MATLAB: Can you find the error in this code that aims to sum the digits of a string

for loopMATLABwhile loop

I wrote this code in order to sum the digits of any string, and to also create a loop that stops the summing process when the length of the sum is equal to a single digit.
What's wrong?
n = '12361927'
function d = sumdigits(n)
  s=0
  for i=1:length(n)
    s=s+str2double(n(i))
  end
end
s=sumdigits(n)
if length(str2num(s))==1
  s=s
else
  while length(str2num(s))~=1
 
    
    if length(str2num(s))==1
      break
    else
      s=sumdigits(s)
      end
  end
  end

Best Answer

Simpler:
str = '12361927'
while numel(str)>1
    num = sum(str-'0')
    str = num2str(num)
end
Prints this in the command window:
str = 12361927
num =  31
str = 31
num =  4
str = 4
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