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  1. /*
  2.  * Lazy generation of Pythagorean Triples
  3.  * Inspired by http://b...content-available-to-author-only...i.com/2014/04/21/getting-lazy-with-c/
  4.  * John Zwinck, 2014-04-22
  5.  * 
  6.  * This demonstrates a different, perhaps more C++ish solution to lazily
  7.  * generate a sequence.  This approach does no dynamic memory allocation.
  8.  * The main idea is to implement a custom forward iterator (STL compatible).
  9.  *
  10.  * Separation of concerns are handled thusly:
  11.  * 1. Generating Pythagorean Triples is done by our triple_iterator.
  12.  * 2. Printing is handled by std::ostream_iterator<>.
  13.  * 3. Limiting the number of triples generated is via std::copy_n().
  14.  *
  15.  * As in the original version, the algorithm is naive.
  16.  */
  17.  
  18. #include <algorithm>
  19. #include <iostream>
  20. #include <iterator>
  21.  
  22. using namespace std;
  23.  
  24. struct triple
  25. {
  26.     int x = 0;
  27.     int y = 0;
  28.     int z = 0;
  29. };
  30.  
  31. ostream& operator<<(ostream& out, const triple& value)
  32. {
  33.     return out << value.x << ", " << value.y << ", " << value.z;
  34. }
  35.  
  36. // unbounded forward iterator producing Pythagorean Triples on demand
  37. class triple_iterator : public iterator<forward_iterator_tag, triple>, triple
  38. {
  39. public:
  40.     triple_iterator() { ++*this; } // initialize to the first valid triple
  41.     const triple& operator*() { return *this; }
  42.  
  43.     // set ourselves to the next Pythagorean Triple
  44.     triple_iterator& operator++()
  45.     {
  46.     	// we must skip certain actions the first time through,
  47.     	// to give the appearance of resuming where we returned the last time
  48.         bool resume = true;
  49.  
  50.         for (; ; ++z) {
  51.             if (!resume) {
  52.                 x = 1;
  53.             }
  54.             for (; x <= z; ++x) {
  55.                 if (!resume) {
  56.                     y = x;
  57.                 }
  58.                 for (; y <= z; ++y) {
  59.                     if (resume) {
  60.                         ++y;
  61.                     }
  62.                     if (x*x + y*y == z*z) {
  63.                         return *this;
  64.                     }
  65.                     resume = false;
  66.                 }
  67.             }
  68.         }
  69.     }
  70. };
  71.  
  72. int main()
  73. {
  74.     // write the first few Pythagorean Triples to stdout
  75.     copy_n(triple_iterator(), 10, ostream_iterator<triple>(cout, "\n"));
  76. }
  77.  
Success #stdin #stdout 0s 3296KB
comments (?)
stdin
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Standard input is empty
stdout
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3, 4, 5
6, 8, 10
5, 12, 13
9, 12, 15
8, 15, 17
12, 16, 20
7, 24, 25
15, 20, 25
10, 24, 26
20, 21, 29
https://ideone.com/T1xVLz
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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