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POPrinting all Possible nCr Combinations in Java
 

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  1. POPrinting all Possible nCr Combinations in Java
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    <p>I'm trying to print out all possibilities of nCr, which are the combinations when order doesn't matter. So 5C1 there are 5 possibilities: 1 , 2, 3, 4, 5. 5C2 there are 10 possibilities: 1 2, 1 3, 1 4, 1 5, 2 3, 2 4, 2 5, 3 4, 3 5, 4 5.</p> <p>I made functions that print what I want for r = 2, r = 3, and r = 4, and I sort of see the pattern, but I cant seem to make a working method for variable r:</p> <pre><code>public void printCombinationsChoose2(int n, int k) //for when k = 2 { for (int a = 1; a &lt; n; a++) { for (int b = a + 1; b &lt;= n; b++) { System.out.println("" + a + " " + b); } } } public void printCombinationsChoose3(int n, int k) //for when k = 3 { for (int a = 1; a &lt; n - 1; a++) { for (int b = a + 1; b &lt; n; b++) { for (int c = b + 1; c &lt;= n; c++) { System.out.println("" + a + " " + b + " " + c); } } } } public void printCombinationsChoose4(int n, int k) //for when k = 4 { for (int a = 1; a &lt; n - 2; a++) { for (int b = a + 1; b &lt; n - 1; b++) { for (int c = b + 1; c &lt; n; c++) { for (int d = c + 1; d &lt;= n; d++) { System.out.println("" + a + " " + b + " " + c + " " + d); } } } } } public void printCombinations(int n, int k) //Doesn't work { int[] nums = new int[k]; for (int i = 1; i &lt;= nums.length; i++) nums[i - 1] = i; int count = 1; while (count &lt;= k) { for (int a = nums[k - count]; a &lt;= n; a++) { nums[k - count] = a; for (int i = 0; i &lt; nums.length; i++) System.out.print("" + nums[i] + " "); System.out.println(); } count++; } } </code></pre> <p>So I think the layout of my last method is right, but I'm just not doing the right things, because when I call <code>printCominbations(5, 2)</code>, it prints</p> <pre><code>1 2 1 3 1 4 1 5 1 5 2 5 3 5 4 5 5 5 </code></pre> <p>when it should be what I said earlier for 5C2.</p> <p><strong>Edit</strong> The last example was bad. This is a better one to illustrate what it's doing wrong: <code>printCombinations(5, 3)</code> gives this:</p> <pre><code>1 2 3 1 2 4 1 2 5 1 2 5 1 3 5 1 4 5 1 5 5 1 5 5 2 5 5 3 5 5 4 5 5 5 5 5 </code></pre> <p>How do I get it to be:</p> <pre><code>1 2 3 1 2 4 1 2 5 1 3 4 1 3 5 1 4 5 2 3 4 2 3 5 2 4 5 3 4 5 </code></pre>
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    1. PO
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    1. USPat Needham
    1. USPat Needham
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      1. POPrinting all Possible nCr Combinations in Java
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    1. CORefer to [Permutation Algorithm][1] [1]: http://stackoverflow.com/questions/3346249/java-permutation-algorithm
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      1. POPrinting all Possible nCr Combinations in Java
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    2. COThat only works for when your r is 2. In my example that doesn't work, the r = 2, but I want it to work for any r
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      1. POPrinting all Possible nCr Combinations in Java
      1. USPat Needham
 

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