find first and last value for unique julian date

Question

i have a data set similar to the following:

bthd = sort(floor(1+(10-1).*rand(10,1)));
bthd2 = sort(floor(1+(10-1).*rand(10,1)));
bthd3 = sort(floor(1+(10-1).*rand(10,1)));

Depth = [bthd;bthd2;bthd3];
Jday = [repmat(733774,10,1);repmat(733775,10,1);repmat(733776,10,1)];

temp = 10+(30-10).*rand(30,1);

Data = [Jday,Depth,temp];

where I have a matrix similar to 'Data' with Julian Date in the first column, depth in the second, and then temperature in the third column. I would like to find what are the first and last values are for each unique Jday. This can be obtained by:

Data = [Jday,Depth,temp];

[~,~,b] = unique(Data(:,1),'rows');

for j = 1:length(unique(b));
    top_temp(j) = temp(find(b == j,1,'first'));
    bottom_temp(j) = temp(find(b == j,1,'last'));
end

However, my data set is extremely large and using this loop results in long running time. Can anyone suggest a vectorized solution to do this?

Answer 1

use diff :

% for example
Jday = [1 1 1 2 2 3 3 3 5 5 6 7 7 7];
last = find( [diff(Jday) 1] );
first = [1 last(1:end-1)+1];
top_temp = temp(first) ;
bottom_temp = temp(last);

Note that this solution assumes Jday is sorted. If this is not the case, you may sort Jday prior to the suggested procedure.

Answer 2

You should be able to accomplish this using the occurrence option of the unique function:

[~, topidx, ~] = unique(Data(:, 1), 'first', 'legacy');
[~, bottomidx, ~] = unique(Data(:, 1), 'last', 'legacy');

top_temp = temp(topidx);
bottom_temp = temp(bottomidx);

The legacy option is needed if you're using MATLAB R2013a. You should be able to remove it if you're running R2012b or earlier.