How I can get a vector from another through swaps?

Hi! I have two vectors obtained as permutations of 8 numbers: A=[1 5 8 7 4 3 2 6] B=[2 1 4 8 5 3 6 7] How I can get the second from the first with the fewest number of swaps? thanks.

Answer 1

[Azzi Abdelmalek]

Do you mean randomly?

A=[1 5 8 7 4 3 2 6] 
B=A(randperm(numel(A)))

[lucia del gaudio]

thanks for the answer, but the problem is that i have the vectors and i would know how many swaps (exchanges) are needed starting from A to obtain B.

Answer 2

[Image Analyst]

Wouldn't that simply be 7, at most? You simply do a swap of each element of A in turn to put it in the correct location as defined by B. It could take up to 7 swaps to get the first 7 numbers in the right location, and the 8th one will have to be in the correct location because how could it not be if all the others are correct. It could be less than 7 but 7 would be the most.

[lucia del gaudio]

sorry but i'm not very pratical with matlab .. How i can do it?

[Jan]

@lucia: What kind of help do you expect? It is your homework and we cannot solve it for you without forcing you to cheat.

[Image Analyst]

Is this homework? It's tagged as homework. If so, use a for loop. I think it's only like 3 or 4 lines of code. See http://www.mathworks.com/matlabcentral/answers/8626-how-do-i-get-help-on-homework-questions-on-matlab-answers

[Image Analyst]

This works.

A=[1 5 8 7 4 3 2 6] 
B=[2 1 4 8 5 3 6 7] % What we should get.
indexes = 1 : length(A);
for k = 1 : length(B)
  % Find out where in A B(k) occurs
  indexInA = find(B(k) == A)
  % Rather than just do an assignment to an output vector, 
  % we're required by the question to do a swap instead.
  otherIndex = setdiff(.........)
  if isempty(otherIndex)
    break; % ' Quit when we're done.
  end
  % Do the swap
  [A(indexInA), A(k)] = deal(..........)
end

It's a for loop like I said. The hint I'm giving is to use setdiff() to find the indexes that you want to swap, and to use deal() to do the actual swapping itself. See if you can figure out what goes inside. To get the other index, realize that it can't be the same index you're working on or any prior to that, and it can't be the source location that you just found.

[lucia del gaudio]

@Jan Simon: I was just trying an help to understand what commands could be used to solve the problem; that I have written is just one example.

@Image Analyst: thanks a lot for the suggestion! I will try to do in this way.