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| 22 | <title>Pointer Increment/Decrement (GNU C Language Manual)</title>
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| 24 | <meta name="description" content="Pointer Increment/Decrement (GNU C Language Manual)">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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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-Drawbacks.html" rel="next" title="Pointer Arithmetic Drawbacks">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Pointer-Increment_002fDecrement"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointer-Arithmetic-Drawbacks.html" accesskey="n" rel="next">Pointer Arithmetic Drawbacks</a>, Previous: <a href="Pointer-Arithmetic-Low-Level.html" accesskey="p" rel="prev">Pointer Arithmetic Low Level</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="Pointer-Increment-and-Decrement"></span><h3 class="section">14.13 Pointer Increment and Decrement</h3>
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| 64 | <span id="index-pointer-increment-and-decrement"></span>
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| 65 | <span id="index-incrementing-pointers"></span>
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| 66 | <span id="index-decrementing-pointers"></span>
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| 67 |
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| 68 | <p>The ‘<samp>++</samp>’ operator adds 1 to a variable. We have seen it for
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| 69 | integers (see <a href="Increment_002fDecrement.html">Increment/Decrement</a>), but it works for pointers too.
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| 70 | For instance, suppose we have a series of positive integers,
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| 71 | terminated by a zero, and we want to add them all up.
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">int
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| 75 | sum_array_till_0 (int *p)
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| 76 | {
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| 77 | int sum = 0;
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| 78 |
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| 79 | for (;;)
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| 80 | {
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| 81 | /* <span class="roman">Fetch the next integer.</span> */
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| 82 | int next = *p++;
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| 83 | /* <span class="roman">Exit the loop if it’s 0.</span> */
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| 84 | if (next == 0)
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| 85 | break;
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| 86 | /* <span class="roman">Add it into running total.</span> */
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| 87 | sum += next;
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| 88 | }
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| 89 |
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| 90 | return sum;
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| 91 | }
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>The statement ‘<samp>break;</samp>’ will be explained further on (see <a href="break-Statement.html">break Statement</a>). Used in this way, it immediately exits the surrounding
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| 95 | <code>for</code> statement.
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| 96 | </p>
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| 97 | <p><code>*p++</code> parses as <code>*(p++)</code>, because a postfix operator always
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| 98 | takes precedence over a prefix operator. Therefore, it dereferences
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| 99 | <code>p</code>, and increments <code>p</code> afterwards. Incrementing a variable
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| 100 | means adding 1 to it, as in <code>p = p + 1</code>. Since <code>p</code> is a
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| 101 | pointer, adding 1 to it advances it by the width of the datum it
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| 102 | points to—in this case, one <code>int</code>. Therefore, each iteration
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| 103 | of the loop picks up the next integer from the series and puts it into
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| 104 | <code>next</code>.
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| 105 | </p>
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| 106 | <p>This <code>for</code>-loop has no initialization expression since <code>p</code>
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| 107 | and <code>sum</code> are already initialized, it has no end-test since the
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| 108 | ‘<samp>break;</samp>’ statement will exit it, and needs no expression to
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| 109 | advance it since that’s done within the loop by incrementing <code>p</code>
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| 110 | and <code>sum</code>. Thus, those three expressions after <code>for</code> are
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| 111 | left empty.
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| 112 | </p>
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| 113 | <p>Another way to write this function is by keeping the parameter value unchanged
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| 114 | and using indexing to access the integers in the table.
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| 115 | </p>
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| 116 | <div class="example">
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| 117 | <pre class="example">int
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| 118 | sum_array_till_0_indexing (int *p)
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| 119 | {
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| 120 | int i;
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| 121 | int sum = 0;
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| 122 |
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| 123 | for (i = 0; ; i++)
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| 124 | {
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| 125 | /* <span class="roman">Fetch the next integer.</span> */
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| 126 | int next = p[i];
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| 127 | /* <span class="roman">Exit the loop if it’s 0.</span> */
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| 128 | if (next == 0)
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| 129 | break;
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| 130 | /* <span class="roman">Add it into running total.</span> */
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| 131 | sum += next;
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| 132 | }
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| 133 |
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| 134 | return sum;
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| 135 | }
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| 136 | </pre></div>
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| 137 |
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| 138 | <p>In this program, instead of advancing <code>p</code>, we advance <code>i</code>
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| 139 | and add it to <code>p</code>. (Recall that <code>p[i]</code> means <code>*(p +
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| 140 | i)</code>.) Either way, it uses the same address to get the next integer.
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| 141 | </p>
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| 142 | <p>It makes no difference in this program whether we write <code>i++</code> or
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| 143 | <code>++i</code>, because the value is not used. All that matters is the
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| 144 | effect, to increment <code>i</code>.
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| 145 | </p>
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| 146 | <p>The ‘<samp>--</samp>’ operator also works on pointers; it can be used
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| 147 | to scan backwards through an array, like this:
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| 148 | </p>
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| 149 | <div class="example">
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| 150 | <pre class="example">int
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| 151 | after_last_nonzero (int *p, int len)
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| 152 | {
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| 153 | /* <span class="roman">Set up <code>q</code> to point just after the last array element.</span> */
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| 154 | int *q = p + len;
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| 155 |
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| 156 | while (q != p)
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| 157 | /* <span class="roman">Step <code>q</code> back until it reaches a nonzero element.</span> */
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| 158 | if (*--q != 0)
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| 159 | /* <span class="roman">Return the index of the element after that nonzero.</span> */
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| 160 | return q - p + 1;
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| 161 |
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| 162 | return 0;
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| 163 | }
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| 164 | </pre></div>
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| 165 |
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| 166 | <p>That function returns the length of the nonzero part of the
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| 167 | array specified by its arguments; that is, the index of the
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| 168 | first zero of the run of zeros at the end.
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| 169 | </p>
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| 170 | <hr>
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| 171 | <div class="header">
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| 172 | <p>
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| 173 | Next: <a href="Pointer-Arithmetic-Drawbacks.html" accesskey="n" rel="next">Pointer Arithmetic Drawbacks</a>, Previous: <a href="Pointer-Arithmetic-Low-Level.html" accesskey="p" rel="prev">Pointer Arithmetic Low Level</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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| 174 | </div>
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| 175 |
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| 176 |
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| 177 |
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| 178 | </body>
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