any character you want in the
string, even null characters. From version 0.6.105, you may also use
<tt>\d<i>XXX</i></tt> where <i>XXX</i> is 0 to 2��-1. If you write two constant
strings after each other, they will be concatenated into one string.</p>

<p>You might be surprised to see that individual characters can have values
up to 2��-1 and wonder how much memory that use. Do not worry, Pike
automatically decides the proper amount of memory for a string, so all
strings with character values in the range 0-255 will be stored with
one byte per character. You should also beware that not all functions
can handle strings which are not stored as one byte per character, so
there are some limits to when this feature can be used.</p>

<p>Although strings are a form of arrays, they are immutable. This means that
there is no way to change an individual character within a string without
creating a new string. This may seem strange, but keep in mind that strings
are shared, so if you would change a character in the string <tt>"foo"</tt>,
you would change *all* <tt>"foo"</tt> everywhere in the program.</p>

<p>However, the Pike compiler will allow you to to write code like you could
change characters within strings, the following code is valid and works:</p>

<example>
string s="hello torld";
s[6]='w';
</example>

<p>However, you should be aware that this does in fact create a new string and
it may need to copy the string <i>s</i> to do so. This means that the above
operation can be quite slow for large strings. You have been warned.
Most of the time, you can use <ref>replace</ref>, <ref>sscanf</ref>,
<ref>`/</ref>
or some other high-level string operation to avoid having to use the above
construction too much.</p>

<p>All the comparison operators plus the operators listed here can be used on strings:</p>

<dl>
<dt> Summation</dt>
<dd> Adding strings together will simply concatenate them.
     <tt>"foo"+"bar"</tt> becomes <tt>"foobar"</tt>.</dd>
<dt> Subtraction</dt>
<dd> Subtracting one string from another will remove all occurrences
     of the second string from the first one. So 
     <tt>"foobarfoogazonk" - "foo"</tt> results in <tt>"bargazonk"</tt>.</dd>
<dt> Indexing</dt>
<dd> Indexing will let you get the ASCII value of any character in a string.
     The first index is zero.</dd>
<dt> Range</dt>
<dd> The range operator will let you copy any part of the string into a
     new string. Example: <tt>"foobar"[2..4]</tt> will return <tt>"oba"</tt>.</dd>
<dt> Division</dt>
<dd> Division will let you divide a string at every occurrence of a word or
     character. For instance if you do <tt>"foobargazonk" / "o"</tt> the
     result would be <tt>({"f","","bargaz","nk"})</tt>. It is also possible
     to divide the string into strings of length N by dividing the string
     by N. If N is converted to a float before dividing, the reminder of
     the division will be included in the result.</dd>
<dt> Multiplication</dt>
<dd> The inverse of the division operator can be accomplished by multiplying
     an array with a string. So if you evaluate
     <tt>({"f","","bargaz","nk"}) * "o"</tt> the result would be
     <tt>"foobargazonk"</tt>.</dd>
<dt> Modulo</dt>
<dd> To complement the division operator, you can do <tt>string</tt> % <tt>int</tt>.
     This operator will simply return the part of the string that was not 
     included in the array returned by <tt>string</tt> / <tt>int</tt></dd>
</dl>

<p>Also, these functions operates on strings:</p>

<dl>
<dt><tt>string <ref>String.capitalize</ref>(string <i>s</i>)</tt></dt>
<dd>Returns <i>s</i> with the first character converted to upper case.</dd>

<dt><tt>int <ref>String.count</ref>(string <i>haystack</i>, string <i>needle</i>)</tt></dt>
<dd>Returns the number of occurances of <i>needle</i> in <i>haystack</i>.
    Equvivalent to <tt><ref>sizeof</ref>(<i>haystack</i>/<i>needle</i>)-1</tt>.</dd>

<dt><tt>int <ref>String.width</ref>(string <i>s</i>)</tt></dt>
<dd>Returns the width <i>s</i> in bits (8, 16 or 32).</dd>

<dt><tt>string <ref>lower_case</ref>(string <i>s</i>)</tt></dt>
<dd>Returns <i>s</i> with all the upper case characters converted to lower case.</dd>

<dt><tt>string <ref>replace</ref>(string <i>s</i>, string <i>from</i>, string <i>to</i>)</tt></dt>
<dd>This function replaces all occurrences of the string <i>from</i>
    in <i>s</i> with <i>to</i> and returns the new string.</dd>

<dt><tt>string <ref>reverse</ref>(string <i>s</i>)</tt></dt>
<dd>This function returns a copy of <i>s</i> with the last byte from <i>s</i>
    first, the second last in second place and so on.</dd>

<dt><tt>int <ref>search</ref>(string <i>haystack</i>, string <i>needle</i>)</tt></dt>
<dd>This function finds the first occurrence of <i>needle</i> in
    <i>haystack</i> and returns where it found it.</dd>

<dt><tt>string <ref>sizeof</ref>(string <i>s</i>)</tt></dt>
<dd>Same as <tt><ref>strlen</ref>(<i>s</i>)</tt>,
returns the length of the string.</dd>

<dt><tt>int <ref>stringp</ref>(mixed <i>s</i>)</tt></dt>
<dd>This function returns 1 if <i>s</i> is a string, 0 otherwise.</dd>

<dt><tt>int <ref>strlen</ref>(string <i>s</i>)</tt></dt>
<dd>Returns the length of the string <i>s</i>.</dd>

<dt><tt>string <ref>upper_case</ref>(string <i>s</i>)</tt></dt>
<dd>This function returns <i>s</i> with all lower case characters converted
    to upper case.</dd>
</dl>
</subsection>
</section>

<section title="Pointer types">

<p>The basic types are, as the name implies, very basic. They are the foundation,
most of the pointer types are merely interesting ways to store the basic
types. The pointer types are <tt>array</tt>, <tt>mapping</tt>,
<tt>multiset</tt>, <tt>program</tt>, <tt>object</tt> and <tt>function</tt>.
They are all <b>pointers</b> which means that they point to something
in memory. This "something" is freed when there are no more pointers to it.
Assigning a variable with a value of a pointer type will not copy this
"something" instead it will only generate a new reference to it. Special care
sometimes has to be taken when giving one of these types as arguments to
a function; the function can in fact modify the "something". If this effect
is not wanted you have to explicitly copy the value. More about this will
be explained later in this chapter.</p>

<subsection title="array">

<p>Arrays are the simplest of the pointer types. An array is merely a block of
memory with a fixed size containing a number of slots which can hold any
type of value. These slots are called <b>elements</b> and are accessible
through the index operator. To write a constant array you enclose the
values you want in the array with <tt>({ })</tt> like this:</p>

<example>
({ })      // Empty array
({ 1 })    // Array containing one element of type int
({ "" })   // Array containing a string
({ "", 1, 3.0 }) // Array of three elements, each of different type
</example>

<p>As you can see, each element in the array can contain any type of value.
Indexing and ranges on arrays works just like on strings, except with
arrays you can change values inside the array with the index operator.
However, there is no way to change the size of the array, so if you want
to append values to the end you still have to add it to another array
which creates a new array. Figure 4.1 shows how the schematics of an array.
As you can see, it is a very simple memory structure.</p>

<!-- <image src=array.fig>fig 4.1 -->

<p>Operators and functions usable with arrays:</p>

<dl>
<dt> indexing ( <tt><i>arr</i> [ <i>c</i> ]</tt> )</dt>
<dd> Indexing an array retrieves or sets a given element in the array.
     The index <i>c</i> has to be an integer. To set an index, simply put
     the whole thing on the left side of an assignment, like this:
     <tt><i>arr</i> [ <i>c</i> ] = <i>new_value</i></tt></dd>

<dt> range ( <tt><i>arr</i> [ <i>from</i> .. <i>to</i> ]</tt> )</dt>
<dd> The range copies the elements <i>from</i>, <i>from</i>+1, , <i>from</i>+2 ... <i>to</i>
     into a new array. The new array will have the size <i>to</i>-<i>from</i>+1.</dd>

<dt> comparing (<tt><i>a</i> == <i>b</i></tt> and <tt><i>a</i> != <i>b</i></tt>)</dt>
<dd> The equal operator returns 1 if <i>a</i> and <i>b</i> are the <b>same</b> arrays.
     It is not enough that they have the same size and same data. They must
     be the same array. For example: <tt>({1}) == ({1})</tt> would return 0, while
     <tt>array(int) a=({1}); return a==a;</tt> would return 1. Note that you cannot
     use the operators <tt>&gt;</tt>, <tt>&gt;=</tt>, <tt>&lt;</tt> or <tt>&lt;=</tt> on arrays.</dd>

<dt> Summation (<tt><i>a</i> + <i>b</i></tt>)</dt>
<dd> As with strings, summation concatenates arrays. <tt>({1})+({2})</tt> returns <tt>({1,2})</tt>.</dd>

<dt> Subtractions (<tt><i>a</i> - <i>b</i></tt>)</dt>
<dd> Subtracting one array from another returns a copy of
    <i>a</i> with all the elements that are also present in <i>b</i> removed.
    So <tt>({1,3,8,3,2}) - ({3,1})</tt> returns <tt>({8,2})</tt>.</dd>

<dt> Intersection (<tt><i>a</i> &amp; <i>b</i></tt>)</dt>
<dd> Intersection returns an array with all values that are present in both
     <i>a</i> and <i>b</i>. The order of the elements will be the same as
     the the order of the elements in <i>a</i>. Example:
     <tt>({1,3,7,9,11,12}) &amp; ({4,11,8,9,1})</tt> will return:
     <tt>({1,9,11})</tt>.</dd>

<dt> Union (<tt><i>a</i> | <i>b</i></tt>)</dt>
<dd> Union works almost as summation, but it only adds elements not
     already present in <i>a</i>. So, <tt>({1,2,3}) | ({1,3,5})</tt> will
     return <tt>({1,2,3,5})</tt>. 
     Note: the order of the elements in <i>a</i> can be changed!</dd>

<dt> Xor (<tt><i>a</i> ^ <i>b</i></tt>)</dt>
<dd>  This is also called symmetric difference. It returns an array with all
     elements present in <i>a</i> or <i>b</i> but the element must NOT
     be present in both. Example: <tt>({1,3,5,6}) ^ ({4,5,6,7})</tt> will
     return <tt>({1,3,4,7})</tt>.</dd>

<dt> Division (<tt><i>a</i> / <i>b</i></tt>)</dt>
<dd> This will split the array <i>a</i> into an array of arrays. If <i>b</i> is
     another array, <i>a</i> will be split at each occurance of that array.
     If <i>b</i> is an integer or float, <i>a</i> will be split between
     every <i>b</i>th element. Examples: <tt>({1,2,3,4,5})/({2,3})</tt> will
     return <tt>({ ({1}), ({4,5}) })</tt> and <tt>({1,2,3,4})/2</tt> will
     return <tt>({ ({1,2}), ({3,4}) })</tt>.</dd>

<dt> Modulo (<tt><i>a</i> % <i>b</i></tt>)</dt>
<dd> This operation is valid only if <i>b</i> is an integer. It will return
     the part of the array that was not included by dividing <i>a</i> by
     <i>b</i>.</dd>

<dt><tt>array <ref>aggregate</ref>(mixed ... <i>elems</i>)</tt></dt>
<dd> This function does the same as the <tt>({ })</tt> operator; it creates an
     array from all arguments given to it. In fact, writing <tt>({1,2,3})</tt>
     is the same as writing <tt>aggregate(1,2,3)</tt>.</dd>

<dt><tt>array <ref>allocate</ref>(int <i>size</i>)</tt></dt>
<dd>This function allocates a new array of size <tt>size</tt>. All the elements
    in the new array will be zeroes.</dd>

<dt><tt>int <ref>arrayp</ref>(mixed <i>a</i>)</tt></dt>
<dd>This function returns 1 if <i>a</i> is an array, 0 otherwise.</dd>

<dt><tt>array <ref>column</ref>(array(mixed) <i>a</i>, mixed <i>ind</i>)</tt></dt>
<dd>This function goes through the array <i>a</i> and indexes every element
    in it on <i>ind</i> and builds an array of the results. So if you have
    an array <i>a</i> in which each element is a also an array. This function
    will take a cross section, by picking out element <i>ind</i> from each
    of the arrays in <i>a</i>. Example:
    <tt>column( ({ ({1,2,3}), ({4,5,6}), ({7,8,9}) }), 2)</tt> will return
    <tt>({3,6,9})</tt>.</dd>

<dt><tt>int <ref>equal</ref>(mixed <i>a</i>, mixed <i>b</i>)</tt></dt>
<dd> This function returns 1 if if <i>a</i> and <i>b</i> look the same. They
     do not have to be pointers to the same array, as long as they are the same
     size and contain equal data.</dd>

<dt><tt>array <ref>filter</ref>(array <i>a</i>, mixed <i>func</i>, mixed ... <i>args</i>)</tt></dt>
<dd><tt>filter</tt> returns every element in <i>a</i> for which
    <i>func</i> returns <b>true</b> when called with that element as
    first argument, and <i>args</i> for the second, third, etc.
    arguments. (Both <i>a</i> and <i>func</i> can be other things; see
    the reference for <tt><ref>filter</ref></tt> for
    details about that.)</dd>

<dt><tt>array <ref>map</ref>(array <i>a</i>, mixed <i>func</i>, mixed ... <i>args</i>)</tt></dt>
<dd>This function works similar to <ref>filter</ref> but returns the
    results of the function <i>func</i> instead of returning the
    elements from <i>a</i> for which <i>func</i> returns <b>true</b>.
    (Like <ref>filter</ref>, this function accepts other things for
    <i>a</i> and <i>func</i>; see the reference for <ref>map</ref>.)</dd>

<dt><tt>array <ref>replace</ref>(array <i>a</i>, mixed <i>from</i>, mixed <i>to</i>)</tt></dt>
<dd>This function will create a copy of <i>a</i> with all elements equal to
    <i>from</i> replaced by <i>to</i>.</dd>

<dt><tt>array <ref>reverse</ref>(array <i>a</i>)</tt></dt>
<dd><tt>Reverse</tt> will create a copy of <i>a</i> with the last element first,
    the last but one second, and so on.</dd>

<dt><tt>array <ref>rows</ref>(array <i>a</i>, array <i>indexes</i>)</tt></dt>
<dd>This function is similar to <ref>column</ref>. It indexes <i>a</i> with
    each element from <i>indexes</i> and returns the results in an array.
    For example: <tt>rows( ({"a","b","c"}), ({ 2,1,2,0}) ) </tt> will return
    <tt>({"c","b","c","a"})</tt>.</dd>

<dt><tt>int <ref>search</ref>(array <i>haystack</i>, mixed <i>needle</i>)</tt></dt>
<dd>This function returns the index of the first occurrence of an element
    equal (tested with <tt>==</tt>) to <i>needle</i> in the array
    <i>haystack</i>.</dd>

<dt><tt>int <ref>sizeof</ref>(mixed <i>arr</i>)</tt></dt>
<dd>This function returns the number of elements in the array <i>arr</i>.</dd>

<dt><tt>array <ref>sort</ref>(array <i>arr</i>, array ... <i>rest</i>)</tt></dt>
<dd>This function sorts <i>arr</i> in smaller-to-larger order. Numbers, floats
    and strings can be sorted. If there are any additional arguments, they
    will be permutated in the same manner as <i>arr</i>. See
    <!-- <ref to=functions> --> functions for more details.</dd>

<dt><tt>array <ref>Array.uniq</ref>(array <i>a</i>)</tt></dt>
<dd>This function returns a copy of the array <i>a</i> with all duplicate
    elements removed. Note that this function can return the elements
    in any order.</dd>
</dl>
</subsection>

<subsection title="mapping">

<p>Mappings are are really just more generic arrays. However, they are slower
and use more memory than arrays, so they cannot replace arrays completely.
What makes mappings special is that they can be indexed on other things than
integers. We can imagine that a mapping looks like this:</p>

<!-- <image src=mapping.fig>fig 4.2 -->

<p>Each index-value pair is floating around freely inside the mapping. There is
exactly one value for each index. We also have a (magical) lookup function.
This lookup function can find any index in the mapping very quickly. Now, if
the mapping is called <i>m</i> and we index it like this:
<tt><i>m</i> [ <i>i</i> ]</tt> the lookup function will quickly find the index
<i>i</i> in the mapping and return the corresponding value. If the index is 
not found, zero is returned instead. 
If we on the other hand assign an index in the mapping the value will
instead be overwritten with the new value. If the index is not found when
assigning, a new index-value pair will be added to the mapping.
Writing a constant mapping is easy:</p>

<example>
([ ])       // Empty mapping
([ 1:2 ])   // Mapping with one index-value pair, the 1 is the index
([ "one":1, "two":2 ]) // Mapping which maps words to numbers
([ 1:({2.0}), "":([]), ]) // Mapping with lots of different types
</example>

<p>As with arrays, mappings can contain any type. The main difference is that
the index can be any type too. Also note that the index-value pairs in a
mapping are not stored in a specific order. You can not refer to the
fourteenth key-index pair, since there is no way of telling which one is
the fourteenth. Because of this, you cannot use the range operator on
mappings.</p>

<p>The following operators and functions are important:</p>

<dl>
<dt> indexing ( <tt><i>m</i> [ <i>ind</i> ]</tt> )</dt>
<dd> As discussed above, indexing is used to retrieve, store and add values
     to the mapping.</dd>
<dt> addition, subtraction, union, intersection and xor</dt>
<dd> All these operators works exactly as on arrays, with the difference that
     they operate on the indices. In those cases when the value can come from
     either mapping, it will be taken from the right side of the operator.
     This makes it easier to add new values to a mapping with <tt>+=</tt>.
     Some examples:<br />
     <tt>([1:3, 3:1]) + ([2:5, 3:7])</tt> returns <tt>([1:3, 2:5, 3:7 ])</tt><br />
     <tt>([1:3, 3:1]) - ([2:5, 3:7])</tt> returns <tt>([1:3])</tt><br />
     <tt>([1:3, 3:1]) | ([2:5, 3:7])</tt> returns <tt>([1:3, 2:5, 3:7 ])</tt><br />
     <tt>([1:3, 3:1]) &amp; ([2:5, 3:7])</tt> returns <tt>([3:7])</tt><br />
     <tt>([1:3, 3:1]) ^ ([2:5, 3:7])</tt> returns <tt>([1:3, 2:5])</tt><br /></dd>

<dt> same ( <tt><i>a</i> == <i>b</i></tt> )</dt>
<dd> Returns 1 if <i>a</i> is <b>the same</b> mapping as <i>b</i>, 0 otherwise.</dd>

<dt> not same ( <tt><i>a</i> != <i>b</i></tt> )</dt>
<dd> Returns 0 if <i>a</i> is <b>the same</b> mapping as <i>b</i>, 1 otherwise.</dd>

<dt><tt>array <ref>indices</ref>(mapping <i>m</i>)</tt></dt>
<dd><tt>Indices</tt> returns an array containing all the indices in the mapping <i>m</i>.</dd>

<dt><tt>mixed <ref>m_delete</ref>(mapping <i>m</i>, mixed <i>ind</i>)</tt></dt>
<dd>This function removes the index-value pair with the index <i>ind</i> from the mapping <i>m</i>.
    It will return the value that was removed.</dd>

<dt><tt>int <ref>mappingp</ref>(mixed <i>m</i>)</tt></dt>
<dd>This function returns 1 if <i>m</i> is a mapping, 0 otherwise.</dd>

<dt><tt>mapping <ref>mkmapping</ref>(array <i>ind</i>, array <i>val</i>)</tt></dt>
<dd>This function constructs a mapping from the two arrays <i>ind</i> and
    <i>val</i>. Element 0 in <i>ind</i> and element 0 in <i>val</i> becomes
    one index-value pair. Element 1 in <i>ind</i> and element 1 in <i>val</i>
    becomes another index-value pair, and so on..</dd>

<dt><tt>mapping <ref>replace</ref>(mapping <i>m</i>, mixed <i>from</i>, mixed <i>to</i>)</tt></dt>
<dd>This function creates a copy of the mapping <i>m</i> with all values equal to
    <i>from</i> replaced by <i>to</i>.</dd>

<dt><tt>mixed <ref>search</ref>(mapping <i>m</i>, mixed <i>val</i>)</tt></dt>
<dd>This function returns the index of the 'first' index-value pair which has the value <i>val</i>.</dd>

<dt><tt>int <ref>sizeof</ref>(mapping <i>m</i>)</tt></dt>
<dd><tt>Sizeof</tt> returns how many index-value pairs there are in the mapping.</dd>

<dt><tt>array <ref>values</ref>(mapping <i>m</i>)</tt></dt>
<dd>This function does the same as <ref>indices</ref>, but returns an array with all the values instead.
    If <ref>indices</ref> and <ref>values</ref> are called on the same mapping after each other, without
    any other mapping operations in between, the returned arrays will be in the same order. They can
    in turn be used as arguments to <ref>mkmapping</ref> to rebuild the mapping <i>m</i> again.</dd>

<dt><tt>int <ref>zero_type</ref>(mixed t)</tt></dt>
<dd>When indexing a mapping and the index is not found, zero is returned. However, problems can arise
    if you have also stored zeroes in the mapping. This function allows you to see the difference between
    the two cases. If <tt>zero_type(<i>m</i> [ <i>ind</i> ])</tt> returns 1, it means that the value was
    not present in the mapping. If the value was present in the mapping, <ref>zero_type</ref> will return
    something else than 1.</dd>
</dl>
</subsection>


<subsection title="multiset">

<p>A multiset is almost the same thing as a mapping. The difference is that there
are no values:</p>

<!-- <image src=multiset.fig>fig 4.3 -->

<p>Instead, the index operator will return 1 if the value was found
in the multiset and 0 if it was not. When assigning an index to a multiset like
this: <tt><i>mset</i>[ <i>ind</i> ] = <i>val</i></tt> the index <i>ind</i>
will be added to the multiset <i>mset</i> if <i>val</i> is <b>true</b>.
Otherwise <i>ind</i> will be removed from the multiset instead.</p>

<p>Writing a constant multiset is similar to writing an array:</p>

<example>
(&lt; &gt;)      // Empty multiset
(&lt; 17 &gt;)  // Multiset with one index: 17
(&lt; "", 1, 3.0, 1 &gt;) // Multiset with four indices
</example>

<p>Note that you can actually have more than one of the same index in a multiset. This is
normally not used, but can be practical at times.</p>
</subsection>

<subsection title="program">

<p>Normally, when we say <b>program</b> we mean something we can execute from
a shell prompt. However, Pike has another meaning for the same word. In Pike
a <tt>program</tt> is the same as a <b>class</b> in C++. A <tt>program</tt>
holds a table of what functions and variables are defined in that program.
It also holds the code itself, debug information and references to other
programs in the form of inherits. A <tt>program</tt> does not hold space
to store any data however.
All the information in a <tt>program</tt> is
gathered when a file or string is run through the Pike compiler. The variable
space needed to execute the code in the program is stored in an <tt>object</tt>
which is the next data type we will discuss.</p>

<!-- <image src=program.fig>fig 4.4 -->

<p>Writing a <tt>program</tt> is easy, in fact, every example we have tried so
far has been a <tt>program</tt>. To load such a program into memory, we can
use <tt>compile_file</tt> which takes a file name, compiles the file
and returns the compiled program. It could look something like this:</p>

<example>
program p = compile_file("hello_world.pike");
</example>

<p>You can also use the <b>cast</b> operator like this:</p>

<example>
program p = (program) "hello_world";
</example>

<p>This will also load the program <tt>hello_world.pike</tt>, the only difference
is that it will cache the result so that next time you do <tt>(program)"hello_world"</tt>
you will receive the _same_ program. If you call <tt>compile_file("hello_world.pike")</tt>
repeatedly you will get a new program each time.</p>

<p>There is also a way to write programs inside programs with the help of the
<tt>class</tt> keyword:</p>

<example>
class class_name {
  inherits, variables and functions
}
</example>

<p>The <tt>class</tt> keyword can be written as a separate entity
outside of all functions, but it is also an expression which returns the
<tt>program</tt> written between the brackets. The <i>class_name</i> is
optional. If used you can later refer to that <tt>program</tt> by the name
<i>class_name</i>.
This is very similar to how classes are written in C++ and can be used
in much the same way. It can also be used to create <b>structs</b>
(or records if you program Pascal).
Let's look at an example:</p>

<example>
class record {
  string title;
  string artist;
  array(string) songs;
}

array(record) records = ({});

void add_empty_record()
{
  records+=({ record() });
}

void show_record(record rec)
{
  write("Record name: "+rec-&gt;title+"\n");
  write("Artist: "+rec-&gt;artist+"\n");
  write("Songs:\n");
  foreach(rec-&gt;songs, string song)
    write("   "+song+"\n");
}
</example>

<p>This could be a small part of a better record register program. It is not
a complete executable program in itself.  In this example we create a
<tt>program</tt> called <tt>record</tt> which has three identifiers.
In <tt>add_empty_record</tt> a new object is created
by calling <tt>record</tt>. This is called <b>cloning</b> and it
allocates space to store the variables defined in the <tt>class record</tt>.
<tt>Show_record</tt> takes one of the records created in
<tt>add_empty_record</tt> and shows the contents of it. As you can see, the arrow operator
is used to access the data allocated in <tt>add_empty_record</tt>.
If you do not understand this section I suggest you go on and read the
next section about <tt>objects</tt> and then come back and read this
section again.</p>

<dl>
<dt> cloning</dt>
<dd> To create a data area for a <tt>program</tt> you need to instantiate or
     <b>clone</b> the program. This is accomplished by using a pointer
     to the <tt>program</tt> as if it was a function and call it. That
     creates a new object and calls the function <tt>create</tt> in the
     new object with the arguments.

<!--
     It is also possible to use the
     functions <tt>new()</tt> and <tt>clone()</tt> which do exactly the
     same thing except you can use a string to specify what program you
     want to clone.
-->
</dd>

<dt> compiling</dt>
<dd> All programs are generated by compiling a string. The string may of
     course be read from a file. For this purpose there are three functions:
<expr>
program <ref>compile</ref>(string p);
program <ref>compile_file</ref>(string filename);
program <ref>compile_string</ref>(string p, string filename);
</expr>
     <ref>compile_file</ref> simply reads the file given as argument, compiles
     it and returns the resulting program. <ref>compile_string</ref> instead
     compiles whatever is in the string <i>p</i>. The second argument,
     <i>filename</i>, is only used in debug printouts when an error occurs
     in the newly made program. Both <ref>compile_file</ref> and
     <ref>compile_string</ref> call <ref>compile</ref> to actually compile
     the string after having called <ref>cpp</ref> on it.</dd>

<dt> casting</dt>
<dd> Another way of compiling files to program is to use the <b>cast</b>
     operator. Casting a string to the type <tt>program</tt> calls a function
     in the master object which will compile the program in question for you.
     The master also keeps the program in a cache, so if you later need the
     same program again it will not be re-compiled.</dd>

<dt> <tt>int <ref>programp</ref>(mixed <i>p</i>)</tt></dt>
<dd> This function returns 1 if <i>p</i> is a program, 0 otherwise.</dd>

<dt> comparisons</dt>
<dd> As with all data types <tt>==</tt> and <tt>!=</tt> can be used to
     see if two programs are the same or not.</dd>
</dl>

<p>The following operators and functions are important:</p>

<dl>
<dt> cloning ( <tt><i>p</i> ( <i>args</i> )</tt> )</dt>
<dd> Creates an object from a program. Discussed in the next section.</dd>

<dt> indexing ( <tt><i>p</i> [ <i>string</i> ]</tt>, or
                <tt><i>p</i> -> <i>identifier</i></tt> )</dt>
<dd> Retreives the value of the named constant from a program.</dd>

<dt> <tt>array(string) <ref>indices</ref>(program <i>p</i>)</tt></dt>
<dd> Returns an array with the names of all non-protected constants in the
     program.</dd>

<dt> <tt>array(mixed) <ref>values</ref>(program <i>p</i>)</tt></dt>
<dd> Returns an array with the values of all non-protected constants in the
     program.</dd>
</dl>

</subsection>


<subsection title="object">

<p>Although programs are absolutely necessary for any application you might
want to write, they are not enough. A <tt>program</tt> doesn't have anywhere
to store data, it just merely outlines how to store data. To actually store
the data you need an <tt>object</tt>. Objects are basically a chunk of memory
with a reference to the program from which it was cloned. Many objects can
be made from one program. The <tt>program</tt> outlines where in the object
different variables are stored.</p>

<!-- <image src=object.fig>fig 4.5 -->

<p>Each object has its own set of variables, and when calling a function in that
object, that function will operate on those variables. If we take a look at
the short example in the section about programs, we see that it would be
better to write it like this:</p>

<example>
class record {
  string title;
  string artist;
  array(string) songs;

  void show()
  {
    write("Record name: "+title+"\n");
    write("Artist: "+artist+"\n");
    write("Songs:\n");
    foreach(songs, string song)
      write("   "+song+"\n");
  }
}

array(record) records = ({});

void add_empty_record()
{
  records+=({ record() });
}

void show_record(object rec)
{
  rec-&gt;show();
}
</example>

<p>Here we can clearly see how the function <tt>show</tt> prints the
contents of the variables in that object. In essence, instead of accessing
the data in the object with the <tt>-&gt;</tt> operator, we call a function
in the object and have it write the information itself. This type of
programming is very flexible, since we can later change how <tt>record</tt>
stores its data, but we do not have to change anything outside of
the <tt>record</tt> program.</p>

<p>Functions and operators relevant to objects:</p>

<dl>
<dt> indexing</dt>
<dd> Objects can be indexed on strings to access identifiers. If the identifier
     is a variable, the value can also be set using indexing. If the identifier
     is a function, a pointer to that function will be returned. If the
     identifier is a constant, the value of that constant will be returned.
     Note that the <tt>-&gt;</tt> operator is actually the same as indexing.
     This means that <tt>o-&gt;foo</tt> is the same as <tt>o["foo"]</tt></dd>

<dt> cloning</dt>
<dd> As discussed in the section about programs, cloning a program is done
     by using a pointer to the program as a function and calling it.
     Whenever you clone an object, all the global variables will be
     initialized. After that the function <tt>create</tt> will be called
     with any arguments you call the program with.</dd>

<dt> <tt>void <ref>destruct</ref>(object <i>o</i>)</tt></dt>
<dd> This function invalidates all references to the object <i>o</i> and
     frees all variables in that object. This function is also called when
     <i>o</i> runs out of references. If there is a function named
     <tt>destroy</tt> in the object, it will be called before the actual
     destruction of the object.</dd>

<dt> <tt>array(string) <ref>indices</ref>(object <i>o</i>)</tt></dt>
<dd> This function returns a list of all identifiers in the object <i>o</i>.</dd>

<dt> <tt>program <ref>object_program</ref>(object <i>o</i>)</tt></dt>
<dd> This function returns the program from which <i>o</i> was cloned.</dd>

<dt> <tt>int <ref>objectp</ref>(mixed <i>o</i>)</tt></dt>
<dd> This function returns 1 if <i>o</i> is an object, 0 otherwise.
     Note that if <i>o</i> has been destructed, this function will return 0.</dd>

<dt> <tt>object <ref>this_object</ref>()</tt></dt>
<dd> This function returns the object in which the interpreter is currently
     executing.</dd>

<dt> <tt>array <ref>values</ref>(object <i>o</i>)</tt></dt>
<dd> This function returns the same as <tt>rows(o,indices(o))</tt>.
     That means it returns all the values of the identifiers in the
     object <i>o</i>.</dd>

<dt> comparing</dt>
<dd> As with all data types <tt>==</tt> and <tt>!=</tt> can be used to
     check if two objects are the same or not.</dd>
</dl>
</subsection>

<subsection title="function">

<p>When indexing an object on a string, and that string is the name of a function
in the object a <tt>function</tt> is returned. Despite its name, a
<tt>function</tt> is really a <b>function pointer</b>.</p>

<!-- <image src=function.fig>fig 4.6 -->

<p>When the function pointer is called, the interpreter sets
<ref>this_object()</ref> to the object in which the function is located and
proceeds to execute the function it points to. Also note that function pointers
can be passed around just like any other data type:</p>

<example>
int foo() { return 1; }
function bar() { return foo; }
int gazonk() { return foo(); }
int teleledningsanka() { return bar()(); }
</example>

<p>In this example, the function bar returns a pointer to the function
<tt>foo</tt>. No indexing is necessary since the function <tt>foo</tt> is
located in the same object. The function <tt>gazonk</tt> simply calls
<tt>foo</tt>. However, note that the word <tt>foo</tt> in that function
is an expression returning a function pointer that is then called. To
further illustrate this, <tt>foo</tt> has been replaced by <tt>bar()</tt>
in the function <tt>teleledningsanka</tt>.</p>

<p>For convenience, there is also a simple way to write a function inside another
function. To do this you use the <tt>lambda</tt> keyword.  The
syntax is the same as for a normal function, except you write
<tt>lambda</tt> instead of the function name:</p>

<example>
lambda ( types ) { statements }
</example>

<p>The major difference is that this is an expression that can be used inside
an other function. Example:</p>

<example>
function bar() { return lambda() { return 1; }; )
</example>

<p>This is the same as the first two lines in the previous example, the keyword
<tt>lambda</tt> allows you to write the function inside <tt>bar</tt>.</p>

<p>Note that unlike C++ and Java you can not use function overloading in Pike.
This means that you cannot have one function called 'foo' which takes an
integer argument and another function 'foo' which takes a float argument.</p>

<p>This is what you can do with a function pointer.</p>

<dl>
<dt> calling ( <i>f</i> ( mixed ... <i>args</i> ) )</dt>
<dd> As mentioned earlier, all function pointers can be called. In this example
     the function <i>f</i> is called with the arguments <i>args</i>.</dd>

<dt> <tt>string <ref>function_name</ref>(function <i>f</i>)</tt></dt>
<dd> This function returns the name of the function <i>f</i> is pointing at.</dd>

<dt> <tt>object <ref>function_object</ref>(function <i>f</i>)</tt></dt>
<dd> This function returns the object the function <i>f</i> is located in.</dd>

<dt> <tt>int <ref>functionp</ref>(mixed <i>f</i>)</tt></dt>
<dd> This function returns 1 if <i>f</i> is a <tt>function</tt>, 0 otherwise.
     If <i>f</i> is located in a destructed object, 0 is returned.</dd>

<dt> <tt>function <ref>this_function</ref>()</tt></dt>
<dd> This function returns a pointer to the function it is called from.
     This is normally only used with <b>lambda</b> functions because they
     do not have a name.</dd>
</dl>
</subsection>
</section>

<section title="Sharing data">

<p>As mentioned in the beginning of this chapter, the assignment operator
(<tt>=</tt>) does not copy anything when you use it on a pointer type.
Instead it just creates another reference to the memory object.
In most situations this does not present a problem, and it speeds up
Pike's performance. However, you must be aware of this when programming.
This can be illustrated with an example:</p>

<example>
int main(int argc, array(string) argv)
{
  array(string) tmp;
  tmp=argv;
  argv[0]="Hello world.\n";
  write(tmp[0]);
}
</example>

<p>This program will of course write <tt>Hello world.</tt></p>

<p>Sometimes you want to create a copy of a mapping, array or object. To
do so you simply call <ref>copy_value</ref> with whatever you want to copy
as argument. Copy_value is recursive, which means that if you have an
array containing arrays, copies will be made of all those arrays.</p>

<p>If you don't want to copy recursively, or you know you don't have to
copy recursively, you can use the plus operator instead. For instance,
to create a copy of an array you simply add an empty array to it, like this:
<tt>copy_of_arr = arr + ({});</tt> If you need to copy a mapping you use
an empty mapping, and for a multiset you use an empty multiset.</p>
</section>


<section title="Variables">

<p>When declaring a variable, you also have to specify what type of variable
it is. For most types, such as <tt>int</tt> and <tt>string</tt> this is
very easy. But there are much more interesting ways to declare variables
than that, let's look at a few examples:</p>

<example>
int x; // x is an integer
int|string x; // x is a string or an integer
array(string) x; // x is an array of strings
array x; // x is an array of mixed
mixed x; // x can be any type
string *x; // x is an array of strings

// x is a mapping from int to string
mapping(string:int) x;

// x implements Stdio.File
Stdio.File x;

// x implements Stdio.File
object(Stdio.File) x;

// x is a function that takes two integer
// arguments and returns a string
function(int,int:string) x;

// x is a function taking any amount of
// integer arguments and returns nothing.
function(int...:void) x;

// x is ... complicated
mapping(string:function(string|int...:mapping(string:array(string)))) x;
</example>

<p>As you can see there are some interesting ways to specify types.
Here is a list of what is possible:</p>

<dl>
<dt> <tt>mixed</tt></dt>
<dd> This means that the variable can contain any type, or the
     function return any value.</dd>

<dt> <tt>array( <i>type</i> )</tt></dt>
<dd> This means an array of elements with the type <i>type</i>.</dd>

<dt> <tt>mapping( <i>key type</i> : <i>value type</i> )</tt></dt>
<dd> This is a mapping where the keys are of type <i>key type</i> and the
      values of <i>value type</i>.</dd>

<dt> <tt>multiset ( <i>type</i> )</tt></dt>
<dd> This means a multiset containing values of the type <i>type</i>.</dd>

<dt> <tt>object ( <i>program</i> )</tt></dt>
<dd> This means an object which 'implements'  the specified program. The
     <i>program</i> can be a class, a constant, or a string.
     If the program is a string it will be casted to a program first.
     See the documentation for <tt>inherit</tt> for more information
     about this casting. The compiler will assume that any function
     or variable accessed in this object has the same type information
     as that function or variable has in <i>program</i>.</dd>

<dt> <tt><i>program</i></tt></dt>
<dd> This too means 'an object which implements <i>program</i>'.
     <i>program</i> can be a class or a constant.</dd>

<dt> <tt>function( <i>argument types</i> : <i>return type</i> )</tt></dt>
<dd> This is a function taking the specified arguments and returning
     <i>return type</i>. The <i>argument types</i> is a comma separated
     list of types that specify the arguments. The argument list can also
     end with <tt>...</tt> to signify that there can be any amount of the
     last type.</dd>

<dt> <tt><i>type1</i> | <i>type2</i></tt></dt>
<dd> This means either <i>type1</i> or <i>type2</i></dd>

<dt> <tt>void</tt></dt>
<dd> Void can only be used in certain places, if used as return type for a
     function it means that the function does not return a value. If used
     in the argument list for a function it means that that argument can
     be omitted. Example: <tt>function(int|void:void)</tt> this means a
     function that may or may not take an integer argument and does not
     return a value.</dd>
</dl>

</section>

</chapter>