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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Pointers.html" rel="up" title="Pointers">
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| 33 | <link href="Pointer-Arithmetic-Low-Level.html" rel="next" title="Pointer Arithmetic Low Level">
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| 58 | <span id="Pointers-and-Arrays"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointer-Arithmetic-Low-Level.html" accesskey="n" rel="next">Pointer Arithmetic Low Level</a>, Previous: <a href="Pointer-Arithmetic.html" accesskey="p" rel="prev">Pointer Arithmetic</a>, Up: <a href="Pointers.html" accesskey="u" rel="up">Pointers</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Pointers-and-Arrays-1"></span><h3 class="section">14.11 Pointers and Arrays</h3>
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| 64 | <span id="index-pointers-and-arrays"></span>
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| 65 | <span id="index-arrays-and-pointers"></span>
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| 66 |
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| 67 | <p>The clean way to refer to an array element is
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| 68 | <code><var>array</var>[<var>index</var>]</code>. Another, complicated way to do the
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| 69 | same job is to get the address of that element as a pointer, then
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| 70 | dereference it: <code>* (&<var>array</var>[0] + <var>index</var>)</code> (or
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| 71 | equivalently <code>* (<var>array</var> + <var>index</var>)</code>). This first gets a
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| 72 | pointer to element zero, then increments it with <code>+</code> to point to
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| 73 | the desired element, then gets the value from there.
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| 74 | </p>
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| 75 | <p>That pointer-arithmetic construct is the <em>definition</em> of square
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| 76 | brackets in C. <code><var>a</var>[<var>b</var>]</code> means, by definition,
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| 77 | <code>*(<var>a</var> + <var>b</var>)</code>. This definition uses <var>a</var> and <var>b</var>
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| 78 | symmetrically, so one must be a pointer and the other an integer; it
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| 79 | does not matter which comes first.
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| 80 | </p>
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| 81 | <p>Since indexing with square brackets is defined in terms of addition
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| 82 | and dereference, that too is symmetrical. Thus, you can write
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| 83 | <code>3[array]</code> and it is equivalent to <code>array[3]</code>. However, it
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| 84 | would be foolish to write <code>3[array]</code>, since it has no advantage
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| 85 | and could confuse people who read the code.
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| 86 | </p>
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| 87 | <p>It may seem like a discrepancy that the definition <code>*(<var>a</var> +
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| 88 | <var>b</var>)</code> requires a pointer, but <code>array[3]</code> uses an array value
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| 89 | instead. Why is this valid? The name of the array, when used by
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| 90 | itself as an expression (other than in <code>sizeof</code>), stands for a
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| 91 | pointer to the arrays’s zeroth element. Thus, <code>array + 3</code>
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| 92 | converts <code>array</code> implicitly to <code>&array[0]</code>, and the result
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| 93 | is a pointer to element 3, equivalent to <code>&array[3]</code>.
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| 94 | </p>
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| 95 | <p>Since square brackets are defined in terms of such addition,
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| 96 | <code>array[3]</code> first converts <code>array</code> to a pointer. That’s why
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| 97 | it works to use an array directly in that construct.
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| 98 | </p>
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| 99 | <hr>
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| 100 | <div class="header">
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| 101 | <p>
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| 102 | Next: <a href="Pointer-Arithmetic-Low-Level.html" accesskey="n" rel="next">Pointer Arithmetic Low Level</a>, Previous: <a href="Pointer-Arithmetic.html" accesskey="p" rel="prev">Pointer Arithmetic</a>, Up: <a href="Pointers.html" accesskey="u" rel="up">Pointers</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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