How to switch two values in a matrix?

Hi. I have a matrix :

x = [ 6.5 7 23 24 25]

x =

      6.5000    7.0000   23.0000   24.0000   25.0000

However, the numbers inside matrix x are not in the correct spots. I need to move some of them. In order to find out which ones to move, I have to look at another matrix which is called c and c equals to:

c = [ 1 2 3 4 5 ; 
      3 2 4 1 5]

c =  
     1     2     3     4     5
     3     2     4     1     5

I want to write a code which can detect which columns in matrix c do not have the same numbers in its two rows. For example in matrix c the answer is 1st, 3rd, and 4th columns. Following is what I have done.

y = find(c(1,:)~=c(2,:));

Now this line of code helps me to find where in numbers do not match.

Now I want a code which can apply the findings of y to swap the values in matrix x.

In other words, the out put of the code that I am looking for should give me

J = blahblah(x);
J = [ 24 7 6.5 23 25];

This is very important and I have no clue how to do it.

Any help would be appreciated.

Answer 1

[Roger Stafford]

If I understand the method in your example correctly, then using your definition of x and c, the following should accomplish the necessary transformation using matlab's 'circshift' function:

   y = find(c(:,1)~=c(2,:));
   J = x;
   J(y) = J(circshift(y,1));

[Changoleon]

 c = [ 1 2 3 4 5; 3 2 4 1 5];

 x = [ 6.5 7 23 24 25];

 y = find(c(1,:)~=c(2,:));

 J = x;

 J(y) = J(circshift(y,1));

Hi, unfortunately this does not work :/ because x is exactly like J

[Roger Stafford]

Are you quite sure of that Changoleon? On my computer J is changed from x by that last line of code which shifts the three values around. Please check your work again.

[Changoleon]

Hi Roger, Would you please copy and paste the following lines of code into your MATLAB?

 x = [ 6.5 7 23 24 25];
 c = [ 1 2 3 4 5 ; 3 2 4 1 5];
 y = find(c(1,:)~=c(2,:));

 J = x;
 J(y) = J(circshift(y,1));

 J
 x

Now if you click run, the values of J and x will appear in command window and they are the same :/ what am I doing wrong? I also get an orange message in the command window saying:

Warning: CIRCSHIFT(X,K) with scalar K and where size(X,1)==1 will change behavior in future versions. To retain current behavior, use CIRCSHIFT(X,[K,0]) instead. > In Untitled2 (line 9)

[Walter Roberson]

>>  x = [ 6.5 7 23 24 25];
 c = [ 1 2 3 4 5 ; 3 2 4 1 5];
 y = find(c(1,:)~=c(2,:));

   J = x;
   J(y) = J(circshift(y,1));

   J
   x
  J =
                          24                         7                       6.5                        23                        25
  x =
                         6.5                         7                        23                        24                        25

R2016b.

Which version are you using?

[Changoleon]

WoW it works for you. It could be the versions then. Mine is R2015b! I'll download the new version and let you know how it went. Thanks

[Changoleon]

Walter,

Hi, I just tried that in my Laptop which has a R2016a and still x and J are same. What else could be wrong? I have attached a screenshot of my code.

[Walter Roberson]

   J(y) = J(circshift(y,[0, 1]));

[Roger Stafford]

@Changoleon. Try this:

   y = find(c(:,1)~=c(2,:));
   J = x;
   J(y) = J(circshift(y,1,2));

The '2' specifies that shifting is along the 2nd dimension (along the row)

[Changoleon]

Thanks! You're the best

Answer 2

[Walter Roberson]

If you have two indices, I, J, at which you need to swap, then

YourMatrix([I, J]) = YourMatrix([J, I]);

will do the swap.

If you indices are expressed through a row vector of length 2, y, then

YourMatrix(y) = YourMatrix(fliplr(y));

Use flipud if it is a row vector.

This will not work as well with length greater than 2: if your vector were an odd length then one of the elements would be set to itself.

[Changoleon]

As you said the values that I want to change could be more than 2. Up to 75. The method should be all automated. How can I do that?

[Walter Roberson]

The question becomes how to match the source and destination.

The general syntax

YourMatrix(ListOfDestiantions) = Yourmatrix(ListOfCorrespodingSources);

reads everything from Yourmatrix(ListOfCorrespodingSources) before doing any assignments at YourMatrix(ListOfDestiantions), so it is safer than using

for K = 1 : length(ListOfCorrespodingSources)
  YourMatrix(ListOfDestinations(K)) = YourMatrix(ListOfCorrespodingSources(K));
end

The "for" version of it messes up on swapping values.