How do I turn the for function into a recursive function?

I have a function:

function [ filled ] = travelDistance( blank )
%TRAVELDISTANCE
%   blank: two-dimensional array comprised of -1s, 0s, and 1s
%   filled: blank that is modified (replace every 0 in blank with its
%   distance to the nearest 1, tarting at 2, traveling along cardinal
%   directions without passing through a -1 value)

filled=blank;
[a,b]=size(blank);

for f=1:1000
  for x=2:a
      for y=2:b
          filled(x,y);
          if filled(x,y)==0
              if (filled(x-1,y)==f||filled(x+1,y)==f||filled(x,y-1)==f||filled(x,y+1)==f)
                  filled(x,y)=f+1;
              end
          else
              filled(x,y);
          end
      end
  end
end

But I don't know how to turn it into a recursive function.

I have tried the following:

function [ filled ] = travelDistance( blank )
%TRAVELDISTANCE
%   blank: two-dimensional array comprised of -1s, 0s, and 1s
%   filled: blank that is modified (replace every 0 in blank with its
%   distance to the nearest 1, tarting at 2, traveling along cardinal
%   directions without passing through a -1 value)

filled=blank;
[a,b]=size(blank);

for f=1:1000
  for x=2:a
      for y=2:b
          filled(x,y);
          if filled(x,y)==0
              if (filled(x-1,y)==f||filled(x+1,y)==f||filled(x,y-1)==f||filled(x,y+1)==f)
                  filled(x,y)=travelDistance(filled(x,y)+1);
              end
          else
              filled(x,y);
          end
      end
  end
end

But it does not produce the correct product.

Answer 1

[Walter Roberson]

See https://www.mathworks.com/matlabcentral/answers/361333-help-i-need-to-convert-numbers-into-english-words-using-recursion#answer_285821

[amateurintraining]

I have attempted to edit my function, using a subfunction with the recursion made with your answer. I am still a little confused on the method but it produces an infinite recursion. Here is the new function:

function [ filled ] = travelDistance( blank )
%TRAVELDISTANCE
%   blank: two-dimensional array comprised of -1s, 0s, and 1s
%   filled: blank that is modified (replace every 0 in blank with its
%   distance to the nearest 1, tarting at 2, traveling along cardinal
%   directions without passing through a -1 value)

filled=blank;
[a,b]=size(blank);

for f=1:1000
  for x=2:a
      for y=2:b
          filled(x,y);
          if filled(x,y)==0
              if (filled(x-1,y)==f||filled(x+1,y)==f||filled(x,y-1)==f||filled(x,y+1)==f)
                  filled(x,y)=helper(f,1,f,x,y);
              end
          else
              filled(x,y);
          end
      end
  end
end
end
function filled = helper(initial,increment,final,x,y)
filled=helper(initial+increment,increment,final,x,y);
end