Calculating primes using dynamic memory allocation - C++ Data Type

C++ examples for Data Type:Pointer

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Description

Calculating primes using dynamic memory allocation

Demo Code

#include <iostream>
#include <iomanip>
#include <cmath>                       // For square root function

int main()/*w w  w  .ja v a2s  . c o m*/
{
  int max {};                       // Number of primes required
  int count {3};                    // Count of primes found
  std::cout << "How many primes would you like? ";
  std::cin >> max;                                           // Read number required and...
  unsigned long long* primes {new unsigned long long[max]};  // ...allocate memory for them

  *primes = 2uLL;                                            // Insert three seed primes...
  *(primes + 1) = 3uLL;
  *(primes + 2) = 5uLL;
  unsigned long long trial {*(primes + 2)};                  // Candidate prime
  bool isprime {false};                                      // Indicates when a prime is found

  unsigned long long limit {};                               // Stores square root of trial
  do
  {
    trial += 2;                                              // Next value for checking
    limit = static_cast<unsigned long long>(std::sqrt(trial));
    int i {};                                             // Index to primes array
    do
    {
      isprime = trial %  *(primes + i) > 0;                  // False for exact division
    } while (primes[++i] <= limit && isprime);

    if (isprime)                                             // We got one...
      *(primes + count++) = trial;                           // ...so save it in primes array
  } while (count < max);

  // Output primes 10 to  a line
  for (int i{} ; i < max ; ++i)
  {
    std::cout << std::setw(10) << *(primes + i);
    if ((i + 1) % 10 == 0)                                   // After every 10th prime...
      std::cout << std::endl;                                // ...start a new line
  }
  std::cout << std::endl;
  delete[] primes;                                           // Free up memory...
  primes = nullptr;                                          // ... and reset the pointer
}

Result


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