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How to Print Prime Numbers, a C++ Example

Posted by Mike on Sep 20th 07:18:20
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The following code uses the Sieve of Eratosthenes method to print all prime numbers in the range of 2 to n, while n being the upper limit, command line integer you feed into the program. You could also just hard code n if you wanted to. Since it's in C++, this code can easily be ported to PHP or other languages that share similar syntax.

  1. int main(int argc, char *argv[]) {
  2.  
  3.         int n;
  4.         n = atoi(argv[1]);
  5.  
  6.         int myarray[n];
  7.  
  8.         // populate the array
  9.         for(int i=0, j=2; j<=n; i++, j++) {
  10.                 myarray[i] = j;
  11.         }
  12.  
  13.         // mark the multiples with 1's
  14.         for(int i=2; i<=n; i++) { // run through each number, 2 through n
  15.                 for(int j=0; j<(n-1); j++) { // run through each element of your array
  16.                         if(myarray[j]>1 && myarray[j]!=i && myarray[j]%i==0) {
  17.                                 myarray[j] = 1; // mark the multiple
  18.                         }
  19.                 }
  20.         }
  21.  
  22.         // print out the elements that don't have 1's
  23.         for(int i=0; i<(n-1); i++) {
  24.                 if(myarray[i] != 1) {
  25.                         printf("%d ", myarray[i]);
  26.                 }
  27.         }
  28.  
  29.         return 1;
  30. }

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