MATLAB: Extract identical values from a data set

arrayfindunique

Hi everyone,
I have a data set for polar coordinates from this command:
[TH,R,Z] = cart2pol(xdata,ydata,zdata);
I then want to find same values of R and corresponding to them values of Z. I have tried these commands:
u=unique(R);
n=histc(R,u);
u(n>1);
find(R==u(n>1))
which gives me an error: Error using == Matrix dimensions must agree.
Error in NSMatlabNPC (line 108) find(R==u(n>1))
And tried to use
find(diff(R)==0)
which returned me an empty matrix 1 by 0.
The problem is probably that I have several identical R of different values and these functions might not be suitable to identify them separately. Are there any other ways I could use to access the same R values?

Best Answer

The error Error using == Matrix dimensions must agree is raised because u(n>1) contains a subset of the elements of R and so these two vectors are of different dimension and when comparing the two with ==, it will fail because of this difference in dimension.
Since R is a distance value (from http://www.mathworks.com/help/matlab/ref/cart2pol.html) then you may have different values that could be considered the same but due to the double data type used you will not be able to group the two as identical using (say) unique. For example,
 1.1234567
 1.1234568
Should the above two numbers be considered the same? If so, then you can group like values in R that should be considered identical, given a tolerance.
We sort the data first, and then do the grouping. If we assume that each of the TH, R, and Z are vectors (as opposed to matrices) we can do
 data = [R Z TH];
Now sort the data as
 sortedData = sortrows(data,1);
In the above, we just sort on the first column of data which corresponds to our R data. We can then iterate over the data and find all those elements that should be considered part of the same group given a tolerance
 myTol       = 0.00001; % use myTol to determine if 2 values should be consided same
 grpStartIdx = 1;       % start index of group
 grpStopIdx  = 1;       % stop index of group
 groupedData = {};      % a cell array for the grouped data
 atGroupIdx  = 0;
 % iterate over each row of sortedData starting at second element
 for k=2:size(sortedData,1)
   % compare the first element with the previous
   if abs(sortedData(k,1)-sortedData(k-1,1)) < myTol
      % the two consecutive elements are considered to be identical
      % so must be part of same group
      grpStopIdx = k;
   else
      % the two consecutive elements are not identical since their difference
      % is greater than myTol so we have come to a new group
      grpStopIdx = k-1;
      % copy all data (R,Z,TH) as a group to the cell array
      atGroupIdx = atGroupIdx + 1;
      groupedData{atGroupIdx} = sortedData(grpStartIdx:grpStopIdx,:);
      % update next group indices
      grpStartIdx = k;
      grpStopIdx  = k;
   end
 end
 % copy the last group
 atGroupIdx = atGroupIdx + 1;
 groupedData{atGroupIdx} = sortedData(grpStartIdx:grpStopIdx,:);
The above may contain a little more code that you wish (and not as neat as using the built-in MATLAB functions), but each group of near-identical R data (as determined by myTol) are combined together in the cell array groupedData for further processing.
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