How to swap two items in multiple lists? C#

Hello everyone!

Lets say I have this data:

List:

ListOfPoints = a
ListOfPoints = b
ListOfPoints = c
ListOfPoints = d

Now what I'm trying to do is: swap two points in every list (a,b,c,d), but my approach is not working.

Here is my code:

List<List<Point3d>> currentGeneration = handoverPopulation.ToList();

foreach (List<Point3d> generation in currentGeneration)

{

Random r = new Random();

int index1;

int index2;

index1 = r.Next(0, generation.Count);

index2 = r.Next(0, generation.Count);

if (index1 != index2)

{

Point3d cache = generation[index1];

generation[index1] = generation[index2];

generation[index2] = cache;

}

Why does it not work? The list before and after swapping are still the same (see Picture below). I tried it with a for Loop and put <Random> out of the Loop, however no difference.

Thanks for your help,

Tülin

Replies to This Discussion

Permalink Reply by David Rutten on September 6, 2016 at 9:19am

I can't see anything intrinsically wrong with the code, and since I can't replicate the problem I can't comment further. Perhaps lists are getting copied somehow? What type is handoverPopulation? How are you assigning outputs?

As a general rule though I'd recommend re-using a Random object as often as possible and construct it with a specific seed. If you don't use seeds then you won't be able to repeat a specific run, and if you construct new random instances quickly then they may end up using the same seed because if you don't supply one, the system clock will be used instead.

Permalink Reply by David Rutten on September 7, 2016 at 3:34am

Oh I see now. You're not duplicating the lists of points so your two references are to the same list in memory. Ie. the swapping works just fine, BOTH outputs are swapped.

Permalink Reply by David Rutten on September 7, 2016 at 3:47am

I'm unable to parse the intent of this code, can you elaborate what it is supposed to do:

int n = population[column].Count;
while (n > 0)
{
byte[] box = new byte[1];
do provider.GetBytes(box);
while (!(box[0] < n * (255 / n)));
int k = (box[0] % n);
n--;
Point3d value = individual[k];
individual[k] = individual[n];
individual[n] = value;
}

Permalink Reply by David Rutten on September 7, 2016 at 3:48am

Is that just an attempt to randomize the individual?

Permalink Reply by David Rutten on September 8, 2016 at 2:55pm

Ok, then I guessed right. The code I posted shows how to randomize the order of an array, basically using the Array.Sort method and an key array of random bytes.

Permalink Reply by David Rutten on September 7, 2016 at 4:02am

I attached a modified version which (amongst other changes) contains a DuplicatePopulation() method. You need to use this if you want to modify individuals in one generation without affecting individuals in other generations.

Attachments: