efficient loop - finding min and max index of certain value

I have an array that has for example the following structure:

ic = [ 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3 3 3 3 3 ....... 60000 60000 60000]'

and i want to find the minimum and maximum index of every value in ic. I created the following loop, but it is rather slow:

min_idx = zeros(1, max(ic))';
max_idx = zeros(1, max(ic))';
for n=1:max(ic);
    id = ic;
    id == n;
    idx = find(id);
    min_id = min(idx); %should return the minimum index of every value in ic
    max_id = max(idx); %should return the miximum index of every value in ic
    min_idx(n) = min_id;
    max_idx(n) = max_id;
end

With this code it takes around 320 sec to finish. How can this be made faster to run?

Answer 1

[Stephen23]

Assuming that each value occurs only within one contiguous block:

ic = [1;1;1;1;1;1;1;1;2;2;2;2;2;2;2;3;3;3;3;3;60000;60000;60000];
tmp = diff(ic(:))~=0;
idx_min = find([true;tmp])

idx_min = 4×1

1 9 16 21

●

idx_max = find([tmp;true])

idx_max = 4×1

8 15 20 23

●

Answer 2

[David Hill]

m=find(diff(ic));
max_idx=[m,length(ic)];
min_idx=[1,m];