MATLAB: How to find the index of splitapply

splitapply

I have this code where I split the data into intervals and I look to find the maximum value for each interval as below. I need to find the indexes of the maximum values (Result) as well to collect more information from the original matrix (data). For simplify I created the following example.

x          = round(Data(:,18),0);
y          = Data(:,17);
[uv,~,idx] = unique(x);
[Result]   = splitapply(@max, y, idx);

How can I find the indexes correspond to the Result to get more information from the original data.

Thank you so much for your help.

Best Answer

The indices within each group or the indices over all of y? If the former,


indices   = splitapply(@maxIndex, y, idx);
function out=maxIndex(z)
  [~,out]=max(z);
end

Related Solutions

MATLAB: Find the max value and index of each interval in a big matrix

To be sure: Would this loop solve your problem?

XX = [1   2  3  4  5  1  3]'; 
YY = [10 10 10 20 20 30 20]';
uXX = unique(XX);
Result = zeros(size(uXX));
Index  = zeros(size(uXX));
for k = 1:numel(uXX)
    match            = find(XX == uXX(k));
    [Result(k), idx] = max(YY(match));
    Index(k)         = match(idx);
end

If so, try if this is faster:

XX = [1   2  3  4  5  1  3]'; 
YY = [10 10 10 20 20 30 20]';
[uv, ~, idx]    = unique(XX);
[Result, Index] = splitapply(@maxIndex, [YY, (1:numel(YY)).'], idx)
function [value, index] = maxIndex(z)
  [value, k] = max(z(:, 1));
  index      = z(k, 2);
end

Here the index related to the YY vector is appended as 2nd column, such that the related information is available inside maxIndex also.

Use timeit or tic/toc to compare the runtime with providing 2 separate arrays:

[uv, ~, idx]    = unique(XX);
[Result, Index] = splitapply(@maxIndex, YY, (1:numel(YY)).', idx)
function [value, index] = maxIndex(Y, IndexY)
  [value, k] = max(Y);
  index      = IndexY(k);
end

MATLAB: How to calculate differences in matrix

I think this does what you intend

A = [
1  245
1  370
1  1555
1  1620
2  255
2  295
2  335
2  1375
2  1415
2  1575
2  1615
2  1655
3  250
3  415
3  520
3  565
3  1405
];
out = [[A, [diff(A(:,2)); 0]]; [max(A(:,1))+1 0 0]];
indexToOutAbsVal = find(diff(out(:,1))~=0);

out(indexToOutAbsVal,3) = abs(A(indexToOutAbsVal,2) - A([1; indexToOutAbsVal(1:end-1)+1],2)); out(end,:) = [];

Best Answer

Related Solutions

MATLAB: Find the max value and index of each interval in a big matrix

MATLAB: How to calculate differences in matrix

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