How to avoid this problem "consider preallocating the memory .for speed" in the below sample code

The code is like this

m = [];
x=[100,200,300.....]

for k= 1:1:142
    if(k >= 142)
        break;
    end
    %
    if x(k)== [5];
        mm = horzcat(mm,xx);
    elseif (x(k+1)  >= 20)
        bb = 10- lj1 ;
        cc = dec2bin(bb,3)-'0';
        mm = horzcat(mm,cc);
        %
    elseif (x(k+2)  >= 10)
        cc = dec2bin(bb,3)-'0';
        mm = horzcat(mm,cc);
        %
    end
end

how the preallocation of 'mm' can be done to increase the speed of execution. Kindly suggest

[KSSV]

Code is not correct...copy the correct code.

[aditya sahu]

SIR @KSSV This is the right code(piece of sample code from the large code)...and the issue is to preallocate the value of 'mm'. As the loop is very large IN THE ORIGINAL CODE 2 problems are coming 1. is it is killing a lot time for executing 2. memory problem...

dd=imread('x.bmp');
qq=reshape(dd',[100],[])';
qq=double(qq);
qq=qq';
ss=dec2bin(qq)-'0';
ss=ss';
pp=dec2bin(qq)-'0';
 xx = [];
 bb = [];
 cc = [];
 mm = [];
  lb = [ 0  21  31]; 
 ub =  [ 20  30  41];
for k=1:3:105
     if(numel(mm) >= 2345) 
        break;
     end
      if(k >= 100) 
        break;
      end
     xx = pp((k+1),6:8);
     mm = horzcat(mm,xx);
    stdif1 = abs(qq(k) -  qq(k+1));
    stdif2 = abs(qq(k+2) -  qq(k+1));

      if (stdif1 >= lb(1)) &&  ( stdif1 <= ub(1)) 
       lj1 = lb(1);
       bb = stdif1 - lj1 ;
       cc = dec2bin(bb,3)-'0'; 
       mm = horzcat(mm,cc);

      elseif(stdif1 >= lb(2)) &&  ( stdif1 <= ub(2)) 
       lj1 = lb(2);
       bb = stdif1 - lj1 ;
       cc = dec2bin(bb,3)-'0'; 
       mm = horzcat(mm,cc);

      else(stdif1 >= lb(3)) &&  ( stdif1 <= ub(3)) 
         lj1 = lb(3);
       bb = stdif1 - lj1 ;
       cc = dec2bin(bb,3)-'0'; 
       mm = horzcat(mm,cc);
      end
  end

mm=reshape(mm,3,[])';

[Steven Lord]

Can you explain the purpose of your code in words? If we know the underlying problem you're trying to solve, we may be able to suggest an alternative approach that avoids growing memory each time through the loop (potentially by avoiding the loop entirely!)

Answer 1

[Stalin Samuel]

Refer the below url which has example of Preallocation

1. pre-allocate memory when using MATLAB
2. Preallocation

Answer 2

[KSSV]

If you are aware of the dimensions of the allocation preallcoate it using:

x = zeros(m,n) ;

If you are not aware of dimensions, and you know that it is either a row or a column vector:

x = zeros([],n) ;    % a row row vector 
x = zeros(m,[]) ;   % a column vector

If you are not sure of dimensions, allocate it as cell:

    x = cell([],n) ; 
    x = cell(m,[]) ;