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variables.html
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<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<main>
<h1 class="main">Variables</h1>
<div class="language-v">name := 'Bob'
age := 20
large_number := i64(9999999999)
println(name)
println(age)
println(large_number)
</div>
<p>Variables are declared and initialized with <code>:=</code>. This is the only
way to declare variables in V. This means that variables always have an initial
value.</p>
<p>The variable's type is inferred from the value on the right hand side.
To choose a different type, use type conversion:
the expression <code>T(v)</code> converts the value <code>v</code> to the
type <code>T</code>.</p>
<p>Unlike most other languages, V only allows defining variables in functions.
By default V does not allow <strong>global variables</strong>. See more <a href="global-variables.html">details</a>.</p>
<p>For consistency across different code bases, all variable and function names
must use the <code>snake_case</code> style, as opposed to type names, which must use <code>PascalCase</code>.</p>
<h2 id="mutable-variables">Mutable variables <a href="variables.html#mutable-variables" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">mut age := 20
println(age)
age = 21
println(age)
</div>
<p>To change the value of the variable use <code>=</code>. In V, variables are
immutable by default.
To be able to change the value of the variable, you have to declare it with <code>mut</code>.</p>
<p>Try compiling the program above after removing <code>mut</code> from the first line.</p>
<h2 id="initialization-vs-assignment">Initialization vs assignment <a href="variables.html#initialization-vs-assignment" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Note the (important) difference between <code>:=</code> and <code>=</code>.
<code>:=</code> is used for declaring and initializing, <code>=</code> is used for assigning.</p>
<div class="language-v">fn main() {
age = 21
}
</div>
<p>This code will not compile, because the variable <code>age</code> is not declared.
All variables need to be declared in V.</p>
<div class="language-v">fn main() {
age := 21
}
</div>
<p>The values of multiple variables can be changed in one line.
In this way, their values can be swapped without an intermediary variable.</p>
<div class="language-v">mut a := 0
mut b := 1
println('${a}, ${b}') // 0, 1
a, b = b, a
println('${a}, ${b}') // 1, 0
</div>
<h2 id="warnings-and-declaration-errors">Warnings and declaration errors <a href="variables.html#warnings-and-declaration-errors" class="header-anchor" aria-hidden="true">#</a></h2>
<p>In development mode the compiler will warn you that you haven't used the variable
(you'll get an "unused variable" warning).
In production mode (enabled by passing the <code>-prod</code> flag to v – <code>v -prod foo.v</code>)
it will not compile at all (like in Go).</p>
<div class="language-v">fn main() {
a := 10
// warning: unused variable `a`
}
</div>
<p>To ignore values returned by a function <code>_</code> can be used</p>
<div class="language-v">fn foo() (int, int) {
return 2, 3
}
fn main() {
c, _ := foo()
print(c)
// no warning about unused variable returned by foo.
}
</div>
<p>Unlike most languages, variable shadowing is not allowed. Declaring a variable with a name
that is already used in a parent scope will cause a compilation error.</p>
<div class="language-v">fn main() {
a := 10
if true {
a := 20 // error: redefinition of `a`
}
}
</div>
<p>While variable shadowing is not allowed, field shadowing is allowed.</p>
<div class="language-v">pub struct Dimension {
width int = -1
height int = -1
}
pub struct Test {
Dimension
width int = 100
// height int
}
fn main() {
test := Test{}
println('${test.width} ${test.height} ${test.Dimension.width}') // 100 -1 -1
}
</div>
</main>
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