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

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aditya sahu
aditya sahu 2017-9-14
评论： Steven Lord 2017-9-14
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
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aditya sahu
aditya sahu 2017-9-14
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
Steven Lord 2017-9-14
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!)

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回答（2 个）

Stalin Samuel
Stalin Samuel 2017-9-14
Refer the below url which has example of Preallocation
  1. pre-allocate memory when using MATLAB
  2. Preallocation
KSSV
KSSV 2017-9-14
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,[]) ;

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