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| 22 | <title>Recursive Fibonacci (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Recursive Fibonacci (GNU C Language Manual)">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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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="Recursive-Fibonacci"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Stack.html" accesskey="n" rel="next">Stack</a>, Up: <a href="The-First-Example.html" accesskey="u" rel="up">The First Example</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="Example_003a-Recursive-Fibonacci"></span><h3 class="section">1.1 Example: Recursive Fibonacci</h3>
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| 64 | <span id="index-recursive-Fibonacci-function"></span>
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| 65 | <span id="index-Fibonacci-function_002c-recursive"></span>
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| 66 |
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| 67 | <p>To introduce the most basic features of C, let’s look at code for a
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| 68 | simple mathematical function that does calculations on integers. This
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| 69 | function calculates the <var>n</var>th number in the Fibonacci series, in
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| 70 | which each number is the sum of the previous two: 1, 1, 2, 3, 5, 8,
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| 71 | 13, 21, 34, 55, ….
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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 | fib (int n)
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| 76 | {
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| 77 | if (n <= 2) /* <span class="roman">This avoids infinite recursion.</span> */
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| 78 | return 1;
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| 79 | else
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| 80 | return fib (n - 1) + fib (n - 2);
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| 81 | }
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| 82 | </pre></div>
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| 83 |
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| 84 | <p>This very simple program illustrates several features of C:
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| 85 | </p>
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| 86 | <ul>
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| 87 | <li> A function definition, whose first two lines constitute the function
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| 88 | header. See <a href="Function-Definitions.html">Function Definitions</a>.
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| 89 |
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| 90 | </li><li> A function parameter <code>n</code>, referred to as the variable <code>n</code>
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| 91 | inside the function body. See <a href="Function-Parameter-Variables.html">Function Parameter Variables</a>.
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| 92 | A function definition uses parameters to refer to the argument
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| 93 | values provided in a call to that function.
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| 94 |
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| 95 | </li><li> Arithmetic. C programs add with ‘<samp>+</samp>’ and subtract with
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| 96 | ‘<samp>-</samp>’. See <a href="Arithmetic.html">Arithmetic</a>.
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| 97 |
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| 98 | </li><li> Numeric comparisons. The operator ‘<samp><=</samp>’ tests for “less than or
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| 99 | equal.” See <a href="Numeric-Comparisons.html">Numeric Comparisons</a>.
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| 100 |
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| 101 | </li><li> Integer constants written in base 10.
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| 102 | See <a href="Integer-Constants.html">Integer Constants</a>.
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| 103 |
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| 104 | </li><li> A function call. The function call <code>fib (n - 1)</code> calls the
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| 105 | function <code>fib</code>, passing as its argument the value <code>n - 1</code>.
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| 106 | See <a href="Function-Calls.html">Function Calls</a>.
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| 107 |
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| 108 | </li><li> A comment, which starts with ‘<samp>/*</samp>’ and ends with ‘<samp>*/</samp>’. The
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| 109 | comment has no effect on the execution of the program. Its purpose is
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| 110 | to provide explanations to people reading the source code. Including
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| 111 | comments in the code is tremendously important—they provide
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| 112 | background information so others can understand the code more quickly.
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| 113 | See <a href="Comments.html">Comments</a>.
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| 114 |
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| 115 | </li><li> Two kinds of statements, the <code>return</code> statement and the
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| 116 | <code>if</code>…<code>else</code> statement. See <a href="Statements.html">Statements</a>.
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| 117 |
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| 118 | </li><li> Recursion. The function <code>fib</code> calls itself; that is called a
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| 119 | <em>recursive call</em>. These are valid in C, and quite common.
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| 120 |
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| 121 | <p>The <code>fib</code> function would not be useful if it didn’t return.
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| 122 | Thus, recursive definitions, to be of any use, must avoid infinite
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| 123 | recursion.
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| 124 | </p>
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| 125 | <p>This function definition prevents infinite recursion by specially
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| 126 | handling the case where <code>n</code> is two or less. Thus the maximum
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| 127 | depth of recursive calls is less than <code>n</code>.
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| 128 | </p></li></ul>
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| 129 |
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| 130 | <table class="menu" border="0" cellspacing="0">
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| 131 | <tr><td align="left" valign="top">• <a href="Function-Header.html" accesskey="1">Function Header</a></td><td> </td><td align="left" valign="top">The function’s name and how it is called.
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| 132 | </td></tr>
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| 133 | <tr><td align="left" valign="top">• <a href="Function-Body.html" accesskey="2">Function Body</a></td><td> </td><td align="left" valign="top">Declarations and statements that implement the function.
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| 134 | </td></tr>
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| 135 | </table>
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| 136 |
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| 137 | <hr>
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| 138 | <div class="header">
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| 139 | <p>
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| 140 | Next: <a href="Stack.html" accesskey="n" rel="next">Stack</a>, Up: <a href="The-First-Example.html" accesskey="u" rel="up">The First Example</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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| 141 | </div>
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