Re: Random numbers with no duplicates

From: David White (no_at_email.provided)
Date: 09/27/04 

Date: Mon, 27 Sep 2004 16:14:30 +1000

"Gary Labowitz" <glabowitz@comcast.net> wrote in message
news:oJydnQObS4S3CsrcRVn-tA@comcast.com...
> "David White" <no@email.provided> wrote in message
> news:n7I5d.2844$pl.50584@nasal.pacific.net.au...
> > That's not right. The last card would always be swapped with itself,
> leaving
> > the array as it is.
> >
> > BTW, another feature of this algorithm is that you only need to fill the
> > array once for any number of repeat uses. Once all values have been
> > selected, they are all still in the array. They are in some
pseudo-random
> > order, but that doesn't matter because the starting order is irrelevant
to
> > the randomness of the selections. Just reset the range to 10 and repeat
> the
> > process.
> If on the last step the tenth card is swapped with a randomly selected
> location, there is a pretty good chance that it will not be swapped with
> itself.

On the last step there's only one location left that can be selected.

> The algorithm is to swap each location in the array (from 0 - (n-1)) with
> randomly selected locations in the range 0 - (n-1).

No, the algorithm is to randomly generate an index in the range 0..N-1
inclusive, where N is the number of values remaining, and swap the value at
the selected location with the value at N-1. Then you decrement N, which
places the selected value outside the selectable range for the next
selection. Here are the relevant parts from a card dealer class:

    CardDeck::CardDeck()
    {
        srand(/* suitable seed */);
        cards_remaining = N; // e.g., 52
        for(int icard = 0; icard < N; ++icard)
            deck[icard] = icard;
    }

    int CardDeck::DealCard()
    {
        if(cards_remaining == 0)
            return -1; // error, deck empty
        int icard = rand() % cards_remaining; // crude, and not ideal, but
good enough for most purposes
        int card = deck[icard];
        deck[icard] = deck[cards_remaining-1];
        deck[cards_remaining-1] = card;
        --cards_remaining;
        return card;
    }

    void CardDeck::Shuffle() // i.e., to re-deal
    {
        cards_remaining = N;
    }

DW

Relevant Pages

  • Re: Help end the testing tedium please
    ... When you call it from Consolidate ... Please assume that both the filename and the ... the file really were test1 thru test60, would the array statement need ... With Selection ...
    (microsoft.public.excel.programming)
  • Re: Help end the testing tedium please
    ... Dim n As Integer ... the file really were test1 thru test60, would the array statement need them ... MsgBox, ... With Selection ...
    (microsoft.public.excel.programming)
  • Re: UPGRADE CONTROL CARD
    ... The problem with some Adaptec cards is that they create the array in such a ... different manner from other Adaptec cards that this method may not work. ... when I went to the newer 2120 card it created the array in a totally ...
    (microsoft.public.windows.server.sbs)
  • Re: Excel Cell Formats
    ... Someone suggested that I create a dummy 2D array containing my ... If the latter, I don't think the code will be too bad; however, if the former, then solution will involve more than just swapping cells or rows around. ... Please note that theoretically, my selection can be the entire 65536x256 of data, though I doubt it that any user would have that much data. ... Can there be other data on the rows (either in front of or after the data that you show being rearrange) that will not move when the designated columns are rearranged? ...
    (microsoft.public.excel.programming)
  • Re: How do you work with simple arrays in VBA?
    ... Your array below gets a compile error "Constant Expression ... Required" on the "raySlides(1 To lSlideCount)". ... those slides, and some of those tasks cause loss of slide selection. ...
    (microsoft.public.powerpoint)