How to Preallocate for speed?

I am trying to preallocate the speed in this particular code and couldn't try to get it right.

It keeps saying Index in position 2 exceeds the array bound and the preallocation doesnt seem to happen.

Do you think the code is right? Could you tell me what I am doing wrong?

Thanks in Advance

function cut_in_small_segmets

global membrane

s=size(membrane);

deltaX=round(s(2)/20);

deltaY=round(s(1)/20);

k=zeros(1000);

for i=1:deltaX

for j=1:deltaY

k=k+s;

small{k}=imadjust(membrane((i-1)*deltaY+1:i*deltaY, (j-1)*deltaX+1:j*deltaX));

%% I get error in the " small{k}=imadjust(membrane((i-1)*deltaY+1:i*deltaY, (j-1)*deltaX+1:j*deltaX));" and it says consider preallocating for speed

end

end

adjusted_pict=[small{1:19}; small{20:38}; small{39:57}; small{58:76}; small{77:95}];

s=size(adjusted_pict);

for i=1:s(1)

for j=1:s(2)

membrane(i,j)=adjusted_pict(i,j);

end

end

end

Answer 1

[David Hill]

s=size(membrane);
deltaX=floor(s(2)/20);%should use floor or you will run into indexing problems below
deltaY=floor(s(1)/20);%should use floor or you will run into indexing problems below
small=cell(deltaX*deltaY);%preallocate small cell array
count=0;
for i=1:deltaX
    for j=1:deltaY
        count=count+1;
        small{count}=imadjust(membrane((i-1)*deltaY+1:i*deltaY, (j-1)*deltaX+1:j*deltaX));
    end
end 

[Madhura Ramani]

Thank You. It worked !!!