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Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">11.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="influences.html">Appendix: Influences</a></li><li class="affix"><a href="undocumented.html">Appendix: As-yet-undocumented Features</a></li><li class="affix"><a href="glossary.html">Appendix: Glossary</a></li></ol>
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<a class="header" href="expressions/array-expr.html#array-and-array-index-expressions" id="array-and-array-index-expressions"><h1>Array and array index expressions</h1></a>
<a class="header" href="expressions/array-expr.html#array-expressions" id="array-expressions"><h2>Array expressions</h2></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>ArrayExpression</em> :<br />
<code>[</code> <code>]</code><br />
| <code>[</code> <a href="expressions.html"><em>Expression</em></a> ( <code>,</code> <a href="expressions.html"><em>Expression</em></a> )<sup>*</sup> <code>,</code><sup>?</sup> <code>]</code><br />
| <code>[</code> <a href="expressions.html"><em>Expression</em></a> <code>;</code> <a href="expressions.html"><em>Expression</em></a> <code>]</code></p>
</blockquote>
<p>An <em><a href="types.html#array-and-slice-types">array</a> expression</em> can be written by
enclosing zero or more comma-separated expressions of uniform type in square
brackets. This produces and array containing each of these values in the
order they are written.</p>
<p>Alternatively there can be exactly two expressions inside the brackets,
separated by a semi-colon. The expression after the <code>;</code> must be a have type
<code>usize</code> and be a <a href="expressions.html#constant-expressions">constant expression</a>,
such as a <a href="tokens.html#literals">literal</a> or a <a href="items/constant-items.html">constant
item</a>. <code>[a; b]</code> creates an array containing <code>b</code>
copies of the value of <code>a</code>. If the expression after the semi-colon has a value
greater than 1 then this requires that the type of <code>a</code> is
<a href="special-types-and-traits.html#copy"><code>Copy</code></a>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
[1, 2, 3, 4];
["a", "b", "c", "d"];
[0; 128]; // array with 128 zeros
[0u8, 0u8, 0u8, 0u8,];
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
#}</code></pre></pre>
<a class="header" href="expressions/array-expr.html#array-and-slice-indexing-expressions" id="array-and-slice-indexing-expressions"><h2>Array and slice indexing expressions</h2></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>IndexExpression</em> :<br />
<a href="expressions.html"><em>Expression</em></a> <code>[</code> <a href="expressions.html"><em>Expression</em></a> <code>]</code></p>
</blockquote>
<p><a href="types.html#array-and-slice-types">Array and slice</a>-typed expressions can be
indexed by writing a square-bracket-enclosed expression of type <code>usize</code> (the
index) after them. When the array is mutable, the resulting <a href="expressions.html#place-expressions-and-value-expressions">memory location</a>
can be assigned to.</p>
<p>For other types an index expression <code>a[b]</code> is equivalent to
<code>*std::ops::Index::index(&a, b)</code>, or
<code>*std::ops::IndexMut::index_mut(&mut a, b)</code> in a mutable place expression
context. Just as with methods, Rust will also insert dereference operations on
<code>a</code> repeatedly to find an implementation.</p>
<p>Indices are zero-based for arrays and slices. Array access is a <a href="expressions.html#constant-expressions">constant
expression</a>, so bounds can be checked at compile-time with a constant index
value. Otherwise a check will be performed at run-time that will put the thread
in a <em>panicked state</em> if it fails.</p>
<pre><pre class="playpen"><code class="language-rust should_panic">
# #![allow(unused_variables)]
#fn main() {
// lint is deny by default.
#![warn(const_err)]
([1, 2, 3, 4])[2]; // Evaluates to 3
let b = [[1, 0, 0], [0, 1, 0], [0, 0, 1]];
b[1][2]; // multidimensional array indexing
let x = (["a", "b"])[10]; // warning: index out of bounds
let n = 10;
let y = (["a", "b"])[n]; // panics
let arr = ["a", "b"];
arr[10]; // warning: index out of bounds
#}</code></pre></pre>
<p>The array index expression can be implemented for types other than arrays and slices
by implementing the <a href="../std/ops/trait.Index.html">Index</a> and <a href="../std/ops/trait.IndexMut.html">IndexMut</a> traits.</p>
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