So you want to replace a certain section of your P_MMSI vector with the same random number? Let's look at your original code:
% Code is
if range(Selected_Data1(:,1)) ==0
for Iter=1:length(Selected_Data1)
Don't use length in this context. From the way you're using Selected_Data1 in your if statement it's a matrix. In that case, if you want to replace a number of elements in P_MMSI equal to the number of rows in Selected_Data1, say that.
rng(1);
P_MMSI(Iter) = rand(1);
You don't need to call rng and rand every iteration in your for loop. Generate and store the number once and use that value over and over.
end
end
Here's how I'd probably write this code, assuming that your selected data has no NaN values. If it does you'd want to check that they are all NaN values using isnan. You could put the code that defines the isColumnConstant variable directly in the if statement condition, but I prefer separating them as I find the descriptive variable name makes it easier to read and understand the code. I haven't tested this but I expect it to work.
% If range is 0, all the elements in the vector must equal the first element
%
% Note that this won't work if Selected_Data1 has no rows or no columns
isColumnConstant = all(Selected_Data1(:, 1) == Selected_Data1(1, 1));
if isColumnConstant
% Generate a random 9-digit value
replacementValue = randi([1e8, 1e9-1])
% Fill in the appropriate elements in P_MMSI with the replacement value
P_MMSI(1:height(Selected_Data1)) = replacementValue;
end
