For loop help needed

Can anyone see a reason why when I implement the following for loop:

for ashift = 1:n
  x = [x(:,1) - x(ashift) x(:,2) - x(n+ashift) ...
  x(:,3) - x(2*n+ashift)];
  x = circshift(x,rowshift)
  rowshift = rowshift - 1;
end

For the instance when n = 4, the output comes in the order of n = 1, n = 2, n = 4, n = 3 (I have checked this many times with manual calclations.

I can't think of any reason why this should be happening. Any help would be greatly appreciated.

Kind regards, Tom

[Azzi Abdelmalek]

Your problem is not clear

[Walter Roberson]

Your x appears to be a matrix, but you are access x(ashift) and x(n_ashift) and x(2*n+ashift), which are linear accesses. Are you sure you want to be switching between 2D coordinates and 1D coordinates in the same expression? If x is n rows down, then the 1D coordinates correspond to x(ashift, 1), x(ashift, 2) and x(ashift, 3) which would be clearer to read.

[Walter Roberson]

What do you initialize rowshift to ?

[Tom]

Yes x is a matrix, it has n columns and always 3 rows (but currently I am sticking with n = 4 for simplicity.

Each row of x represents the xyz coordinates of a vector, and essentially what I need is to place each vector in turn at the origin of coordinates (i.e. make all elements of its row 0) and shift every other vector by the same amount. Once this is done I am placing the vector at the origin at the top of the x matrix with circshift.

I am trying to do this for the 1st row, then the 2nd, 3rd and 4th in turn. But for some reason the 3rd and 4th runs are being switched.

Can you possibly see any reason this might be happening? Or alternatively (if it's not too much to ask) of a simpler way this could be done?

Thanks, Tom

[Tom]

rowshift is initialised to 0

Answer 1

[Sean de Wolski]

Use the debugger!

• Put this in a script or function and save the file.
• Put a break point on the first line (click on the dash next to the line number)
• Run the file and use dbstep or the Step icon to step through the loop and investigate what is happening on each line during each iteration.

[Tom]

This could be helpful, but could you perhaps be more explicit as to how this is done? I have never tried it before.

[Walter Roberson]

Sean did give step by step instructions. Notice the "click on the dash next to the line number", and "dbstep or the Step icon"

[Tom]

Sorry yes I have now managed to use the debugger, but unfortunately still can't find the problem.

[Sean de Wolski]

On each line run the line at the command prompt and figure out what each value is doing.

For example

x(:,1) - x(ashift) x(:,2) - x(n+ashift) x(:,3) - x(2*n+ashift)]

Notice I took the ';' off the end so it prints.

Not lets look inside at it

x(n+ashift)

Divide and conquer until you figure out what's going on.