MATLAB: How to manipulate a matrix under certain conditions to fills gaps with numbers.

MATLABmatrix manipulation

M is a small matrix. i am working on Big Matrix. My focus is on central (Middle) column. Using mid column, i want to fill (repalce '0' with '3' & '4') according to their positions in the mid colums with to respect to all rows. 
M = [0     3     0
     0     3     0
     3     0     0
     0     0     0
     0     3     0
     3     3     3
     0     0     0
     0     0     0
     0     0     0
     0     3     0
     3     3     0
     0     0     0
     3     0     0
     0     3     0
     0     3     3
     0     0     0
     0     0     0
     0     3     0
     0     0     0]
In first occurence of 3 and next occurence of 3 should be filled with 3s. carry on the same mid column, in next occurence from first to next one (6th to 10th rows)should be filled with 4s. Continue by same column, 13 th and 14th rows should be filled as 4 while next occurence of 3 and next 3s gaps( 17th to 19th rows) should be filled with 3
I want to get the 'required colum' where 0's are replaced with bold numbers 3s and 4s with respect to their positions of rows in the mid colum of Matrix M.
Your help and cooperation will be highly appreacitead. Regards in advance. 
  Mid column              Required 
      3                      3 
      3                      3 
      0                      3 
      0                      3 
      3                      3 
      3                      3 
      0                      4 
      0                      4 
      0                      4 
      0                      4 
      3                      3 
      3                      3 
      0                      4 
      0                      4 
      3                      3 
      3                      3 
      0                      3 
      0                      3 
      0                      3 
      3                      3 
      0                      0

Best Answer

Let MM - your 'required colum'.
MM =[3,3,0,0,3,3,0,0,0,3,3,0,0,3,3,0,0,3,0]';
 
m = MM;
m(m == 0) = nan;
%lo - define the indices of the values that need to be changed: 
lo = fillmissing(m,'previous') == 3 & fillmissing(m,'next') == 3 & isnan(m); 
% ii - assign numbers to intervals:
ii = cumsum([0;diff(lo)>0]).*lo; 
%{
p - count the amount of each value of vector ii:
1 element -  the number of zeros (0's);
2 element the number of values in the first interval.
3 element the number of values in the second interval, etc.:
%}
p = accumarray(ii(:)+1,1);
% Let a - the values for to be changed (vector with length equal max(ii), in our case - 4):
a = [3,4,4,3]'; % HERE fixed..
% Condition for intervals with length equal 1
a(p == 1) = 3;
% Replacement:
MM(lo) = a(ii(lo));
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