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   1
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   4 <h1><a name="c" id="c">C</a></h1>
   5 <div class="level1">
   6
   7 <p>
   8
   9 <a href="http://en.wikipedia.org/wiki/C_(programming_language)" class="interwiki iw_wp" title="http://en.wikipedia.org/wiki/C_(programming_language)">C</a>
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  18 <!-- SECTION "C" [1-52] -->
  19 <h2><a name="how_to_print_the_current_time" id="how_to_print_the_current_time">How to print the current time</a></h2>
  20 <div class="level2">
  21
  22 <p>
  23
  24 Using <a href="http://www.cplusplus.com/reference/clibrary/ctime/strftime/" class="urlextern" title="http://www.cplusplus.com/reference/clibrary/ctime/strftime/"  rel="nofollow">strftime</a>.
  25 </p>
  26 <dl class="file">
  27 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=0" title="Download Snippet" class="mediafile mf_c">show_time.c</a></dt>
  28 <dd><pre class="code file c"><span class="co2">#include &lt;stdio.h&gt;</span>
  29 <span class="co2">#include &lt;time.h&gt;</span>
  30 &nbsp;
  31 <span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
  32 <span class="br0">&#123;</span>
  33         time_t  currtime<span class="sy0">;</span>
  34         <span class="kw4">char</span> charTime<span class="br0">&#91;</span>100<span class="br0">&#93;</span> <span class="sy0">=</span> <span class="br0">&#123;</span>0<span class="br0">&#125;</span><span class="sy0">;</span>
  35         time<span class="br0">&#40;</span><span class="sy0">&amp;</span>currtime<span class="br0">&#41;</span><span class="sy0">;</span>
  36         strftime<span class="br0">&#40;</span>charTime<span class="sy0">,</span><span class="kw4">sizeof</span><span class="br0">&#40;</span>charTime<span class="br0">&#41;</span><span class="sy0">-</span><span class="nu0">1</span><span class="sy0">,</span><span class="st0">&quot;%c&quot;</span><span class="sy0">,</span>localtime<span class="br0">&#40;</span><span class="sy0">&amp;</span>currtime<span class="br0">&#41;</span><span class="br0">&#41;</span><span class="sy0">;</span>
  37         <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html"><span class="kw3">printf</span></a><span class="br0">&#40;</span><span class="st0">&quot;time: %s<span class="es1">\n</span>&quot;</span><span class="sy0">,</span> charTime<span class="br0">&#41;</span><span class="sy0">;</span>
  38         <span class="coMULTI">/* &quot;time: Sat Apr 17 11:43:47 2010&quot; */</span>
  39 <span class="br0">&#125;</span></pre>
  40 </dd></dl>
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  47 </div>
  48 <!-- SECTION "How to print the current time" [53-462] -->
  49 <h2><a name="how_to_install_colorgcc" id="how_to_install_colorgcc">How to install colorgcc</a></h2>
  50 <div class="level2">
  51 <pre class="code bash">$ <span class="kw2">sudo</span> <span class="kw2">apt-get</span> <span class="kw2">install</span> colorgcc
  52 $ <span class="kw3">cd</span> <span class="sy0">/</span>usr<span class="sy0">/</span>local<span class="sy0">/</span>bin
  53 $ <span class="kw2">sudo</span> <span class="kw2">ln</span> <span class="re5">-s</span> <span class="sy0">/</span>usr<span class="sy0">/</span>bin<span class="sy0">/</span>colorgcc <span class="kw2">gcc</span>
  54 $ <span class="kw2">sudo</span> <span class="kw2">ln</span> <span class="re5">-s</span> <span class="sy0">/</span>usr<span class="sy0">/</span>bin<span class="sy0">/</span>colorgcc <span class="kw2">g++</span></pre>
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  61 </div>
  62 <!-- SECTION "How to install colorgcc" [463-642] -->
  63 <h2><a name="tail_recursion_without_return" id="tail_recursion_without_return">Tail Recursion Without Return</a></h2>
  64 <div class="level2">
  65
  66 <p>
  67
  68 In a function with a <a href="http://en.wikipedia.org/wiki/Tail_recursion" class="interwiki iw_wp" title="http://en.wikipedia.org/wiki/Tail_recursion">Tail Recursion</a> …
  69 </p>
  70 <dl class="file">
  71 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=2" title="Download Snippet" class="mediafile mf_c">mdc.c</a></dt>
  72 <dd><pre class="code file c"><span class="kw4">int</span> mdc<span class="br0">&#40;</span><span class="kw4">int</span> x<span class="sy0">,</span> <span class="kw4">int</span> y<span class="br0">&#41;</span>
  73 <span class="br0">&#123;</span>
  74         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>y<span class="br0">&#41;</span> <span class="kw1">return</span> x<span class="sy0">;</span>
  75         <span class="kw1">return</span> mdc<span class="br0">&#40;</span>y<span class="sy0">,</span> x <span class="sy0">%</span> y<span class="br0">&#41;</span><span class="sy0">;</span>
  76 <span class="br0">&#125;</span></pre>
  77 </dd></dl>
  78
  79 <p>
  80 … we can delete de <a href="http://en.wikipedia.org/wiki/Tail_recursion" class="interwiki iw_wp" title="http://en.wikipedia.org/wiki/Tail_recursion">Tail_recursion</a> return(s) …
  81 </p>
  82 <dl class="file">
  83 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=3" title="Download Snippet" class="mediafile mf_c">mdc_changed.c</a></dt>
  84 <dd><pre class="code file c"><span class="kw4">int</span> mdc<span class="br0">&#40;</span><span class="kw4">int</span> x<span class="sy0">,</span> <span class="kw4">int</span> y<span class="br0">&#41;</span>
  85 <span class="br0">&#123;</span>
  86         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>y<span class="br0">&#41;</span> <span class="kw1">return</span> x<span class="sy0">;</span>
  87         mdc<span class="br0">&#40;</span>y<span class="sy0">,</span> x <span class="sy0">%</span> y<span class="br0">&#41;</span><span class="sy0">;</span>
  88 <span class="br0">&#125;</span></pre>
  89 </dd></dl>
  90
  91 <p>
  92 … This works because the return value goes to eax register and, as the other function calls don&#039;t change its value, they return the same as the last call done.
  93 </p>
  94
  95 <p>
  96 Another example:
  97 </p>
  98 <dl class="file">
  99 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=4" title="Download Snippet" class="mediafile mf_c">ackermann.c</a></dt>
 100 <dd><pre class="code file c"><span class="kw4">int</span> ackermann<span class="br0">&#40;</span><span class="kw4">int</span> m<span class="sy0">,</span> <span class="kw4">int</span> n<span class="br0">&#41;</span>
 101 <span class="br0">&#123;</span>
 102         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>m<span class="br0">&#41;</span> <span class="kw1">return</span> n <span class="sy0">+</span> <span class="nu0">1</span><span class="sy0">;</span>
 103         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>n<span class="br0">&#41;</span> <span class="kw1">return</span> ackermann<span class="br0">&#40;</span>m <span class="sy0">-</span> 1<span class="sy0">,</span> 1<span class="br0">&#41;</span><span class="sy0">;</span>
 104         <span class="kw1">return</span> ackermann<span class="br0">&#40;</span>m <span class="sy0">-</span> 1<span class="sy0">,</span> ackermann<span class="br0">&#40;</span>m<span class="sy0">,</span> n <span class="sy0">-</span> 1<span class="br0">&#41;</span><span class="br0">&#41;</span><span class="sy0">;</span>
 105 <span class="br0">&#125;</span></pre>
 106 </dd></dl>
 107
 108 <p>
 109 This time we have two tail recursive calls, so we need an else.
 110 </p>
 111 <dl class="file">
 112 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=5" title="Download Snippet" class="mediafile mf_c">ackermann_changed.c</a></dt>
 113 <dd><pre class="code file c"><span class="kw4">int</span> ackermann<span class="br0">&#40;</span><span class="kw4">int</span> m<span class="sy0">,</span> <span class="kw4">int</span> n<span class="br0">&#41;</span>
 114 <span class="br0">&#123;</span>
 115         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>m<span class="br0">&#41;</span> <span class="kw1">return</span> n <span class="sy0">+</span> <span class="nu0">1</span><span class="sy0">;</span>
 116         <span class="kw1">if</span><span class="br0">&#40;</span><span class="sy0">!</span>n<span class="br0">&#41;</span> ackermann<span class="br0">&#40;</span>m <span class="sy0">-</span> 1<span class="sy0">,</span> 1<span class="br0">&#41;</span><span class="sy0">;</span>
 117         <span class="kw1">else</span> ackermann<span class="br0">&#40;</span>m <span class="sy0">-</span> 1<span class="sy0">,</span> ackermann<span class="br0">&#40;</span>m<span class="sy0">,</span> n <span class="sy0">-</span> 1<span class="br0">&#41;</span><span class="br0">&#41;</span><span class="sy0">;</span>
 118 <span class="br0">&#125;</span></pre>
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 126 </div>
 127 <!-- SECTION "Tail Recursion Without Return" [643-1568] -->
 128 <h2><a name="minimal_factorial_program" id="minimal_factorial_program">Minimal Factorial Program</a></h2>
 129 <div class="level2">
 130 <dl class="file">
 131 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=6" title="Download Snippet" class="mediafile mf_c">f.c</a></dt>
 132 <dd><pre class="code file c">main<span class="br0">&#40;</span>n<span class="br0">&#41;</span><span class="br0">&#123;</span><span class="kw1">return</span> <span class="sy0">!</span>n<span class="sy0">?:</span>n<span class="sy0">*</span>main<span class="br0">&#40;</span>n<span class="sy0">-</span>1<span class="br0">&#41;</span><span class="sy0">;</span><span class="br0">&#125;</span></pre>
 133 </dd></dl>
 134 <pre class="code bash">$ <span class="kw2">gcc</span> <span class="re5">-o</span> f f.c
 135 $ .<span class="sy0">/</span>f<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 136 1
 137 $ .<span class="sy0">/</span>f .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 138 2
 139 $ .<span class="sy0">/</span>f . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 140 6
 141 $ .<span class="sy0">/</span>f . . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 142 24
 143 $ .<span class="sy0">/</span>f . . . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 144 <span class="nu0">120</span></pre>
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 150
 151 </div>
 152 <!-- SECTION "Minimal Factorial Program" [1569-1806] -->
 153 <h3><a name="explanation" id="explanation">Explanation</a></h3>
 154 <div class="level3">
 155 <ul>
 156 <li class="level1"><div class="li"> main return type defaults to int</div>
 157 </li>
 158 <li class="level1"><div class="li"> main first parameter defaults to int and is the number of command line arguments</div>
 159 </li>
 160 <li class="level1"><div class="li"> ”!n” is the same as “n == 0”</div>
 161 </li>
 162 <li class="level1"><div class="li"> “x ? : y” is the same as “x ? x : y”</div>
 163 </li>
 164 <li class="level1"><div class="li"> $? is the return value of the program</div>
 165 </li>
 166 </ul>
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 172
 173 </div>
 174 <!-- SECTION "Explanation" [1807-2068] -->
 175 <h2><a name="minimal_fibonacci_program" id="minimal_fibonacci_program">Minimal Fibonacci Program</a></h2>
 176 <div class="level2">
 177
 178 <p>
 179
 180 Same idea of the factorial.
 181 </p>
 182 <dl class="file">
 183 <dt><a href="/~kssilveira/site/doku.php?do=export_code&amp;id=c&amp;codeblock=8" title="Download Snippet" class="mediafile mf_c">f.c</a></dt>
 184 <dd><pre class="code file c">main<span class="br0">&#40;</span>n<span class="br0">&#41;</span><span class="br0">&#123;</span><span class="kw1">return</span> n<span class="sy0">&lt;</span>2<span class="sy0">?:</span>main<span class="br0">&#40;</span>n<span class="sy0">-</span>1<span class="br0">&#41;</span><span class="sy0">+</span>main<span class="br0">&#40;</span>n<span class="sy0">-</span>2<span class="br0">&#41;</span><span class="sy0">;</span><span class="br0">&#125;</span></pre>
 185 </dd></dl>
 186 <pre class="code bash">$ <span class="kw2">gcc</span> <span class="re5">-o</span> f f.c
 187 $ .<span class="sy0">/</span>f<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 188 1
 189 $ .<span class="sy0">/</span>f .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 190 2
 191 $ .<span class="sy0">/</span>f . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 192 3
 193 $ .<span class="sy0">/</span>f . . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 194 5
 195 $ .<span class="sy0">/</span>f . . . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 196 8
 197 $ .<span class="sy0">/</span>f . . . . .<span class="sy0">||</span><span class="kw3">echo</span> <span class="re4">$?</span>
 198 <span class="nu0">13</span></pre>
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 205 </div>
 206 <!-- SECTION "Minimal Fibonacci Program" [2069-] -->