matlab structures finding duplicates in one field to search through another field - matlab

Question

I have a large structure with many fields but I need to find the index of the min magnitude at each time interval.
Structure(:).Time = [ 1, 1, 1, 1, 1, 11, 11, 21, 21, 21, 31, 31, 31, 31, 31, ...]
Structure(:).Mag = [ 11, 16, 9, 4, 6, 111, 10, 8, 15, 3, 0, 95, 52, 16, 7, ...]
So the solution should be:
Solutionindex = [ 4, 7, 10, 11, ...]
To correspond with time = 1, Mag = 4; time = 11, Mag = 10; time = 21, Mag = 3; time = 31, Mag = 0.

Answer 1

This sounds like a job for accumarray (and its trusty sidekick unique)!
% Sample data:
Structure = struct('Time', { 1, 1, 1, 1, 1, 11, 11, 21, 21, 21, 31, 31, 31, 31, 31}, ...
'Mag', {11, 16, 9, 4, 6, 111, 10, 8, 15, 3, 0, 95, 52, 16, 7});
[timeVals, ~, index] = unique([Structure(:).Time]); % Find an index for unique times
nTimes = cumsum(accumarray(index, 1)); % Count the number of each unique time
Solutionindex = accumarray(index, [Structure(:).Mag].', [], #(x) find(x == min(x), 1)) + ...
[0; nTimes(1:(end-1))];
And the result:
Solutionindex =
4
7
10
11

Answer 2

With unique, you can get the different time intervals, then some logic and find. In find the second argument is number of indices to return. This is set to 1 to return the first index. If the last index is wanted, add , 'last' behind the 1.
Time = [ 1, 1, 1, 1, 1, 11, 11, 21, 21, 21, 31, 31, 31, 31, 31];
Mag = [ 11, 16, 9, 4, 6, 111, 10, 8, 15, 3, 0, 95, 52, 16, 7];
[uniques,idx] = unique(Time);
Solutionindex = zeros(1,length(uniques));
for ii=1:length(uniques)
Solutionindex(ii) = find(Mag(Time==uniques(ii)) == min(Mag(Time==uniques(ii))),1)+idx(ii)-1;
end
Result:
Solutionindex =
4 7 10 11