How can I randomize an array?

Question

Here's the part of the program I'm having problems with:

// precondition: board is initialized
// postcondition: board is shuffled by randomly swapping 20 values
void shuffle(int board[][NCOLS]) {
    int num = rand();
    num = num %6 + 1;
    for (int i = 0; i < 20; i++) {

    }
}

Pretty sure I have it wrong already, I think I may need the rand function but I'm not sure how the for loop would work.

Basically there are 6 pictures and they're in 4 columns, it's a memory game and as of the moment they stay in the same place. I need to make it so that they are random and flipped on the side where you can't see them but I can't figure it out.

I have no idea how to randomize columns especially when they're under the name of simply board and NCOLS.

Are you looking for en.cppreference.com/w/cpp/algorithm/random_shuffle ? — Igor Tandetnik
– Igor Tandetnik, Commented Apr 26, 2015 at 23:34

MSalters · Accepted Answer · 2015-04-27 00:03:55Z

2

I can see why this is hard - random_shuffle prefers 1D arrays, and you have a 2D array. Luckily, since arrays are contiguous, that means a 2D array can also be accessed as a 1D array - it's just NCOLS x NROWS elements in memory:

auto begin = &(board[0][0]);
auto end   = begin + NCOLS*NROWS;

answered Apr 27, 2015 at 0:03

MSalters

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4 Comments

Nick Over a year ago

Where would I put that though MSalters? I'm sorry, I'm a bit new to programming, I'm taking just an introductory course to see how it is. To initalize it i would just do shuffle(board); as well right? but do i put that in the for loop? it doesn't seem to do anything

MSalters Over a year ago

Well, random_suffle already loops for you. It just needs to know which elements it needs to shuffle, so the code above goes right before the call to random_shuffle. Look at Igor Tandetnik's comment, he already linked to the appropriate documentation.

Nick Over a year ago

How do I use random_shuffle in this situation? So far what i made doesn't work and looks like this: random_shuffle(board.begin(), board.end()); which only give the error that board needs a class type

MSalters Over a year ago

@Nick: That error is precisely why I provided you with the two statements above. Since you're using a primitive type (not a class type), you don't have the convenient object.begin() syntax.

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