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                            <h1 class="main">Statements &amp; expressions</h1>
<h2 id="if">If <a href="statements-&-expressions.html#if" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">a := 10
b := 20
if a &lt; b {
    println('${a} &lt; ${b}')
} else if a &gt; b {
    println('${a} &gt; ${b}')
} else {
    println('${a} == ${b}')
}
</div>
<p><code>if</code> statements are pretty straightforward and similar to most other languages.
Unlike other C-like languages,
there are no parentheses surrounding the condition and the braces are always required.</p>
<h3 id="if-expressions"><code>If</code> expressions <a href="statements-&-expressions.html#if-expressions" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Unlike C, V does not have a ternary operator, that would allow you to do: <code>x = c ? 1 : 2</code> .
Instead, it has a bit more verbose, but also clearer to read, ability to use <code>if</code> as an
expression. The direct translation in V of the ternary construct above, assuming <code>c</code> is a
boolean condition, would be: <code>x = if c { 1 } else { 2 }</code>.</p>
<p>Here is another example:</p>
<div class="language-v">num := 777
s := if num % 2 == 0 { 'even' } else { 'odd' }
println(s)
// &quot;odd&quot;
</div>
<p>You can use multiple statements in each of the branches of an <code>if</code> expression, followed by a final
value, that will become the value of the entire <code>if</code> expression, when it takes that branch:</p>
<div class="language-v">n := arguments().len
x := if n &gt; 2 {
    dump(arguments())
    42
} else {
    println('something else')
    100
}
dump(x)
</div>
<h3 id="if-unwrapping"><code>If</code> unwrapping <a href="statements-&-expressions.html#if-unwrapping" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Anywhere you can use <code>or {}</code>, you can also use &quot;if unwrapping&quot;. This binds the unwrapped value
of an expression to a variable when that expression is not none nor an error.</p>
<div class="language-v">m := {
    'foo': 'bar'
}

// handle missing keys
if v := m['foo'] {
    println(v) // bar
} else {
    println('not found')
}
</div>
<div class="language-v">fn res() !int {
    return 42
}

// functions that return a result type
if v := res() {
    println(v)
}
</div>
<div class="language-v">struct User {
    name string
}

arr := [User{'John'}]

// if unwrapping with assignment of a variable
u_name := if v := arr[0] {
    v.name
} else {
    'Unnamed'
}
println(u_name) // John
</div>
<h3 id="type-checks-and-casts">Type checks and casts <a href="statements-&-expressions.html#type-checks-and-casts" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can check the current type of a sum type using <code>is</code> and its negated form <code>!is</code>.</p>
<p>You can do it either in an <code>if</code>:</p>
<div class="language-v">struct Abc {
    val string
}

struct Xyz {
    foo string
}

type Alphabet = Abc | Xyz

x := Alphabet(Abc{'test'}) // sum type
if x is Abc {
    // x is automatically cast to Abc and can be used here
    println(x)
}
if x !is Abc {
    println('Not Abc')
}
</div>
<p>or using <code>match</code>:</p>
<div class="language-v">match x {
    Abc {
        // x is automatically cast to Abc and can be used here
        println(x)
    }
    Xyz {
        // x is automatically cast to Xyz and can be used here
        println(x)
    }
}
</div>
<p>This works also with struct fields:</p>
<div class="language-v">struct MyStruct {
    x int
}

struct MyStruct2 {
    y string
}

type MySumType = MyStruct | MyStruct2

struct Abc {
    bar MySumType
}

x := Abc{
    bar: MyStruct{123} // MyStruct will be converted to MySumType type automatically
}
if x.bar is MyStruct {
    // x.bar is automatically cast
    println(x.bar)
} else if x.bar is MyStruct2 {
    new_var := x.bar as MyStruct2
    // ... or you can use `as` to create a type cast an alias manually:
    println(new_var)
}
match x.bar {
    MyStruct {
        // x.bar is automatically cast
        println(x.bar)
    }
    else {}
}
</div>
<p>Mutable variables can change, and doing a cast would be unsafe.
However, sometimes it's useful to type cast despite mutability.
In such cases the developer must mark the expression with the <code>mut</code> keyword
to tell the compiler that they know what they're doing.</p>
<p>It works like this:</p>
<div class="language-v">mut x := MySumType(MyStruct{123})
if mut x is MyStruct {
    // x is cast to MyStruct even if it's mutable
    // without the mut keyword that wouldn't work
    println(x)
}
// same with match
match mut x {
    MyStruct {
        // x is cast to MyStruct even if it's mutable
        // without the mut keyword that wouldn't work
        println(x)
    }
}
</div>
<h2 id="match">Match <a href="statements-&-expressions.html#match" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">os := 'windows'
print('V is running on ')
match os {
    'darwin' { println('macOS.') }
    'linux' { println('Linux.') }
    else { println(os) }
}
</div>
<p>A match statement is a shorter way to write a sequence of <code>if - else</code> statements.
When a matching branch is found, the following statement block will be run.
The else branch will be run when no other branches match.</p>
<div class="language-v">number := 2
s := match number {
    1 { 'one' }
    2 { 'two' }
    else { 'many' }
}
</div>
<p>A match statement can also to be used as an <code>if - else if - else</code> alternative:</p>
<div class="language-v">match true {
    2 &gt; 4 { println('if') }
    3 == 4 { println('else if') }
    2 == 2 { println('else if2') }
    else { println('else') }
}
// 'else if2' should be printed
</div>
<p>or as an <code>unless</code> alternative: <a href="https://www.tutorialspoint.com/ruby/ruby_if_else.htm">unless Ruby</a></p>
<div class="language-v">match false {
    2 &gt; 4 { println('if') }
    3 == 4 { println('else if') }
    2 == 2 { println('else if2') }
    else { println('else') }
}
// 'if' should be printed
</div>
<p>A match expression returns the value of the final expression from the matching branch.</p>
<div class="language-v">enum Color {
    red
    blue
    green
}

fn is_red_or_blue(c Color) bool {
    return match c {
        .red, .blue { true } // comma can be used to test multiple values
        .green { false }
    }
}
</div>
<p>A match statement can also be used to branch on the variants of an <code>enum</code>
by using the shorthand <code>.variant_here</code> syntax. An <code>else</code> branch is not allowed
when all the branches are exhaustive.</p>
<div class="language-v">c := `v`
typ := match c {
    `0`...`9` { 'digit' }
    `A`...`Z` { 'uppercase' }
    `a`...`z` { 'lowercase' }
    else { 'other' }
}
println(typ)
// 'lowercase'
</div>
<p>A match statement also can match the variant types of a <code>sumtype</code>. Note that
in that case, the match is exhaustive, since all variant types are mentioned
explicitly, so there is no need for an <code>else{}</code> branch.</p>
<div class="language-v">struct Dog {}
struct Cat {}
struct Veasel {}
type Animal = Dog | Cat | Veasel
a := Animal(Veasel{})
match a {
    Dog { println('Bay') }
    Cat { println('Meow') }
    Veasel { println('Vrrrrr-eeee') } // see: https://www.youtube.com/watch?v=qTJEDyj2N0Q
}
</div>
<p>You can also use ranges as <code>match</code> patterns. If the value falls within the range
of a branch, that branch will be executed.</p>
<p>Note that the ranges use <code>...</code> (three dots) rather than <code>..</code> (two dots). This is
because the range is <em>inclusive</em> of the last element, rather than exclusive
(as <code>..</code> ranges are). Using <code>..</code> in a match branch will throw an error.</p>
<div class="language-v">const start = 1

const end = 10

c := 2
num := match c {
    start...end {
        1000
    }
    else {
        0
    }
}
println(num)
// 1000
</div>
<p>Constants can also be used in the range branch expressions.</p>
<blockquote class="tip">
<p>[!NOTE]
<code>match</code> as an expression is not usable in <code>for</code> loop and <code>if</code> statements.</p>
</blockquote>
<h2 id="in-operator">In operator <a href="statements-&-expressions.html#in-operator" class="header-anchor" aria-hidden="true">#</a></h2>
<p><code>in</code> allows to check whether an array or a map contains an element.
To do the opposite, use <code>!in</code>.</p>
<div class="language-v">nums := [1, 2, 3]
println(1 in nums) // true
println(4 !in nums) // true
</div>
<blockquote class="tip">
<p>[!NOTE]
<code>in</code> checks if map contains a key, not a value.</p>
</blockquote>
<div class="language-v">m := {
    'one': 1
    'two': 2
}

println('one' in m) // true
println('three' !in m) // true
</div>
<p>It's also useful for writing boolean expressions that are clearer and more compact:</p>
<div class="language-v">enum Token {
    plus
    minus
    div
    mult
}

struct Parser {
    token Token
}

parser := Parser{}
if parser.token == .plus || parser.token == .minus || parser.token == .div || parser.token == .mult {
    // ...
}
if parser.token in [.plus, .minus, .div, .mult] {
    // ...
}
</div>
<p>V optimizes such expressions,
so both <code>if</code> statements above produce the same machine code and no arrays are created.</p>
<h2 id="for-loop">For loop <a href="statements-&-expressions.html#for-loop" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V has only one looping keyword: <code>for</code>, with several forms.</p>
<h3 id="forin"><code>for</code>/<code>in</code> <a href="statements-&-expressions.html#forin" class="header-anchor" aria-hidden="true">#</a></h3>
<p>This is the most common form. You can use it with an array, map or
numeric range.</p>
<h4 id="array-for">Array <code>for</code> <a href="statements-&-expressions.html#array-for" class="header-anchor" aria-hidden="true">#</a></h4>
<div class="language-v">numbers := [1, 2, 3, 4, 5]
for num in numbers {
    println(num)
}
names := ['Sam', 'Peter']
for i, name in names {
    println('${i}) ${name}')
    // Output: 0) Sam
    //         1) Peter
}
</div>
<p>The <code>for value in arr</code> form is used for going through elements of an array.
If an index is required, an alternative form <code>for index, value in arr</code> can be used.</p>
<p>Note that the value is read-only.
If you need to modify the array while looping, you need to declare the element as mutable:</p>
<div class="language-v">mut numbers := [0, 1, 2]
for mut num in numbers {
    num++
}
println(numbers) // [1, 2, 3]
</div>
<p>When an identifier is just a single underscore, it is ignored.</p>
<h4 id="custom-iterators">Custom iterators <a href="statements-&-expressions.html#custom-iterators" class="header-anchor" aria-hidden="true">#</a></h4>
<p>Types that implement a <code>next</code> method returning an <code>Option</code> can be iterated
with a <code>for</code> loop.</p>
<div class="language-v">struct SquareIterator {
    arr []int
mut:
    idx int
}

fn (mut iter SquareIterator) next() ?int {
    if iter.idx &gt;= iter.arr.len {
        return none
    }
    defer {
        iter.idx++
    }
    return iter.arr[iter.idx] * iter.arr[iter.idx]
}

nums := [1, 2, 3, 4, 5]
iter := SquareIterator{
    arr: nums
}
for squared in iter {
    println(squared)
}
</div>
<p>The code above prints:</p>
<pre><code>1
4
9
16
25
</code></pre>
<h4 id="map-for">Map <code>for</code> <a href="statements-&-expressions.html#map-for" class="header-anchor" aria-hidden="true">#</a></h4>
<div class="language-v">m := {
    'one': 1
    'two': 2
}
for key, value in m {
    println('${key} -&gt; ${value}')
    // Output: one -&gt; 1
    //         two -&gt; 2
}
</div>
<p>Either key or value can be ignored by using a single underscore as the identifier.</p>
<div class="language-v">m := {
    'one': 1
    'two': 2
}
// iterate over keys
for key, _ in m {
    println(key)
    // Output: one
    //         two
}
// iterate over values
for _, value in m {
    println(value)
    // Output: 1
    //         2
}
</div>
<h4 id="range-for">Range <code>for</code> <a href="statements-&-expressions.html#range-for" class="header-anchor" aria-hidden="true">#</a></h4>
<div class="language-v">// Prints '01234'
for i in 0 .. 5 {
    print(i)
}
</div>
<p><code>low..high</code> means an <em>exclusive</em> range, which represents all values
from <code>low</code> up to <em>but not including</em> <code>high</code>.</p>
<blockquote class="tip">
<p>[!NOTE]
This exclusive range notation and zero-based indexing follow principles of
logical consistency and error reduction. As Edsger W. Dijkstra outlines in
'Why Numbering Should Start at Zero'
(<a href="https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html">EWD831</a>),
zero-based indexing aligns the index with the preceding elements in a sequence,
simplifying handling and minimizing errors, especially with adjacent subsequences.
This logical and efficient approach shapes our language design, emphasizing clarity
and reducing confusion in programming.</p>
</blockquote>
<h3 id="condition-for">Condition <code>for</code> <a href="statements-&-expressions.html#condition-for" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">mut sum := 0
mut i := 0
for i &lt;= 100 {
    sum += i
    i++
}
println(sum) // &quot;5050&quot;
</div>
<p>This form of the loop is similar to <code>while</code> loops in other languages.
The loop will stop iterating once the boolean condition evaluates to false.
Again, there are no parentheses surrounding the condition, and the braces are always required.</p>
<h3 id="bare-for">Bare <code>for</code> <a href="statements-&-expressions.html#bare-for" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">mut num := 0
for {
    num += 2
    if num &gt;= 10 {
        break
    }
}
println(num) // &quot;10&quot;
</div>
<p>The condition can be omitted, resulting in an infinite loop.</p>
<h3 id="c-for">C <code>for</code> <a href="statements-&-expressions.html#c-for" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">for i := 0; i &lt; 10; i += 2 {
    // Don't print 6
    if i == 6 {
        continue
    }
    println(i)
}
</div>
<p>Finally, there's the traditional C style <code>for</code> loop. It's safer than the <code>while</code> form
because with the latter it's easy to forget to update the counter and get
stuck in an infinite loop.</p>
<p>Here <code>i</code> doesn't need to be declared with <code>mut</code> since it's always going to be mutable by definition.</p>
<h3 id="labelled-break-&amp;-continue">Labelled break &amp; continue <a href="#labelled-break-&amp;-continue" class="header-anchor" aria-hidden="true">#</a></h3>
<p><code>break</code> and <code>continue</code> control the innermost <code>for</code> loop by default.
You can also use <code>break</code> and <code>continue</code> followed by a label name to refer to an outer <code>for</code>
loop:</p>
<div class="language-v">outer: for i := 4; true; i++ {
    println(i)
    for {
        if i &lt; 7 {
            continue outer
        } else {
            break outer
        }
    }
}
</div>
<p>The label must immediately precede the outer loop.
The above code prints:</p>
<pre><code>4
5
6
7
</code></pre>
<h2 id="defer">Defer <a href="statements-&-expressions.html#defer" class="header-anchor" aria-hidden="true">#</a></h2>
<p>A defer statement defers the execution of a block of statements
until the surrounding function returns.</p>
<div class="language-v">import os

fn read_log() {
    mut ok := false
    mut f := os.open('log.txt') or { panic(err) }
    defer {
        f.close()
    }
    // ...
    if !ok {
        // defer statement will be called here, the file will be closed
        return
    }
    // ...
    // defer statement will be called here, the file will be closed
}
</div>
<p>If the function returns a value the <code>defer</code> block is executed <em>after</em> the return
expression is evaluated:</p>
<div class="language-v">import os

enum State {
    normal
    write_log
    return_error
}

// write log file and return number of bytes written

fn write_log(s State) !int {
    mut f := os.create('log.txt')!
    defer {
        f.close()
    }
    if s == .write_log {
        // `f.close()` will be called after `f.write()` has been
        // executed, but before `write_log()` finally returns the
        // number of bytes written to `main()`
        return f.writeln('This is a log file')
    } else if s == .return_error {
        // the file will be closed after the `error()` function
        // has returned - so the error message will still report
        // it as open
        return error('nothing written; file open: ${f.is_opened}')
    }
    // the file will be closed here, too
    return 0
}

fn main() {
    n := write_log(.return_error) or {
        println('Error: ${err}')
        0
    }
    println('${n} bytes written')
}
</div>
<p>To access the result of the function inside a <code>defer</code> block the <code>$res()</code> expression can be used.
<code>$res()</code> is only used when a single value is returned, while on multi-return the <code>$res(idx)</code>
is parameterized.</p>
<div class="language-v">fn (mut app App) auth_middleware() bool {
    defer {
        if !$res() {
            app.response.status_code = 401
            app.response.body = 'Unauthorized'
        }
    }
    header := app.get_header('Authorization')
    if header == '' {
        return false
    }
    return true
}

fn (mut app App) auth_with_user_middleware() (bool, string) {
    defer {
        if !$res(0) {
            app.response.status_code = 401
            app.response.body = 'Unauthorized'
        } else {
            app.user = $res(1)
        }
    }
    header := app.get_header('Authorization')
    if header == '' {
        return false, ''
    }
    return true, 'TestUser'
}
</div>
<h2 id="goto">Goto <a href="statements-&-expressions.html#goto" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V allows unconditionally jumping to a label with <code>goto</code>. The label name must be contained
within the same function as the <code>goto</code> statement. A program may <code>goto</code> a label outside
or deeper than the current scope. <code>goto</code> allows jumping past variable initialization or
jumping back to code that accesses memory that has already been freed, so it requires
<code>unsafe</code>.</p>
<div class="language-v">if x {
    // ...
    if y {
        unsafe {
            goto my_label
        }
    }
    // ...
}
my_label:
</div>
<p><code>goto</code> should be avoided, particularly when <code>for</code> can be used instead.
<a href="statements-&-expressions.html#labelled-break-&-continue">Labelled break/continue</a> can be used to break out of
a nested loop, and those do not risk violating memory-safety.</p>

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