MATLAB: How to change a for loop to a recursive function

cell arraysfor loopfunctionMATLABrecursionrecursive

Hi!
I'm new to Matlab but I'm trying my best to improve as quickly as possible. I'm trying to make a function just by using recursion, which is really new to me. I've got this
res=[0,res]
for N=(length(res):-1:2)
    if res(N)>9
        res(N)=res(N)-10;
        res(N-1)=res(N-1)+1;
    end
end
which in theory will make array 'res' into single digit cells. The value of each original cell never surpasses 18.
This is not for any school project or anything marked. This is an enquiry to improve my matlab skills.
Can you help me? What is the best way to transform this into a recursive function?
Thank you in advance,
EKB

Best Answer

I'm not sure why you would want to replace your loop by a recursive function as it's often less efficient. It's certainly possible, you would call you recursive function with an array shorter by one element, with a carry at each step. Something like:
function res = singledigit(res)
   res = recurse([0 res], 0);
end
function res = recurse(res, carry)
   res(end) = res(end) + carry;
   carry = floor(res/10);
   res(end) = mod(res(end), 10);
   if numel(res) > 1
      res(1:end-1) = recurse(res(1:end-1), carry);
   end
end
I don't see that as any better as your loop. What would be better however would be to vectorise the calculation. As you know that no value is never greater than 18, the carry will never be more than 1 and adding the carry before doing a modulus will never bring any value above 19. Thus the following is guaranteed to work:
res = mod([0 res] + [floor(res/10) 0], 10);
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