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<title>8.19 Extended indexing operators </title>
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<h2 class="section" id="s:index-operators"><a class="section-anchor" href="#s:index-operators" aria-hidden="true"></a>8.19 Extended indexing operators </h2>
<ul>
<li><a href="indexops.html#ss%3Amultiindexing">8.19.1 Multi-index notation</a>
</li></ul>
<p>
(Introduced in 4.06)</p><div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018"><a class="syntax" id="dot-ext"><span class="c010">dot-ext</span></a></td><td class="c015">::=</td><td class="c017">
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="#dot-operator-char"><span class="c010">dot-operator-char</span></a> { <a class="syntax" href="lex.html#operator-char"><span class="c010">operator-char</span></a> }
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" id="dot-operator-char"><span class="c010">dot-operator-char</span></a></td><td class="c015">::=</td><td class="c017">
<span class="c004">!</span> ∣ <span class="c004">?</span> ∣ <a class="syntax" href="lex.html#core-operator-char"><span class="c010">core-operator-char</span></a> ∣ <span class="c004">%</span> ∣ <span class="c004">:</span>
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">.</span> [<a class="syntax" href="names.html#module-path"><span class="c010">module-path</span></a> <span class="c004">.</span>] <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> ( <span class="c004">(</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">)</span> ∣ <span class="c004">[</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">]</span> ∣ <span class="c004">{</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">}</span> ) [ <span class="c004"><-</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> ]
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<span class="c010">operator-name</span></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">.</span> <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> (<span class="c004">()</span> ∣ <span class="c004">[]</span> ∣ <span class="c004">{}</span>) [<span class="c004"><-</span>]
</td></tr>
<tr><td class="c018"> </td></tr>
</table></td></tr>
</table></div><p>This extension provides syntactic sugar for getting and setting elements
for user-defined indexed types. For instance, we can define python-like
dictionaries with
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> Dict = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">include</span> Hashtbl
<span class="ocamlkeyword">let</span> ( .%{} ) tabl index = find tabl index
<span class="ocamlkeyword">let</span> ( .%{}<- ) tabl index value = add tabl index value
<span class="ocamlkeyword">end</span>
<span class="ocamlkeyword">let</span> dict =
<span class="ocamlkeyword">let</span> dict = Dict.create 10 <span class="ocamlkeyword">in</span>
<span class="ocamlkeyword">let</span> () =
dict.Dict.%{<span class="ocamlstring">"one"</span>} <- 1;
<span class="ocamlkeyword">let</span> <span class="ocamlkeyword">open</span> Dict <span class="ocamlkeyword">in</span>
dict.%{<span class="ocamlstring">"two"</span>} <- 2 <span class="ocamlkeyword">in</span>
dict</div></div>
</div><div class="caml-example toplevel">
<div class="ocaml">
<div class="pre caml-input"> dict.Dict.%{<span class="ocamlstring">"one"</span>};;</div>
<div class="pre caml-output ok">- : int = 1</div></div>
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> <span class="ocamlkeyword">open</span> Dict <span class="ocamlkeyword">in</span> dict.%{<span class="ocamlstring">"two"</span>};;</div>
<div class="pre caml-output ok">- : int = 2</div></div>
</div>
<h3 class="subsection" id="ss:multiindexing"><a class="section-anchor" href="#ss:multiindexing" aria-hidden="true"></a>8.19.1 Multi-index notation</h3>
<div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">.</span> [<a class="syntax" href="names.html#module-path"><span class="c010">module-path</span></a> <span class="c004">.</span>] <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> <span class="c004">(</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> {<span class="c004">;</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> }<sup>+</sup> <span class="c004">)</span> [ <span class="c004"><-</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> ]
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">.</span> [<a class="syntax" href="names.html#module-path"><span class="c010">module-path</span></a> <span class="c004">.</span>] <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> <span class="c004">[</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> {<span class="c004">;</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> }<sup>+</sup> <span class="c004">]</span> [ <span class="c004"><-</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> ]
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> <span class="c004">.</span> [<a class="syntax" href="names.html#module-path"><span class="c010">module-path</span></a> <span class="c004">.</span>] <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> <span class="c004">{</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> {<span class="c004">;</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> }<sup>+</sup> <span class="c004">}</span> [ <span class="c004"><-</span> <a class="syntax" href="expr.html#expr"><span class="c010">expr</span></a> ]
</td></tr>
<tr><td class="c018"> </td></tr>
<tr><td class="c018">
<span class="c010">operator-name</span></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">.</span> <a class="syntax" href="#dot-ext"><span class="c010">dot-ext</span></a> (<span class="c004">(;..)</span> ∣ <span class="c004">[;..]</span> ∣ <span class="c004">{;..}</span>) [<span class="c004"><-</span>]
</td></tr>
<tr><td class="c018"> </td></tr>
</table></td></tr>
</table></div><p>Multi-index are also supported through a second variant of indexing operators</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> (.%[;..]) = Bigarray.Genarray.get
<span class="ocamlkeyword">let</span> (.%{;..}) = Bigarray.Genarray.get
<span class="ocamlkeyword">let</span> (.%(;..)) = Bigarray.Genarray.get</div></div>
</div><p>which is called when an index literals contain a semicolon separated list
of expressions with two and more elements:</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">let</span> sum x y = x.%[1;2;3] + y.%[1;2]
<span class="ocamlcomment">(* is equivalent to *)</span>
<span class="ocamlkeyword">let</span> sum x y = (.%[;..]) x [|1;2;3|] + (.%[;..]) y [|1;2|]</div></div>
</div><p>In particular this multi-index notation makes it possible to uniformly handle
indexing Genarray and other implementations of multidimensional arrays.</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> A = Bigarray.Genarray
<span class="ocamlkeyword">let</span> (.%{;..}) = A.get
<span class="ocamlkeyword">let</span> (.%{;..}<- ) = A.set
<span class="ocamlkeyword">let</span> (.%{ }) a k = A.get a [|k|]
<span class="ocamlkeyword">let</span> (.%{ }<-) a k x = A.set a [|k|] x
<span class="ocamlkeyword">let</span> syntax_compare vec mat t3 t4 =
vec.%{0} = A.get vec [|0|]
&& mat.%{0;0} = A.get mat [|0;0|]
&& t3.%{0;0;0} = A.get t3 [|0;0;0|]
&& t4.%{0;0;0;0} = t4.{0,0,0,0}</div></div>
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