My matlab code is attempting to find the indices in a 601 by 1 matrix that correspond to a given value but says the left and right sides have a different number of elements
pH_fine = pH(1):0.01:pH(end);
pH_labvals = [7.72,9.87,7.4,7.63,7.06,6.85,8.29,9.37,11.1];
index_labvals = [];
a = find(pH_fine == 8); %This works perfectly
for i = 1:length(pH_labvals)
index_labvals(i) = find(pH_fine == pH_labvals(i)); %This throws an error
end
Your problem is that the find(pH_fine == pH_labvals(i)) on the right side sometimes doesn't find any match, and returns an empty result for an index, specifically a 1-by-0 row vector. This doesn't match the size of the left side, which is indexing a 1-by-1 element from your vector index_labvals.
You need to check first if the result of find is empty, and decide what you will put in the index vector in that case, like a 0 or NaN. You will also need to deal with find giving you a vector of indices if pH_labvals has the same value repeated. If you simply want to remove repeated values, you could use unique like so:
pH_labvals = unique(pH_labvals, 'stable');
If you're wondering why you're getting an empty result from find, you should read through this post about the perils of floating-point comparison. One possible solution, assuming pH_labvals contains non-repeated values with 2 decimal places of precision, is to first round your pH_fine vector to 2 decimal places:
pH_fine = round(pH(1):0.01:pH(end), 2);
This should allow you to avoid the errors from floating-point comparison.
An alternative approach is to use interp1 for table lookup:
pH = [1,14]; % Not sure what values you use here, it doesn't matter for the example.
pH_fine = pH(1):0.01:pH(end);
pH_labvals = [7.72,9.87,7.4,7.63,7.06,6.85,8.29,9.37,11.1];
index_labvals = interp1(pH_fine,1:numel(pH_fine),pH_labvals,'nearest')
Here, we're finding the nearest index within pH_fine that matches each of the values in pH_labvals. 1:numel(pH_fine) are the indices into pH_fine.
Note that there's no need for a loop, as interp1 will lookup all pH_labvals at once.
