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c++srand

Strange srand behaviour inside for loop; c++

Just writing a program to shuffle a deck of cards, and got different behaviour depending on whether the RNG was seeded inside or outside of the for loop; ie.

  for(int i = 0; i < 52; i++)
{
  srand(time(0));  
  Card temp = deck[i];
  int toSwap = rand()%52;
  deck[i] = deck[toSwap];
  deck[toSwap] = temp;
}

gives the output

Nine of Hearts
Ace of Clubs
Two of Clubs
Three of Clubs
Four of Clubs

etc, but

void DeckOfCards::shuffle()
{
  srand(time(0));  
  for(int i = 0; i < 52; i++)
  {
  Card temp = deck[i];
  int toSwap = rand()%52;
  deck[i] = deck[toSwap];
  deck[toSwap] = temp;
  }
  currentCard =0;
}

leads to

Ace of Hearts
Queen of Spades
Four of Hearts
Seven of Clubs
Five of Hearts

(the correct functionality). Anyone know why reseeding the RNG would cause this?

Solution

  • As time(NULL) changes only every second, the RNG seed will be the same if the for-loop doesn't take more than a second to complete.