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| 22 | <title>Accessing Array Elements (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Accessing-Array-Elements"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Declaring-an-Array.html" accesskey="n" rel="next">Declaring an Array</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</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="Accessing-Array-Elements-1"></span><h3 class="section">16.1 Accessing Array Elements</h3>
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| 64 | <span id="index-accessing-array-elements"></span>
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| 65 | <span id="index-array-elements_002c-accessing"></span>
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| 66 |
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| 67 | <p>If the variable <code>a</code> is an array, the <var>n</var>th element of
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| 68 | <code>a</code> is <code>a[<var>n</var>]</code>. You can use that expression to access
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| 69 | an element’s value or to assign to it:
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">x = a[5];
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| 73 | a[6] = 1;
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| 74 | </pre></div>
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| 75 |
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| 76 | <p>Since the variable <code>a</code> is an lvalue, <code>a[<var>n</var>]</code> is also an
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| 77 | lvalue.
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| 78 | </p>
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| 79 | <p>The lowest valid index in an array is 0, <em>not</em> 1, and the highest
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| 80 | valid index is one less than the number of elements.
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| 81 | </p>
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| 82 | <p>The C language does not check whether array indices are in bounds, so
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| 83 | if the code uses an out-of-range index, it will access memory outside the
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| 84 | array.
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| 85 | </p>
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| 86 | <p><strong>Warning:</strong> Using only valid index values in C is the
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| 87 | programmer’s responsibility.
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| 88 | </p>
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| 89 | <p>Array indexing in C is not a primitive operation: it is defined in
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| 90 | terms of pointer arithmetic and dereferencing. Now that we know
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| 91 | <em>what</em> <code>a[i]</code> does, we can ask <em>how</em> <code>a[i]</code> does
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| 92 | its job.
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| 93 | </p>
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| 94 | <p>In C, <code><var>x</var>[<var>y</var>]</code> is an abbreviation for
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| 95 | <code>*(<var>x</var>+<var>y</var>)</code>. Thus, <code>a[i]</code> really means
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| 96 | <code>*(a+i)</code>. See <a href="Pointers-and-Arrays.html">Pointers and Arrays</a>.
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| 97 | </p>
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| 98 | <p>When an expression with array type (such as <code>a</code>) appears as part
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| 99 | of a larger C expression, it is converted automatically to a pointer
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| 100 | to element zero of that array. For instance, <code>a</code> in an
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| 101 | expression is equivalent to <code>&a[0]</code>. Thus, <code>*(a+i)</code> is
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| 102 | computed as <code>*(&a[0]+i)</code>.
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| 103 | </p>
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| 104 | <p>Now we can analyze how that expression gives us the desired element of
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| 105 | the array. It makes a pointer to element 0 of <code>a</code>, advances it
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| 106 | by the value of <code>i</code>, and dereferences that pointer.
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| 107 | </p>
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| 108 | <p>Another equivalent way to write the expression is <code>(&a[0])[i]</code>.
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| 109 | </p>
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| 110 | <hr>
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| 111 | <div class="header">
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| 112 | <p>
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| 113 | Next: <a href="Declaring-an-Array.html" accesskey="n" rel="next">Declaring an Array</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</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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