where is the mistake?

function oo(w) 
   if isstring(w)
   


count =0;

while count<15
  
   
    if strcmp(w(count),'r')
      disp('yes')
        
    
        
  count=count+1;
  
    
    
    end
end
   end

Answer 1

[David Hill]

Why not just:

w='abcderfrgrhhr';
l=w=='r'
l =

  1×13 logical array

   0   0   0   0   0   1   0   1   0   1   0   0   1

Answer 2

[DGM]

If you want it to do what your code implies:

w = 'a4jkltr6mramlkrffgajr';
oo(w)

function oo(w)
    if isstring(w) % strings are not chars 
        w = char(w);
    end
    if ischar(w)
        for c = 1:numel(w) % don't use a while loop, don't hard-code variable length
            if w(c) == 'r' % don't need strcmp for single char tests
                disp('yes') % if all you print is 'yes'
            else
                disp('no') % then there's no way to know where it occurred
            end
        end
    end
end

If instead you want a function that's actually useful for something, @David Hill already posted a useful answer.

w = 'a4jkltr6mramlkrffgajr';
idx = letterisr(w)

function out = letterisr(w) % use a meaningful name and provide output
    if isstring(w) % strings are not chars 
        w = char(w);
    end
    if ischar(w)
        out = w == 'r';
    end
end