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                        <main class="v-doc">
                            <h1 class="main">V Types</h1>
<h2 id="primitive-types">Primitive types <a href="v-types.html#primitive-types" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">bool

string

i8    i16  int  i64      i128 (soon)
u8    u16  u32  u64      u128 (soon)

rune // represents a Unicode code point

f32 f64

isize, usize // platform-dependent, the size is how many bytes it takes to reference any location in memory

voidptr // this one is mostly used for [C interoperability](#v-and-c)
</div>
<blockquote class="tip">
<p>[!NOTE]
Unlike C and Go, <code>int</code> is always a 32 bit integer.</p>
</blockquote>
<p>There is an exception to the rule that all operators
in V must have values of the same type on both sides. A small primitive type
on one side can be automatically promoted if it fits
completely into the data range of the type on the other side.
These are the allowed possibilities:</p>
<div class="language-v">   i8 → i16 → int → i64
                  ↘     ↘
                    f32 → f64
                  ↗     ↗
   u8 → u16 → u32 → u64 ⬎
      ↘     ↘     ↘      ptr
   i8 → i16 → int → i64 ⬏
</div>
<p>An <code>int</code> value for example can be automatically promoted to <code>f64</code>
or <code>i64</code> but not to <code>u32</code>. (<code>u32</code> would mean loss of the sign for
negative values).
Promotion from <code>int</code> to <code>f32</code>, however, is currently done automatically
(but can lead to precision loss for large values).</p>
<p>Literals like <code>123</code> or <code>4.56</code> are treated in a special way. They do
not lead to type promotions, however they default to <code>int</code> and <code>f64</code>
respectively, when their type has to be decided:</p>
<div class="language-v">u := u16(12)
v := 13 + u    // v is of type `u16` - no promotion
x := f32(45.6)
y := x + 3.14  // y is of type `f32` - no promotion
a := 75        // a is of type `int` - default for int literal
b := 14.7      // b is of type `f64` - default for float literal
c := u + a     // c is of type `int` - automatic promotion of `u`'s value
d := b + x     // d is of type `f64` - automatic promotion of `x`'s value
</div>
<h2 id="strings">Strings <a href="v-types.html#strings" class="header-anchor" aria-hidden="true">#</a></h2>
<p>In V, strings are encoded in UTF-8, and are immutable (read-only) by default:</p>
<div class="language-v">s := 'hello 🌎' // the `world` emoji takes 4 bytes, and string length is reported in bytes
assert s.len == 10

arr := s.bytes() // convert `string` to `[]u8`
assert arr.len == 10

s2 := arr.bytestr() // convert `[]u8` to `string`
assert s2 == s

name := 'Bob'
assert name.len == 3
// indexing gives a byte, u8(66) == `B`
assert name[0] == u8(66)
// slicing gives a string 'ob'
assert name[1..3] == 'ob'

// escape codes
// escape special characters like in C
windows_newline := '\r\n'
assert windows_newline.len == 2

// arbitrary bytes can be directly specified using `\x##` notation where `#` is
// a hex digit
aardvark_str := '\x61ardvark'
assert aardvark_str == 'aardvark'
assert '\xc0'[0] == u8(0xc0)

// or using octal escape `\###` notation where `#` is an octal digit
aardvark_str2 := '\141ardvark'
assert aardvark_str2 == 'aardvark'

// Unicode can be specified directly as `\u####` where # is a hex digit
// and will be converted internally to its UTF-8 representation
star_str := '\u2605' // ★
assert star_str == '★'
// UTF-8 can be specified this way too, as individual bytes.
assert star_str == '\xe2\x98\x85'
</div>
<p>Since strings are immutable, you cannot directly change characters in a string:</p>
<div class="language-v">mut s := 'hello 🌎'
s[0] = `H` // not allowed
</div>
<blockquote class="tip">
<p>error: cannot assign to <code>s[i]</code> since V strings are immutable</p>
</blockquote>
<p>Note that indexing a string normally will produce a <code>u8</code> (byte), not a <code>rune</code> nor another <code>string</code>.
Indexes correspond to <em>bytes</em> in the string, not Unicode code points.
If you want to convert the <code>u8</code> to a <code>string</code>, use the <code>.ascii_str()</code> method on the <code>u8</code>:</p>
<div class="language-v">country := 'Netherlands'
println(country[0]) // Output: 78
println(country[0].ascii_str()) // Output: N
</div>
<p>However, you can easily get the runes for a string with the <code>runes()</code> method, which will return an
array of the UTF-8 characters from the string.  You can then index this array.  Just be aware that
there may be fewer indexes available on the <code>rune</code> array than on the bytes in the string, if there
<em>are</em> any non-ASCII characters.</p>
<div class="language-v">mut s := 'hello 🌎'
// there are 10 bytes in the string (as shown earlier), but only 7 runes, since the `world` emoji
// only counts as one `rune` (one Unicode character)
assert s.runes().len == 7
println(s.runes()[6])
</div>
<p>If you want the code point from a specific <code>string</code> index or other more advanced UTF-8 processing
and conversions, refer to the
<a href="https://modules.vlang.io/encoding.utf8.html">vlib/encoding/utf8</a> module.</p>
<p>Both single and double quotes can be used to denote strings. For consistency, <code>vfmt</code> converts double
quotes to single quotes unless the string contains a single quote character.</p>
<p>Prepend <code>r</code> for raw strings. Escapes are not handled, so you will get exacly what you type:</p>
<div class="language-v">s := r'hello\nworld' // the `\n` will be preserved as two characters
println(s) // &quot;hello\nworld&quot;
</div>
<p>Strings can be easily converted to integers:</p>
<div class="language-v">s := '42'
n := s.int() // 42

// all int literals are supported
assert '0xc3'.int() == 195
assert '0o10'.int() == 8
assert '0b1111_0000_1010'.int() == 3850
assert '-0b1111_0000_1010'.int() == -3850
</div>
<p>For more advanced <code>string</code> processing and conversions, refer to the
<a href="https://modules.vlang.io/strconv.html">vlib/strconv</a> module.</p>
<h3 id="string-interpolation">String interpolation <a href="v-types.html#string-interpolation" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Basic interpolation syntax is pretty simple - use <code>${</code> before a variable name and <code>}</code> after. The
variable will be converted to a string and embedded into the literal:</p>
<div class="language-v">name := 'Bob'
println('Hello, ${name}!') // Hello, Bob!
</div>
<p>It also works with fields: <code>'age = ${user.age}'</code>. You may also use more complex expressions:
<code>'can register = ${user.age &gt; 13}'</code>.</p>
<p>Format specifiers similar to those in C's <code>printf()</code> are also supported. <code>f</code>, <code>g</code>, <code>x</code>, <code>o</code>, <code>b</code>,
etc. are optional and specify the output format. The compiler takes care of the storage size, so
there is no <code>hd</code> or <code>llu</code>.</p>
<p>To use a format specifier, follow this pattern:</p>
<p><code>${varname:[flags][width][.precision][type]}</code></p>
<ul>
<li><p>flags: may be zero or more of the following: <code>-</code> to left-align output within the field, <code>0</code> to use
<code>0</code> as the padding character instead of the default <code>space</code> character.</p>
<blockquote class="tip">
<p><strong>Note</strong></p>
<p>V does not currently support the use of <code>'</code> or <code>#</code> as format flags, and V supports but
doesn't need <code>+</code> to right-align since that's the default.</p>
</blockquote>
</li>
<li><p>width: may be an integer value describing the minimum width of total field to output.</p>
</li>
<li><p>precision: an integer value preceded by a <code>.</code> will guarantee that many digits after the decimal
point without any insignificant trailing zeros. If displaying insignificant zero's is desired,
append a <code>f</code> specifier to the precision value (see examples below). Applies only to float
variables and is ignored for integer variables.</p>
</li>
<li><p>type: <code>f</code> and <code>F</code> specify the input is a float and should be rendered as such, <code>e</code> and <code>E</code> specify
the input is a float and should be rendered as an exponent (partially broken), <code>g</code> and <code>G</code> specify
the input is a float--the renderer will use floating point notation for small values and exponent
notation for large values, <code>d</code> specifies the input is an integer and should be rendered in base-10
digits, <code>x</code> and <code>X</code> require an integer and will render it as hexadecimal digits, <code>o</code> requires an
integer and will render it as octal digits, <code>b</code> requires an integer and will render it as binary
digits, <code>s</code> requires a string (almost never used).</p>
<blockquote class="tip">
<p><strong>Note</strong></p>
<p>When a numeric type can render alphabetic characters, such as hex strings or special values
like <code>infinity</code>, the lowercase version of the type forces lowercase alphabetics and the
uppercase version forces uppercase alphabetics.</p>
</blockquote>
<blockquote class="tip">
<p><strong>Note</strong></p>
<p>In most cases, it's best to leave the format type empty. Floats will be rendered by
default as <code>g</code>, integers will be rendered by default as <code>d</code>, and <code>s</code> is almost always redundant.
There are only three cases where specifying a type is recommended:</p>
</blockquote>
</li>
<li><p>format strings are parsed at compile time, so specifying a type can help detect errors then</p>
</li>
<li><p>format strings default to using lowercase letters for hex digits and the <code>e</code> in exponents. Use a
uppercase type to force the use of uppercase hex digits and an uppercase <code>E</code> in exponents.</p>
</li>
<li><p>format strings are the most convenient way to get hex, binary or octal strings from an integer.</p>
</li>
</ul>
<p>See
<a href="https://en.wikipedia.org/wiki/Printf_format_string#Format_placeholder_specification">Format Placeholder Specification</a>
for more information.</p>
<div class="language-v">x := 123.4567
println('[${x:.2}]') // round to two decimal places =&gt; [123.46]
println('[${x:10}]') // right-align with spaces on the left =&gt; [   123.457]
println('[${int(x):-10}]') // left-align with spaces on the right =&gt; [123       ]
println('[${int(x):010}]') // pad with zeros on the left =&gt; [0000000123]
println('[${int(x):b}]') // output as binary =&gt; [1111011]
println('[${int(x):o}]') // output as octal =&gt; [173]
println('[${int(x):X}]') // output as uppercase hex =&gt; [7B]

println('[${10.0000:.2}]') // remove insignificant 0s at the end =&gt; [10]
println('[${10.0000:.2f}]') // do show the 0s at the end, even though they do not change the number =&gt; [10.00]
</div>
<p>V also has <code>r</code> and <code>R</code> switches, which will repeat the string the specified amount of times.</p>
<div class="language-v">println('[${'abc':3r}]') // [abcabcabc]
println('[${'abc':3R}]') // [ABCABCABC]
</div>
<h3 id="string-operators">String operators <a href="v-types.html#string-operators" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">name := 'Bob'
bobby := name + 'by' // + is used to concatenate strings
println(bobby) // &quot;Bobby&quot;
mut s := 'hello '
s += 'world' // `+=` is used to append to a string
println(s) // &quot;hello world&quot;
</div>
<p>All operators in V must have values of the same type on both sides. You cannot concatenate an
integer to a string:</p>
<div class="language-v">age := 10
println('age = ' + age) // not allowed
</div>
<blockquote class="tip">
<p>error: infix expr: cannot use <code>int</code> (right expression) as <code>string</code></p>
</blockquote>
<p>We have to either convert <code>age</code> to a <code>string</code>:</p>
<div class="language-v">age := 11
println('age = ' + age.str())
</div>
<p>or use string interpolation (preferred):</p>
<div class="language-v">age := 12
println('age = ${age}')
</div>
<p>See all methods of <a href="https://modules.vlang.io/index.html#string">string</a>
and related modules <a href="https://modules.vlang.io/strings.html">strings</a>,
<a href="https://modules.vlang.io/strconv.html">strconv</a>.</p>
<h2 id="runes">Runes <a href="v-types.html#runes" class="header-anchor" aria-hidden="true">#</a></h2>
<p>A <code>rune</code> represents a single UTF-32 encoded Unicode character and is an alias for <code>u32</code>.
To denote them, use <code>`</code> (backticks) :</p>
<div class="language-v">rocket := `🚀`
</div>
<p>A <code>rune</code> can be converted to a UTF-8 string by using the <code>.str()</code> method.</p>
<div class="language-v">rocket := `🚀`
assert rocket.str() == '🚀'
</div>
<p>A <code>rune</code> can be converted to UTF-8 bytes by using the <code>.bytes()</code> method.</p>
<div class="language-v">rocket := `🚀`
assert rocket.bytes() == [u8(0xf0), 0x9f, 0x9a, 0x80]
</div>
<p>Hex, Unicode, and Octal escape sequences also work in a <code>rune</code> literal:</p>
<div class="language-v">assert `\x61` == `a`
assert `\141` == `a`
assert `\u0061` == `a`

// multibyte literals work too
assert `\u2605` == `★`
assert `\u2605`.bytes() == [u8(0xe2), 0x98, 0x85]
assert `\xe2\x98\x85`.bytes() == [u8(0xe2), 0x98, 0x85]
assert `\342\230\205`.bytes() == [u8(0xe2), 0x98, 0x85]
</div>
<p>Note that <code>rune</code> literals use the same escape syntax as strings, but they can only hold one unicode
character. Therefore, if your code does not specify a single Unicode character, you will receive an
error at compile time.</p>
<p>Also remember that strings are indexed as bytes, not runes, so beware:</p>
<div class="language-v">rocket_string := '🚀'
assert rocket_string[0] != `🚀`
assert 'aloha!'[0] == `a`
</div>
<p>A string can be converted to runes by the <code>.runes()</code> method.</p>
<div class="language-v">hello := 'Hello World 👋'
hello_runes := hello.runes() // [`H`, `e`, `l`, `l`, `o`, ` `, `W`, `o`, `r`, `l`, `d`, ` `, `👋`]
assert hello_runes.string() == hello
</div>
<h2 id="numbers">Numbers <a href="v-types.html#numbers" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">a := 123
</div>
<p>This will assign the value of 123 to <code>a</code>. By default <code>a</code> will have the
type <code>int</code>.</p>
<p>You can also use hexadecimal, binary or octal notation for integer literals:</p>
<div class="language-v">a := 0x7B
b := 0b01111011
c := 0o173
</div>
<p>All of these will be assigned the same value, 123. They will all have type
<code>int</code>, no matter what notation you used.</p>
<p>V also supports writing numbers with <code>_</code> as separator:</p>
<div class="language-v">num := 1_000_000 // same as 1000000
three := 0b0_11 // same as 0b11
float_num := 3_122.55 // same as 3122.55
hexa := 0xF_F // same as 255
oct := 0o17_3 // same as 0o173
</div>
<p>If you want a different type of integer, you can use casting:</p>
<div class="language-v">a := i64(123)
b := u8(42)
c := i16(12345)
</div>
<p>Assigning floating point numbers works the same way:</p>
<div class="language-v">f := 1.0
f1 := f64(3.14)
f2 := f32(3.14)
</div>
<p>If you do not specify the type explicitly, by default float literals
will have the type of <code>f64</code>.</p>
<p>Float literals can also be declared as a power of ten:</p>
<div class="language-v">f0 := 42e1 // 420
f1 := 123e-2 // 1.23
f2 := 456e+2 // 45600
</div>
<h2 id="arrays">Arrays <a href="v-types.html#arrays" class="header-anchor" aria-hidden="true">#</a></h2>
<p>An array is a collection of data elements of the same type. An array literal is a
list of expressions surrounded by square brackets. An individual element can be
accessed using an <em>index</em> expression. Indexes start from <code>0</code>:</p>
<div class="language-v">mut nums := [1, 2, 3]
println(nums) // `[1, 2, 3]`
println(nums[0]) // `1`
println(nums[1]) // `2`

nums[1] = 5
println(nums) // `[1, 5, 3]`
</div>
<p><a id='array-operations'></a></p>
<p>An element can be appended to the end of an array using the push operator <code>&lt;&lt;</code>.
It can also append an entire array.</p>
<div class="language-v">mut nums := [1, 2, 3]
nums &lt;&lt; 4
println(nums) // &quot;[1, 2, 3, 4]&quot;

// append array
nums &lt;&lt; [5, 6, 7]
println(nums) // &quot;[1, 2, 3, 4, 5, 6, 7]&quot;
</div>
<div class="language-v">mut names := ['John']
names &lt;&lt; 'Peter'
names &lt;&lt; 'Sam'
// names &lt;&lt; 10  &lt;-- This will not compile. `names` is an array of strings.
</div>
<p><code>val in array</code> returns true if the array contains <code>val</code>. See <a href="statements-&-expressions.html#in-operator"><code>in</code> operator</a>.</p>
<div class="language-v">names := ['John', 'Peter', 'Sam']
println('Alex' in names) // &quot;false&quot;
</div>
<h3 id="array-fields">Array Fields <a href="v-types.html#array-fields" class="header-anchor" aria-hidden="true">#</a></h3>
<p>There are two fields that control the &quot;size&quot; of an array:</p>
<ul>
<li><code>len</code>: <em>length</em> - the number of pre-allocated and initialized elements in the array</li>
<li><code>cap</code>: <em>capacity</em> - the amount of memory space which has been reserved for elements,
but not initialized or counted as elements. The array can grow up to this size without
being reallocated. Usually, V takes care of this field automatically but there are
cases where the user may want to do manual optimizations (see <a href="v-types.html#array-initialization">below</a>).</li>
</ul>
<div class="language-v">mut nums := [1, 2, 3]
println(nums.len) // &quot;3&quot;
println(nums.cap) // &quot;3&quot; or greater
nums = [] // The array is now empty
println(nums.len) // &quot;0&quot;
</div>
<p><code>data</code> is a field (of type <code>voidptr</code>) with the address of the first
element. This is for low-level <a href="memory-unsafe-code.html"><code>unsafe</code></a> code.</p>
<blockquote class="tip">
<p>[!NOTE]
Fields are read-only and can't be modified by the user.</p>
</blockquote>
<h3 id="array-initialization">Array Initialization <a href="v-types.html#array-initialization" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The type of an array is determined by the first element:</p>
<ul>
<li><code>[1, 2, 3]</code> is an array of ints (<code>[]int</code>).</li>
<li><code>['a', 'b']</code> is an array of strings (<code>[]string</code>).</li>
</ul>
<p>The user can explicitly specify the type for the first element: <code>[u8(16), 32, 64, 128]</code>.
V arrays are homogeneous (all elements must have the same type).
This means that code like <code>[1, 'a']</code> will not compile.</p>
<p>The above syntax is fine for a small number of known elements but for very large or empty
arrays there is a second initialization syntax:</p>
<div class="language-v">mut a := []int{len: 10000, cap: 30000, init: 3}
</div>
<p>This creates an array of 10000 <code>int</code> elements that are all initialized with <code>3</code>. Memory
space is reserved for 30000 elements. The parameters <code>len</code>, <code>cap</code> and <code>init</code> are optional;
<code>len</code> defaults to <code>0</code> and <code>init</code> to the default initialization of the element type (<code>0</code>
for numerical type, <code>''</code> for <code>string</code>, etc). The run time system makes sure that the
capacity is not smaller than <code>len</code> (even if a smaller value is specified explicitly):</p>
<div class="language-v">arr := []int{len: 5, init: -1}
// `arr == [-1, -1, -1, -1, -1]`, arr.cap == 5

// Declare an empty array:
users := []int{}
</div>
<p>Setting the capacity improves performance of pushing elements to the array
as reallocations can be avoided:</p>
<div class="language-v">mut numbers := []int{cap: 1000}
println(numbers.len) // 0
// Now appending elements won't reallocate
for i in 0 .. 1000 {
    numbers &lt;&lt; i
}
</div>
<blockquote class="tip">
<p>[!NOTE]
The above code uses a <a href="statements-&-expressions.html#range-for">range <code>for</code></a> statement.</p>
</blockquote>
<p>You can initialize the array by accessing the <code>index</code> variable which gives
the index as shown here:</p>
<div class="language-v">count := []int{len: 4, init: index}
assert count == [0, 1, 2, 3]

mut square := []int{len: 6, init: index * index}
// square == [0, 1, 4, 9, 16, 25]
</div>
<h3 id="array-types">Array Types <a href="v-types.html#array-types" class="header-anchor" aria-hidden="true">#</a></h3>
<p>An array can be of these types:</p>
<table>
<thead>
<tr>
<th>Types</th>
<th>Example Definition</th>
</tr>
</thead>
<tbody>
<tr>
<td>Number</td>
<td><code>[]int,[]i64</code></td>
</tr>
<tr>
<td>String</td>
<td><code>[]string</code></td>
</tr>
<tr>
<td>Rune</td>
<td><code>[]rune</code></td>
</tr>
<tr>
<td>Boolean</td>
<td><code>[]bool</code></td>
</tr>
<tr>
<td>Array</td>
<td><code>[][]int</code></td>
</tr>
<tr>
<td>Struct</td>
<td><code>[]MyStructName</code></td>
</tr>
<tr>
<td>Channel</td>
<td><code>[]chan f64</code></td>
</tr>
<tr>
<td>Function</td>
<td><code>[]MyFunctionType</code> <code>[]fn (int) bool</code></td>
</tr>
<tr>
<td>Interface</td>
<td><code>[]MyInterfaceName</code></td>
</tr>
<tr>
<td>Sum Type</td>
<td><code>[]MySumTypeName</code></td>
</tr>
<tr>
<td>Generic Type</td>
<td><code>[]T</code></td>
</tr>
<tr>
<td>Map</td>
<td><code>[]map[string]f64</code></td>
</tr>
<tr>
<td>Enum</td>
<td><code>[]MyEnumType</code></td>
</tr>
<tr>
<td>Alias</td>
<td><code>[]MyAliasTypeName</code></td>
</tr>
<tr>
<td>Thread</td>
<td><code>[]thread int</code></td>
</tr>
<tr>
<td>Reference</td>
<td><code>[]&amp;f64</code></td>
</tr>
<tr>
<td>Shared</td>
<td><code>[]shared MyStructType</code></td>
</tr>
<tr>
<td>Option</td>
<td><code>[]?f64</code></td>
</tr>
</tbody>
</table>
<p><strong>Example Code:</strong></p>
<p>This example uses <a href="structs.html">Structs</a> and <a href="type-declarations.html#sum-types">Sum Types</a> to create an array
which can handle different types (e.g. Points, Lines) of data elements.</p>
<div class="language-v">struct Point {
    x int
    y int
}

struct Line {
    p1 Point
    p2 Point
}

type ObjectSumType = Line | Point

mut object_list := []ObjectSumType{}
object_list &lt;&lt; Point{1, 1}
object_list &lt;&lt; Line{
    p1: Point{3, 3}
    p2: Point{4, 4}
}
dump(object_list)
/*
object_list: [ObjectSumType(Point{
    x: 1
    y: 1
}), ObjectSumType(Line{
    p1: Point{
        x: 3
        y: 3
    }
    p2: Point{
        x: 4
        y: 4
    }
})]
*/
</div>
<h3 id="multidimensional-arrays">Multidimensional Arrays <a href="v-types.html#multidimensional-arrays" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Arrays can have more than one dimension.</p>
<p>2d array example:</p>
<div class="language-v">mut a := [][]int{len: 2, init: []int{len: 3}}
a[0][1] = 2
println(a) // [[0, 2, 0], [0, 0, 0]]
</div>
<p>3d array example:</p>
<div class="language-v">mut a := [][][]int{len: 2, init: [][]int{len: 3, init: []int{len: 2}}}
a[0][1][1] = 2
println(a) // [[[0, 0], [0, 2], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
</div>
<h3 id="array-methods">Array methods <a href="v-types.html#array-methods" class="header-anchor" aria-hidden="true">#</a></h3>
<p>All arrays can be easily printed with <code>println(arr)</code> and converted to a string
with <code>s := arr.str()</code>.</p>
<p>Copying the data from the array is done with <code>.clone()</code>:</p>
<div class="language-v">nums := [1, 2, 3]
nums_copy := nums.clone()
</div>
<p>Arrays can be efficiently filtered and mapped with the <code>.filter()</code> and
<code>.map()</code> methods:</p>
<div class="language-v">nums := [1, 2, 3, 4, 5, 6]
even := nums.filter(it % 2 == 0)
println(even) // [2, 4, 6]
// filter can accept anonymous functions
even_fn := nums.filter(fn (x int) bool {
    return x % 2 == 0
})
println(even_fn)
</div>
<div class="language-v">words := ['hello', 'world']
upper := words.map(it.to_upper())
println(upper) // ['HELLO', 'WORLD']
// map can also accept anonymous functions
upper_fn := words.map(fn (w string) string {
    return w.to_upper()
})
println(upper_fn) // ['HELLO', 'WORLD']
</div>
<p><code>it</code> is a builtin variable which refers to the element currently being
processed in filter/map methods.</p>
<p>Additionally, <code>.any()</code> and <code>.all()</code> can be used to conveniently test
for elements that satisfy a condition.</p>
<div class="language-v">nums := [1, 2, 3]
println(nums.any(it == 2)) // true
println(nums.all(it &gt;= 2)) // false
</div>
<p>There are further built-in methods for arrays:</p>
<ul>
<li><code>a.repeat(n)</code> concatenates the array elements <code>n</code> times</li>
<li><code>a.insert(i, val)</code> inserts a new element <code>val</code> at index <code>i</code> and
shifts all following elements to the right</li>
<li><code>a.insert(i, [3, 4, 5])</code> inserts several elements</li>
<li><code>a.prepend(val)</code> inserts a value at the beginning, equivalent to <code>a.insert(0, val)</code></li>
<li><code>a.prepend(arr)</code> inserts elements of array <code>arr</code> at the beginning</li>
<li><code>a.trim(new_len)</code> truncates the length (if <code>new_length &lt; a.len</code>, otherwise does nothing)</li>
<li><code>a.clear()</code> empties the array without changing <code>cap</code> (equivalent to <code>a.trim(0)</code>)</li>
<li><code>a.delete_many(start, size)</code> removes <code>size</code> consecutive elements from index <code>start</code>
– triggers reallocation</li>
<li><code>a.delete(index)</code> equivalent to <code>a.delete_many(index, 1)</code></li>
<li><code>a.delete_last()</code> removes the last element</li>
<li><code>a.first()</code> equivalent to <code>a[0]</code></li>
<li><code>a.last()</code> equivalent to <code>a[a.len - 1]</code></li>
<li><code>a.pop()</code> removes the last element and returns it</li>
<li><code>a.reverse()</code> makes a new array with the elements of <code>a</code> in reverse order</li>
<li><code>a.reverse_in_place()</code> reverses the order of elements in <code>a</code></li>
<li><code>a.join(joiner)</code> concatenates an array of strings into one string
using <code>joiner</code> string as a separator</li>
</ul>
<p>See all methods of <a href="https://modules.vlang.io/index.html#array">array</a></p>
<p>See also <a href="https://modules.vlang.io/arrays.html">vlib/arrays</a>.</p>
<h4 id="sorting-arrays">Sorting Arrays <a href="v-types.html#sorting-arrays" class="header-anchor" aria-hidden="true">#</a></h4>
<p>Sorting arrays of all kinds is very simple and intuitive. Special variables <code>a</code> and <code>b</code>
are used when providing a custom sorting condition.</p>
<div class="language-v">mut numbers := [1, 3, 2]
numbers.sort() // 1, 2, 3
numbers.sort(a &gt; b) // 3, 2, 1
</div>
<div class="language-v">struct User {
    age  int
    name string
}

mut users := [User{21, 'Bob'}, User{20, 'Zarkon'}, User{25, 'Alice'}]
users.sort(a.age &lt; b.age) // sort by User.age int field
users.sort(a.name &gt; b.name) // reverse sort by User.name string field
</div>
<p>V also supports custom sorting, through the <code>sort_with_compare</code> array method.
Which expects a comparing function which will define the sort order.
Useful for sorting on multiple fields at the same time by custom sorting rules.
The code below sorts the array ascending on <code>name</code> and descending <code>age</code>.</p>
<div class="language-v">struct User {
    age  int
    name string
}

mut users := [User{21, 'Bob'}, User{65, 'Bob'}, User{25, 'Alice'}]

custom_sort_fn := fn (a &amp;User, b &amp;User) int {
    // return -1 when a comes before b
    // return 0, when both are in same order
    // return 1 when b comes before a
    if a.name == b.name {
        if a.age &lt; b.age {
            return 1
        }
        if a.age &gt; b.age {
            return -1
        }
        return 0
    }
    if a.name &lt; b.name {
        return -1
    } else if a.name &gt; b.name {
        return 1
    }
    return 0
}
users.sort_with_compare(custom_sort_fn)
</div>
<h3 id="array-slices">Array Slices <a href="v-types.html#array-slices" class="header-anchor" aria-hidden="true">#</a></h3>
<p>A slice is a part of a parent array. Initially it refers to the elements
between two indices separated by a <code>..</code> operator. The right-side index must
be greater than or equal to the left side index.</p>
<p>If a right-side index is absent, it is assumed to be the array length. If a
left-side index is absent, it is assumed to be 0.</p>
<div class="language-v">nums := [0, 10, 20, 30, 40]
println(nums[1..4]) // [10, 20, 30]
println(nums[..4]) // [0, 10, 20, 30]
println(nums[1..]) // [10, 20, 30, 40]
</div>
<p>In V slices are arrays themselves (they are not distinct types). As a result
all array operations may be performed on them. E.g. they can be pushed onto an
array of the same type:</p>
<div class="language-v">array_1 := [3, 5, 4, 7, 6]
mut array_2 := [0, 1]
array_2 &lt;&lt; array_1[..3]
println(array_2) // `[0, 1, 3, 5, 4]`
</div>
<p>A slice is always created with the smallest possible capacity <code>cap == len</code> (see
<a href="v-types.html#array-initialization"><code>cap</code> above</a>) no matter what the capacity or length
of the parent array is. As a result it is immediately reallocated and copied to another
memory location when the size increases thus becoming independent from the
parent array (<em>copy on grow</em>). In particular pushing elements to a slice
does not alter the parent:</p>
<div class="language-v">mut a := [0, 1, 2, 3, 4, 5]

// Create a slice, that reuses the *same memory* as the parent array
// initially, without doing a new allocation:
mut b := unsafe { a[2..4] } // the contents of `b`, reuses the memory, used by the contents of `a`.

b[0] = 7 // Note that `b[0]` and `a[2]` refer to *the same element* in memory.
println(a) // `[0, 1, 7, 3, 4, 5]` - changing `b[0]` above, changed `a[2]` too.

// the content of `b` will get reallocated, to have room for the `9` element:
b &lt;&lt; 9
// The content of `b`, is now reallocated, and fully independent from the content of `a`.

println(a) // `[0, 1, 7, 3, 4, 5]` - no change, since the content of `b` was reallocated,
// to a larger block, before the appending.

println(b) // `[7, 3, 9]` - the contents of `b`, after the reallocation, and appending of the `9`.
</div>
<p>Appending to the parent array, may or may not make it independent from its child slices.
The behaviour depends on the <em>parent's capacity</em> and is predictable:</p>
<div class="language-v">mut a := []int{len: 5, cap: 6, init: 2}
mut b := unsafe { a[1..4] } // the contents of `b` uses part of the same memory, that is used by `a` too

a &lt;&lt; 3
// still no reallocation of `a`, since `a.len` still fits in `a.cap`
b[2] = 13 // `a[3]` is modified, through the slice `b`.

a &lt;&lt; 4
// the content of `a` has been reallocated now, and is independent from `b` (`cap` was exceeded by `len`)
b[1] = 3 // no change in `a`

println(a) // `[2, 2, 2, 13, 2, 3, 4]`
println(b) // `[2, 3, 13]`
</div>
<p>You can call .clone() on the slice, if you <em>do</em> want to have an independent copy right away:</p>
<div class="language-v">mut a := [0, 1, 2, 3, 4, 5]
mut b := a[2..4].clone()
b[0] = 7 // Note: `b[0]` is NOT referring to `a[2]`, as it would have been, without the `.clone()`
println(a) // [0, 1, 2, 3, 4, 5]
println(b) // [7, 3]
</div>
<h4 id="slices-with-negative-indexes">Slices with negative indexes <a href="v-types.html#slices-with-negative-indexes" class="header-anchor" aria-hidden="true">#</a></h4>
<p>V supports array and string slices with negative indexes.
Negative indexing starts from the end of the array towards the start,
for example <code>-3</code> is equal to <code>array.len - 3</code>.
Negative slices have a different syntax from normal slices, i.e. you need
to add a <code>gate</code> between the array name and the square bracket: <code>a#[..-3]</code>.
The <code>gate</code> specifies that this is a different type of slice and remember that
the result is &quot;locked&quot; inside the array.
The returned slice is always a valid array, though it may be empty:</p>
<div class="language-v">a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
println(a#[-3..]) // [7, 8, 9]
println(a#[-20..]) // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
println(a#[-20..-8]) // [0, 1]
println(a#[..-3]) // [0, 1, 2, 3, 4, 5, 6]

// empty arrays
println(a#[-20..-10]) // []
println(a#[20..10]) // []
println(a#[20..30]) // []
</div>
<h3 id="array-method-chaining">Array method chaining <a href="v-types.html#array-method-chaining" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can chain the calls of array methods like <code>.filter()</code> and <code>.map()</code> and use
the <code>it</code> built-in variable to achieve a classic <code>map/filter</code> functional paradigm:</p>
<div class="language-v">// using filter, map and negatives array slices
files := ['pippo.jpg', '01.bmp', '_v.txt', 'img_02.jpg', 'img_01.JPG']
filtered := files.filter(it#[-4..].to_lower() == '.jpg').map(it.to_upper())
// ['PIPPO.JPG', 'IMG_02.JPG', 'IMG_01.JPG']
</div>
<h2 id="fixed-size-arrays">Fixed size arrays <a href="v-types.html#fixed-size-arrays" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V also supports arrays with fixed size. Unlike ordinary arrays, their
length is constant. You cannot append elements to them, nor shrink them.
You can only modify their elements in place.</p>
<p>However, access to the elements of fixed size arrays is more efficient,
they need less memory than ordinary arrays, and unlike ordinary arrays,
their data is on the stack, so you may want to use them as buffers if you
do not want additional heap allocations.</p>
<p>Most methods are defined to work on ordinary arrays, not on fixed size arrays.
You can convert a fixed size array to an ordinary array with slicing:</p>
<div class="language-v">mut fnums := [3]int{} // fnums is a fixed size array with 3 elements.
fnums[0] = 1
fnums[1] = 10
fnums[2] = 100
println(fnums) // =&gt; [1, 10, 100]
println(typeof(fnums).name) // =&gt; [3]int

fnums2 := [1, 10, 100]! // short init syntax that does the same (the syntax will probably change)

anums := fnums[..] // same as `anums := fnums[0..fnums.len]`
println(anums) // =&gt; [1, 10, 100]
println(typeof(anums).name) // =&gt; []int
</div>
<p>Note that slicing will cause the data of the fixed size array to be copied to
the newly created ordinary array.</p>
<h2 id="maps">Maps <a href="v-types.html#maps" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">mut m := map[string]int{} // a map with `string` keys and `int` values
m['one'] = 1
m['two'] = 2
println(m['one']) // &quot;1&quot;
println(m['bad_key']) // &quot;0&quot;
println('bad_key' in m) // Use `in` to detect whether such key exists
println(m.keys()) // ['one', 'two']
m.delete('two')
</div>
<p>Maps can have keys of type string, rune, integer, float or voidptr.</p>
<p>The whole map can be initialized using this short syntax:</p>
<div class="language-v">numbers := {
    'one': 1
    'two': 2
}
println(numbers)
</div>
<p>If a key is not found, a zero value is returned by default:</p>
<div class="language-v">sm := {
    'abc': 'xyz'
}
val := sm['bad_key']
println(val) // ''
</div>
<div class="language-v">intm := {
    1: 1234
    2: 5678
}
s := intm[3]
println(s) // 0
</div>
<p>It's also possible to use an <code>or {}</code> block to handle missing keys:</p>
<div class="language-v">mm := map[string]int{}
val := mm['bad_key'] or { panic('key not found') }
</div>
<p>You can also check, if a key is present, and get its value, if it was present, in one go:</p>
<div class="language-v">m := {
    'abc': 'def'
}
if v := m['abc'] {
    println('the map value for that key is: ${v}')
}
</div>
<p>The same option check applies to arrays:</p>
<div class="language-v">arr := [1, 2, 3]
large_index := 999
val := arr[large_index] or { panic('out of bounds') }
println(val)
// you can also do this, if you want to *propagate* the access error:
val2 := arr[333]!
println(val2)
</div>
<p>V also supports nested maps:</p>
<div class="language-v">mut m := map[string]map[string]int{}
m['greet'] = {
    'Hello': 1
}
m['place'] = {
    'world': 2
}
m['code']['orange'] = 123
print(m)
</div>
<p>Maps are ordered by insertion, like dictionaries in Python. The order is a
guaranteed language feature. This may change in the future.</p>
<p>See all methods of
<a href="https://modules.vlang.io/builtin.html#map">map</a>
and
<a href="https://modules.vlang.io/maps.html">maps</a>.</p>
<h2 id="map-update-syntax">Map update syntax <a href="v-types.html#map-update-syntax" class="header-anchor" aria-hidden="true">#</a></h2>
<p>As with structs, V lets you initialise a map with an update applied on top of
another map:</p>
<div class="language-v">const base_map = {
    'a': 4
    'b': 5
}

foo := {
    ...base_map
    'b': 88
    'c': 99
}

println(foo) // {'a': 4, 'b': 88, 'c': 99}
</div>
<p>This is functionally equivalent to cloning the map and updating it, except that
you don't have to declare a mutable variable:</p>
<div class="language-v">// same as above (except mutable)
mut foo := base_map.clone()
foo['b'] = 88
foo['c'] = 99
</div>

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