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  1. // Lab 01, testing with valgrind
  2.  
  3. // This program uses a Mersenne Twister to generate pseudo-random numbers.
  4. // The fixed seed means it will generate the same sequence every time, on
  5. // any computer system which properly implements the mt19937 algorithm.
  6.  
  7. // The random numbers are used as indices into a block of memory (of size
  8. // 100), but the random numbers will be in the range [0, 110)!  If it can
  9. // successfully initialize 10 elements of the array, it will then generate
  10. // more random numbers and display the values at those indices.  If any of
  11. // this second batch of random indices didn't occur in the first 10, the
  12. // program would be trying to output an uninitialized value.
  13.  
  14. // Eventually something will go wrong.  Your task is to use valgrind to
  15. // determine which problem occurs, and which line of code causes the first
  16. // problem to be reported.
  17.  
  18. // Make sure you don't modify this file!  Adding or removing lines of
  19. // code could cause you to report the wrong line number for your answers.
  20. // The first initialization of an element of data should occur on line 60,
  21. // and the first output of a value from the array should occur on line 72.
  22.  
  23. // To run this test, type the following commands:
  24. //   make valgrind
  25. //   valgrind ./lab1_bad_valgrind |& more
  26.  
  27. // You've probably never seen "|& more" before.  This means "when you run
  28. // the specified program, send (pipe, vertical bar) the normal and error
  29. // output (the ampersand) to the program more".  More is a "paginator"
  30. // that will stop after every page.  This means you don't have to scroll
  31. // back to find the first error, it will display one page and stop, waiting
  32. // for you to decide when to display "more" of the output.  Press the space
  33. // bar to go onto the next page, or "q" (with no quote marks) to quit.
  34.  
  35. // You can also use this if building your program is producing a lot of
  36. // errors and you're having trouble scrolling back.  You can type:
  37. //   make |& more
  38. // If there's no errors, you probably won't have to hit the spacebar or q,
  39. // since there should be less than one page of output.  You can also use
  40. // more with a filename, such as the command:
  41. //   more lab1_bad.cpp
  42.  
  43.  
  44. #include <iostream>
  45. #include <vector>
  46. #include <random>
  47. using namespace std;
  48.  
  49.  
  50. constexpr size_t SIZE = 100;
  51. constexpr size_t MOD = 110;
  52.  
  53.  
  54. int main() {
  55.     mt19937 mt(72521278);
  56.     int *data = new int[SIZE];
  57.     int count = 0;
  58.  
  59.     // Initialize some of the elements, possibly segfaulting
  60.     data[mt() % MOD] = count++;
  61.     data[mt() % MOD] = count++;
  62.     data[mt() % MOD] = count++;
  63.     data[mt() % MOD] = count++;
  64.     data[mt() % MOD] = count++;
  65.     data[mt() % MOD] = count++;
  66.     data[mt() % MOD] = count++;
  67.     data[mt() % MOD] = count++;
  68.     data[mt() % MOD] = count++;
  69.     data[mt() % MOD] = count++;
  70.  
  71.     // Output some of the elements, possibly getting an uninitialized value
  72.     cout << data[mt() % MOD] << endl;
  73.     cout << data[mt() % MOD] << endl;
  74.     cout << data[mt() % MOD] << endl;
  75.     cout << data[mt() % MOD] << endl;
  76.     cout << data[mt() % MOD] << endl;
  77.     cout << data[mt() % MOD] << endl;
  78.     cout << data[mt() % MOD] << endl;
  79.     cout << data[mt() % MOD] << endl;
  80.     cout << data[mt() % MOD] << endl;
  81.     cout << data[mt() % MOD] << endl;
  82.  
  83.     delete [] data;
  84.     return 0;
  85. } // main()
Success #stdin #stdout 0s 5320KB
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https://ideone.com/0sZ14v
language:
C++ (gcc 8.3)
created:
2 days ago
visibility:
public

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