Printing an an array of integers in random order without repeating in C++

Question

I'm attempting to implement the printRandom() function which should repeatedly select and remove a random entry from the array C and then prints out the selected value until all values in the array C have been printed. They can only be printed one time each.

I can only use cstdlib and iostream. This is my code for the function thus far:

void printRandom(int C[], int n, int seed) {
srand(seed);
int test[10] = { 21,-34,2,-42,89,24,11,4,13,-18 };

for (int i = 0; i < 10; i++)
{
    int r = rand() % 10;
    for (int j = 0; j < 10; j++) {
        if (r != C[j]) {
            C[i] = test[r];
            cout << C[i] << ' ';
        }
    }

}
}

This is the main function I am working with:

int main() {
  int A[10], B[10] ;

  A[0] =  21 ;
  A[1] = -34 ;
  A[2] =   2 ;
  A[3] = -42 ;
  A[4] =  89 ;
  A[5] =  24 ;
  A[6] =  11 ;
  A[7] =   4 ;
  A[8] =  13 ;
  A[9] = -18 ;

  for (int i=0 ; i < 10 ; i++) {
     B[i] = A[i] ;
  }
  printRandom(B,10,38173410) ;

  for (int i=0 ; i < 10 ; i++) {
    B[i] = A[i] ;
 }
  printRandom(B,10,83103131) ;

 for (int i=0 ; i < 10 ; i++) {
    B[i] = A[i] ;
 }
  printRandom(B,10,77192102) ;

return 0 ;
}

Right now I'm getting a large block of numbers

-34 -34 -34 -34 -34 -34 -34 -34 -34 -34 24 24 24 24 24 24 24 24 24 24 13 13 13 13 13 13 13 13 13 13 89 89 89 89 89 89 89 89 89 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 2 2 2 2 2 2 2 2 2 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 4 4 4 4 4 4 4 4 4 4 13 13 13 13 13 13 13 13 13 13 -18 -18 -18 -18 -18 -18 -18 -18 -18 -18 -34 -34 -34 -34 -34 -34 -34 -34 -34 -34 2 2 2 2 2 2 2 2 2 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 4 4 4 4 4 4 4 4 4 4 11 11 11 11 11 11 11 11 11 11 -34 -34 -34 -34 -34 -34 -34 -34 -34 -34 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 21 21 21 21 21 21 21 21 21 21 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 13 13 13 13 13 13 13 13 13 13 -34 -34 -34 -34 -34 -34 -34 -34 -34 -34 89 89 89 89 89 89 89 89 89 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 13 13 13 13 13 13 13 13 13 13 -42 -42 -42 -42 -42 -42 -42 -42 -42 -42 13 13 13 13 13 13 13 13 13 13 89 89 89 89 89 89 89 89 89 89 13 13 13 13 13 13 13 13 13 13

I took some time off of C++ and was wondering if you could help or at least point me in the right direction. Thank you!

@tobi303: Yes, except "I can only use cstdlib and iostream." — Fred Larson
– Fred Larson, Commented Sep 19, 2017 at 14:14
This test, if (r != C[j]), doesn't make any sense. r is an index, but you're comparing it to a value in the array. — Michael Burr
– Michael Burr, Commented Sep 19, 2017 at 14:21
@tobi303 Do note that std::random_shuffle is deprecated currently and will be removed in C++17. std::shuffle is the better option to use. — NathanOliver
– NathanOliver, Commented Sep 19, 2017 at 14:42

463035818_is_not_an_ai · Accepted Answer · 2017-09-19 14:33:49Z

1

Apart from possible bugs your method is rather inefficient, as for example you roll random numbers a lot of times to find the last missing element, when this actually does not require a dice roll at all.

You can avoid this "reroll until index is valid" if you keep track of what indices have been used already. In pseudo code:

void printRandom(int C[], int n, int seed) { 

     D = C;
     size = n;

     for i = 1:n {
          x = rand(  [0... size)  );

          print element at D[x]

          D[x] = D[size-1];

          size--;
     }
}

edited Sep 19, 2017 at 14:33

answered Sep 19, 2017 at 14:27

463035818_is_not_an_ai

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