Iterate though a matrix with repeated values and preform operations on corresponding columns.

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Matlab User
Matlab User on 27 Jan 2016
Edited: Stephen on 9 Feb 2016
Hello,
I have a matrix of three columns and many rows, the third column has several (ordered) repeated values for example A=[0.5 0.2 1; 0.6 1.2 1; 0.3 0.23 1; 0.52 0.64 2; 0.56 0.7 2] etc. I would like to iterate through the matrix, and for each repeated 3rd column preform a operation on the associated first and second column values. My current code is as follows:
IDs=A(:,3);
[rows, columns] = size(IDs);
Count=0;
for i=1:rows
for j=1:columns
if IDs==IDs(i,j)
P(i,j) = polyfit(A(i,1),A(j,2),1);
Count=Count+1;
end
end
T=unique(IDs);
D=[P T];
end
This doesn't return any value of P, although it runs without errors. Any help would be much appreciated. Thanks..

Accepted Answer

Stephen
Stephen on 27 Jan 2016
Edited: Stephen on 27 Jan 2016
There are multiple issues with your code, but they all boil down to the superfluous looping over rows and columns. Doing so means polyfit is applied to scalar values, which is totally useless.
Try this instead:
A = [0.5 0.2 1; 0.6 1.2 1; 0.3 0.23 1; 0.52 0.64 2; 0.56 0.7 2]
X = accumarray(A(:,3),A(:,1),[],@(v){v})
Y = accumarray(A(:,3),A(:,2),[],@(v){v})
C = cellfun(@(x,y)polyfit(x,y,1),X,Y,'UniformOutput',false);
this gives the points fitted in cell arrays X and Y, and the fitted polyfit output (coefficients) in the cell array C. For the example data, it gives these coefficients:
>> C{:}
ans =
2.75000 -0.74000
ans =
1.50000 -0.14000
Plot in a Loop
I also plotted these coefficients and the original data, to check them:
F = 'rg';
V = 0:0.01:1;
for k = 1:numel(C)
plot(X{k},Y{k},['o',F(k)])
hold on
plot(V,polyval(C{k},V),F(k))
end
Plot at Once
You could also plot it without any (explicit) loop:
V = 0:0.01:1;
Z = [X,Y]';
Z(3,:) = {'o'};
plot(Z{:})
hold on
W(2,:) = cellfun(@(c)polyval(c,V),C,'UniformOutput',false);
W(1,:) = {V};
plot(W{:})
This has the advantage that it uses the axes/figure ColorOrder property, so you do not need to specify the colors yourself:
  9 Comments
Stephen
Stephen on 9 Feb 2016
@Matlab User: so you tried "this", but you are not going to tell us what "this" is. Sorry but my mind-reading ability is a bit rusty. Perhaps you tried something, but how am I supposed to know what you tried if you don't actually show it to us? Perhaps it caused an error? Or the output just seemed wrong? You see, without you showing us we have no way of knowing what you did, or what happened when you did "this".
In any case: you have data in cells. So loop over the cell arrays.

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