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<p><a href="https://projecteuler.net/problem=21">Problem 21</a> solution
<a href="#details">More...</a></p>
<div class="textblock"><code>#include &lt;stdio.h&gt;</code><br />
<code>#include &lt;stdlib.h&gt;</code><br />
<code>#include &lt;time.h&gt;</code><br />
</div><div class="textblock"><div class="dynheader">
Include dependency graph for sol1.c:</div>
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Functions</h2></td></tr>
<tr class="memitem:aacf4b7e708651d2164e86958f2c29c93"><td class="memItemLeft" align="right" valign="top">unsigned long&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="../../df/d1a/problem__21_2sol1_8c.html#aacf4b7e708651d2164e86958f2c29c93">sum_of_divisors</a> (unsigned int <a class="el" href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a>)</td></tr>
<tr class="separator:aacf4b7e708651d2164e86958f2c29c93"><td class="memSeparator" colspan="2">&#160;</td></tr>
<tr class="memitem:a3c04138a5bfe5d72780bb7e82a18e627"><td class="memItemLeft" align="right" valign="top">int&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="../../df/d1a/problem__21_2sol1_8c.html#a3c04138a5bfe5d72780bb7e82a18e627">main</a> (int argc, char **argv)</td></tr>
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<a name="details" id="details"></a><h2 class="groupheader">Detailed Description</h2>
<div class="textblock"><p><a href="https://projecteuler.net/problem=21">Problem 21</a> solution </p>
<dl class="section author"><dt>Author</dt><dd><a href="https://github.com/kvedala">Krishna Vedala</a> </dd></dl>
</div><h2 class="groupheader">Function Documentation</h2>
<a id="a3c04138a5bfe5d72780bb7e82a18e627"></a>
<h2 class="memtitle"><span class="permalink"><a href="#a3c04138a5bfe5d72780bb7e82a18e627">&#9670;&nbsp;</a></span>main()</h2>
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<td class="memname">int main </td>
<td>(</td>
<td class="paramtype">int&#160;</td>
<td class="paramname"><em>argc</em>, </td>
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<td class="paramkey"></td>
<td></td>
<td class="paramtype">char **&#160;</td>
<td class="paramname"><em>argv</em>&#160;</td>
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<p>Main function </p>
<p>&lt; We use an array of flags to check if a number at the index was: not-processed = 0 is amicable = 1 not amicable = -1</p>
<div class="fragment"><div class="line"><a name="l00037"></a><span class="lineno"> 37</span>&#160;{</div>
<div class="line"><a name="l00038"></a><span class="lineno"> 38</span>&#160; <span class="keywordtype">unsigned</span> <span class="keywordtype">long</span> sum = 0;</div>
<div class="line"><a name="l00039"></a><span class="lineno"> 39</span>&#160; <span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> <a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a> = 500;</div>
<div class="line"><a name="l00040"></a><span class="lineno"> 40</span>&#160; <span class="keywordflow">if</span> (argc == 2)</div>
<div class="line"><a name="l00041"></a><span class="lineno"> 41</span>&#160; <a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a> = atoi(argv[1]);</div>
<div class="line"><a name="l00042"></a><span class="lineno"> 42</span>&#160;<span class="comment"></span> </div>
<div class="line"><a name="l00043"></a><span class="lineno"> 43</span>&#160;<span class="comment"> /**&lt;</span></div>
<div class="line"><a name="l00044"></a><span class="lineno"> 44</span>&#160;<span class="comment"> * We use an array of flags to check if a number at the index was:</span></div>
<div class="line"><a name="l00045"></a><span class="lineno"> 45</span>&#160;<span class="comment"> * not-processed = 0</span></div>
<div class="line"><a name="l00046"></a><span class="lineno"> 46</span>&#160;<span class="comment"> * is amicable = 1</span></div>
<div class="line"><a name="l00047"></a><span class="lineno"> 47</span>&#160;<span class="comment"> * not amicable = -1</span></div>
<div class="line"><a name="l00048"></a><span class="lineno"> 48</span>&#160;<span class="comment"> **/</span></div>
<div class="line"><a name="l00049"></a><span class="lineno"> 49</span>&#160; <span class="keywordtype">char</span> *flags = (<span class="keywordtype">char</span> *)calloc(<a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a>, <span class="keyword">sizeof</span>(<span class="keywordtype">char</span>));</div>
<div class="line"><a name="l00050"></a><span class="lineno"> 50</span>&#160; </div>
<div class="line"><a name="l00051"></a><span class="lineno"> 51</span>&#160; clock_t start_time = clock();</div>
<div class="line"><a name="l00052"></a><span class="lineno"> 52</span>&#160; <span class="keywordtype">int</span> i;</div>
<div class="line"><a name="l00053"></a><span class="lineno"> 53</span>&#160; <span class="comment">/* there are no such numbers till 10. Lets search from there on */</span></div>
<div class="line"><a name="l00054"></a><span class="lineno"> 54</span>&#160; <span class="keywordflow">for</span> (i = 10; i &lt; <a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a>; i++)</div>
<div class="line"><a name="l00055"></a><span class="lineno"> 55</span>&#160; {</div>
<div class="line"><a name="l00056"></a><span class="lineno"> 56</span>&#160; <span class="keywordflow">if</span> (flags[i] != 0)</div>
<div class="line"><a name="l00057"></a><span class="lineno"> 57</span>&#160; <span class="comment">/* already processed, skip */</span></div>
<div class="line"><a name="l00058"></a><span class="lineno"> 58</span>&#160; <span class="keywordflow">continue</span>;</div>
<div class="line"><a name="l00059"></a><span class="lineno"> 59</span>&#160; </div>
<div class="line"><a name="l00060"></a><span class="lineno"> 60</span>&#160; <span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> b = <a class="code" href="../../df/d1a/problem__21_2sol1_8c.html#aacf4b7e708651d2164e86958f2c29c93">sum_of_divisors</a>(i);</div>
<div class="line"><a name="l00061"></a><span class="lineno"> 61</span>&#160; <span class="keywordflow">if</span> (b &gt;= <a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a>)</div>
<div class="line"><a name="l00062"></a><span class="lineno"> 62</span>&#160; flags[i] = -1;</div>
<div class="line"><a name="l00063"></a><span class="lineno"> 63</span>&#160; <span class="keywordflow">else</span> <span class="keywordflow">if</span> (flags[b] == -1)</div>
<div class="line"><a name="l00064"></a><span class="lineno"> 64</span>&#160; <span class="keywordflow">continue</span>;</div>
<div class="line"><a name="l00065"></a><span class="lineno"> 65</span>&#160; </div>
<div class="line"><a name="l00066"></a><span class="lineno"> 66</span>&#160; <span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> c = <a class="code" href="../../df/d1a/problem__21_2sol1_8c.html#aacf4b7e708651d2164e86958f2c29c93">sum_of_divisors</a>(b);</div>
<div class="line"><a name="l00067"></a><span class="lineno"> 67</span>&#160; <span class="keywordflow">if</span> (c == i &amp;&amp; b != i)</div>
<div class="line"><a name="l00068"></a><span class="lineno"> 68</span>&#160; {</div>
<div class="line"><a name="l00069"></a><span class="lineno"> 69</span>&#160; <span class="comment">/* found amicable */</span></div>
<div class="line"><a name="l00070"></a><span class="lineno"> 70</span>&#160; flags[b] = 1;</div>
<div class="line"><a name="l00071"></a><span class="lineno"> 71</span>&#160; flags[i] = 1;</div>
<div class="line"><a name="l00072"></a><span class="lineno"> 72</span>&#160; sum += b + i;</div>
<div class="line"><a name="l00073"></a><span class="lineno"> 73</span>&#160;<span class="preprocessor">#ifdef DEBUG</span></div>
<div class="line"><a name="l00074"></a><span class="lineno"> 74</span>&#160; printf(<span class="stringliteral">&quot;Amicable: %4d : %4d\n&quot;</span>, i, b);</div>
<div class="line"><a name="l00075"></a><span class="lineno"> 75</span>&#160;<span class="preprocessor">#endif</span></div>
<div class="line"><a name="l00076"></a><span class="lineno"> 76</span>&#160; }</div>
<div class="line"><a name="l00077"></a><span class="lineno"> 77</span>&#160; <span class="keywordflow">else</span></div>
<div class="line"><a name="l00078"></a><span class="lineno"> 78</span>&#160; {</div>
<div class="line"><a name="l00079"></a><span class="lineno"> 79</span>&#160; flags[i] = -1;</div>
<div class="line"><a name="l00080"></a><span class="lineno"> 80</span>&#160; <span class="keywordflow">if</span> (b &lt; <a class="code" href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a>)</div>
<div class="line"><a name="l00081"></a><span class="lineno"> 81</span>&#160; flags[b] = -1;</div>
<div class="line"><a name="l00082"></a><span class="lineno"> 82</span>&#160; }</div>
<div class="line"><a name="l00083"></a><span class="lineno"> 83</span>&#160; }</div>
<div class="line"><a name="l00084"></a><span class="lineno"> 84</span>&#160; </div>
<div class="line"><a name="l00085"></a><span class="lineno"> 85</span>&#160; clock_t end_time = clock();</div>
<div class="line"><a name="l00086"></a><span class="lineno"> 86</span>&#160; </div>
<div class="line"><a name="l00087"></a><span class="lineno"> 87</span>&#160; printf(<span class="stringliteral">&quot;\nTime taken: %.4g millisecond\n&quot;</span>,</div>
<div class="line"><a name="l00088"></a><span class="lineno"> 88</span>&#160; 1e3 * (end_time - start_time) / CLOCKS_PER_SEC);</div>
<div class="line"><a name="l00089"></a><span class="lineno"> 89</span>&#160; printf(<span class="stringliteral">&quot;Sum of all numbers = %lu\n&quot;</span>, sum);</div>
<div class="line"><a name="l00090"></a><span class="lineno"> 90</span>&#160; </div>
<div class="line"><a name="l00091"></a><span class="lineno"> 91</span>&#160; free(flags);</div>
<div class="line"><a name="l00092"></a><span class="lineno"> 92</span>&#160; <span class="keywordflow">return</span> 0;</div>
<div class="line"><a name="l00093"></a><span class="lineno"> 93</span>&#160;}</div>
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<h2 class="memtitle"><span class="permalink"><a href="#aacf4b7e708651d2164e86958f2c29c93">&#9670;&nbsp;</a></span>sum_of_divisors()</h2>
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<td class="memname">unsigned long sum_of_divisors </td>
<td>(</td>
<td class="paramtype">unsigned int&#160;</td>
<td class="paramname"><em>N</em></td><td>)</td>
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<p>function to return the sum of proper divisors of N </p>
<div class="fragment"><div class="line"><a name="l00014"></a><span class="lineno"> 14</span>&#160;{</div>
<div class="line"><a name="l00015"></a><span class="lineno"> 15</span>&#160; <span class="keywordtype">unsigned</span> <span class="keywordtype">long</span> sum = 1 + <a class="code" href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a>; <span class="comment">/* 1 and itself are always a divisor */</span></div>
<div class="line"><a name="l00016"></a><span class="lineno"> 16</span>&#160; <span class="comment">/* divisors are symmertically distributed about the square-root */</span></div>
<div class="line"><a name="l00017"></a><span class="lineno"> 17</span>&#160; <span class="keywordflow">for</span> (<span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> i = 2; i * i &lt; <a class="code" href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a>; i++)</div>
<div class="line"><a name="l00018"></a><span class="lineno"> 18</span>&#160; {</div>
<div class="line"><a name="l00019"></a><span class="lineno"> 19</span>&#160; <span class="keywordflow">if</span> ((<a class="code" href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a> % i) != 0)</div>
<div class="line"><a name="l00020"></a><span class="lineno"> 20</span>&#160; <span class="comment">/* i is not a divisor of N */</span></div>
<div class="line"><a name="l00021"></a><span class="lineno"> 21</span>&#160; <span class="keywordflow">continue</span>;</div>
<div class="line"><a name="l00022"></a><span class="lineno"> 22</span>&#160; </div>
<div class="line"><a name="l00023"></a><span class="lineno"> 23</span>&#160; <span class="comment">// #ifdef DEBUG</span></div>
<div class="line"><a name="l00024"></a><span class="lineno"> 24</span>&#160; <span class="comment">// printf(&quot;%4d, %4d,&quot;, i, N / i);</span></div>
<div class="line"><a name="l00025"></a><span class="lineno"> 25</span>&#160; <span class="comment">// #endif</span></div>
<div class="line"><a name="l00026"></a><span class="lineno"> 26</span>&#160; </div>
<div class="line"><a name="l00027"></a><span class="lineno"> 27</span>&#160; sum += i + (<a class="code" href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a> / i);</div>
<div class="line"><a name="l00028"></a><span class="lineno"> 28</span>&#160; }</div>
<div class="line"><a name="l00029"></a><span class="lineno"> 29</span>&#160; <span class="comment">// #ifdef DEBUG</span></div>
<div class="line"><a name="l00030"></a><span class="lineno"> 30</span>&#160; <span class="comment">// printf(&quot;\nSum of divisors of %4d: %4d\n&quot;, N, sum);</span></div>
<div class="line"><a name="l00031"></a><span class="lineno"> 31</span>&#160; <span class="comment">// #endif</span></div>
<div class="line"><a name="l00032"></a><span class="lineno"> 32</span>&#160; <span class="keywordflow">return</span> sum;</div>
<div class="line"><a name="l00033"></a><span class="lineno"> 33</span>&#160;}</div>
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<div class="ttc" id="aproblem__13_2sol1_8c_html_a0240ac851181b84ac374872dc5434ee4"><div class="ttname"><a href="../../db/d01/problem__13_2sol1_8c.html#a0240ac851181b84ac374872dc5434ee4">N</a></div><div class="ttdeci">#define N</div><div class="ttdef"><b>Definition:</b> sol1.c:111</div></div>
<div class="ttc" id="aproblem__21_2sol1_8c_html_aacf4b7e708651d2164e86958f2c29c93"><div class="ttname"><a href="../../df/d1a/problem__21_2sol1_8c.html#aacf4b7e708651d2164e86958f2c29c93">sum_of_divisors</a></div><div class="ttdeci">unsigned long sum_of_divisors(unsigned int N)</div><div class="ttdef"><b>Definition:</b> sol1.c:13</div></div>
<div class="ttc" id="aproblem__23_2sol1_8c_html_a337e545af040d754dc012b42523ee1ee"><div class="ttname"><a href="../../d7/ddb/problem__23_2sol1_8c.html#a337e545af040d754dc012b42523ee1ee">MAX_N</a></div><div class="ttdeci">unsigned long MAX_N</div><div class="ttdef"><b>Definition:</b> sol1.c:13</div></div>
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