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| 22 | <title>Function Body (GNU C Language Manual)</title>
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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="Recursive-Fibonacci.html" rel="up" title="Recursive Fibonacci">
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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="Function-Body"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Function-Header.html" accesskey="p" rel="prev">Function Header</a>, Up: <a href="Recursive-Fibonacci.html" accesskey="u" rel="up">Recursive Fibonacci</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="Function-Body-1"></span><h4 class="subsection">1.1.2 Function Body</h4>
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| 64 | <span id="index-function-body"></span>
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| 65 | <span id="index-recursion"></span>
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| 66 |
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| 67 | <p>The rest of the function definition is called the <em>function body</em>.
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| 68 | Like every function body, this one starts with ‘<samp>{</samp>’, ends with
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| 69 | ‘<samp>}</samp>’, and contains zero or more <em>statements</em> and
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| 70 | <em>declarations</em>. Statements specify actions to take, whereas
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| 71 | declarations define names of variables, functions, and so on. Each
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| 72 | statement and each declaration ends with a semicolon (‘<samp>;</samp>’).
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| 73 | </p>
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| 74 | <p>Statements and declarations often contain <em>expressions</em>; an
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| 75 | expression is a construct whose execution produces a <em>value</em> of
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| 76 | some data type, but may also take actions through “side effects”
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| 77 | that alter subsequent execution. A statement, by contrast, does not
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| 78 | have a value; it affects further execution of the program only through
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| 79 | the actions it takes.
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| 80 | </p>
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| 81 | <p>This function body contains no declarations, and just one statement,
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| 82 | but that one is a complex statement in that it contains nested
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| 83 | statements. This function uses two kinds of statements:
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| 84 | </p>
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| 85 | <dl compact="compact">
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| 86 | <dt><code>return</code></dt>
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| 87 | <dd><p>The <code>return</code> statement makes the function return immediately.
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| 88 | It looks like this:
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">return <var>value</var>;
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>Its meaning is to compute the expression <var>value</var> and exit the
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| 95 | function, making it return whatever value that expression produced.
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| 96 | For instance,
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| 97 | </p>
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| 98 | <div class="example">
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| 99 | <pre class="example">return 1;
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| 100 | </pre></div>
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| 101 |
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| 102 | <p>returns the integer 1 from the function, and
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| 103 | </p>
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| 104 | <div class="example">
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| 105 | <pre class="example">return fib (n - 1) + fib (n - 2);
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| 106 | </pre></div>
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| 107 |
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| 108 | <p>returns a value computed by performing two function calls
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| 109 | as specified and adding their results.
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| 110 | </p>
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| 111 | </dd>
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| 112 | <dt><code><code>if</code>…<code>else</code></code></dt>
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| 113 | <dd><p>The <code>if</code>…<code>else</code> statement is a <em>conditional</em>.
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| 114 | Each time it executes, it chooses one of its two substatements to execute
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| 115 | and ignores the other. It looks like this:
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| 116 | </p>
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| 117 | <div class="example">
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| 118 | <pre class="example">if (<var>condition</var>)
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| 119 | <var>if-true-statement</var>
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| 120 | else
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| 121 | <var>if-false-statement</var>
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| 122 | </pre></div>
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| 123 |
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| 124 | <p>Its meaning is to compute the expression <var>condition</var> and, if it’s
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| 125 | “true,” execute <var>if-true-statement</var>. Otherwise, execute
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| 126 | <var>if-false-statement</var>. See <a href="if_002delse-Statement.html">if-else Statement</a>.
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| 127 | </p>
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| 128 | <p>Inside the <code>if</code>…<code>else</code> statement, <var>condition</var> is
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| 129 | simply an expression. It’s considered “true” if its value is
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| 130 | nonzero. (A comparison operation, such as <code>n <= 2</code>, produces the
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| 131 | value 1 if it’s “true” and 0 if it’s “false.” See <a href="Numeric-Comparisons.html">Numeric Comparisons</a>.) Thus,
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| 132 | </p>
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| 133 | <div class="example">
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| 134 | <pre class="example">if (n <= 2)
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| 135 | return 1;
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| 136 | else
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| 137 | return fib (n - 1) + fib (n - 2);
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| 138 | </pre></div>
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| 139 |
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| 140 | <p>first tests whether the value of <code>n</code> is less than or equal to 2.
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| 141 | If so, the expression <code>n <= 2</code> has the value 1. So execution
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| 142 | continues with the statement
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| 143 | </p>
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| 144 | <div class="example">
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| 145 | <pre class="example">return 1;
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| 146 | </pre></div>
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| 147 |
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| 148 | <p>Otherwise, execution continues with this statement:
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| 149 | </p>
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| 150 | <div class="example">
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| 151 | <pre class="example">return fib (n - 1) + fib (n - 2);
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| 152 | </pre></div>
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| 153 |
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| 154 | <p>Each of these statements ends the execution of the function and
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| 155 | provides a value for it to return. See <a href="return-Statement.html">return Statement</a>.
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| 156 | </p></dd>
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| 157 | </dl>
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| 158 |
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| 159 | <p>Calculating <code>fib</code> using ordinary integers in C works only for
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| 160 | <var>n</var> < 47, because the value of <code>fib (47)</code> is too large to fit
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| 161 | in type <code>int</code>. The addition operation that tries to add
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| 162 | <code>fib (46)</code> and <code>fib (45)</code> cannot deliver the correct result.
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| 163 | This occurrence is called <em>integer overflow</em>.
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| 164 | </p>
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| 165 | <p>Overflow can manifest itself in various ways, but one thing that can’t
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| 166 | possibly happen is to produce the correct value, since that can’t fit
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| 167 | in the space for the value. See <a href="Integer-Overflow.html">Integer Overflow</a>.
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| 168 | </p>
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| 169 | <p>See <a href="Functions.html">Functions</a>, for a full explanation about functions.
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| 170 | </p>
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| 171 | <hr>
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| 172 | <div class="header">
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| 173 | <p>
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| 174 | Previous: <a href="Function-Header.html" accesskey="p" rel="prev">Function Header</a>, Up: <a href="Recursive-Fibonacci.html" accesskey="u" rel="up">Recursive Fibonacci</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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| 175 | </div>
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| 176 |
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| 177 |
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| 178 |
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| 179 | </body>
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| 180 | </html>
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