find first line that satisfies condition; skip next few; then find next line that satisfies condition

I have a long column of #'s. I want to find and replace values >4.8 with 10000 so that later code can find these spots. This works ok:

stim_time = find(TTL_column>4.8);
TTL_column(stim_time) = 10000; 

However, there are ~3-6 points all grouped together that have almost the same time stamp (e.g., 15870,15871,15872,15873). I only want to replace the first one of these with the #10000 (e.g., only the value at 15870 = 10000). I am totally lost on how I'd get this code to replace only the first spot in each group and not the whole group, since each group has an unequal number of points. Any help would be greatly appreciated!

stim_time =

       15870
       15871
       15872
       15873
       49863
       49864
       49865
       49866
       49867

Answer 1

[Matt J]

idx = diff([0;TTL_column(:)>4.8])>0;
TTL_column(idx) = 10000; 

[Kathleen Hupfeld]

thanks SO much! that works perfectly. is there also a straightforward way to count how many substitutions were made?

[Kathleen Hupfeld]

or a way to count how many substitutions were made that's more efficient than this? :)

total = 0; 
for i = 1 : length(idx)
	if idx(i)==1 
		total=total+1;
	end
end
total 

[Matt J]

sum(idx)