how to preallocate a variable

Question

sonam s 2014-3-16

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/121796-how-to-preallocate-a-variable

回答： dpb 2014-3-16

采纳的回答： dpb

在 MATLAB Online 中打开

a1=('a':'z').'; b1=('A':'Z').';

for i=1:52

    if(i<=26)
        a(i)=a1(i);
        i=i+1;
    end;
    if(i>26)
        a(i)=b1(i-26);
        i=i+1;
    end;
end;

*in the above code it is getting warning as "a might be growing inside a loop.consider preallocating for speed." *

what is meant by that and please help me how to rectify it

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Answer 1

dpb 2014-3-16

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https://ww2.mathworks.cn/matlabcentral/answers/121796-how-to-preallocate-a-variable#answer_128675

在 MATLAB Online 中打开

It means you're building an array ( a in this case) an element at a time. To do that means that each pass thru the loop the array is augmented by another element causing the previous array to be copied to a new one, the new value inserted and the the old one deallocated. That's a lot of excess overhead when one knows a priori the size of the resulting array.

To preallocate, before you start the loop, just write

a=zeros(52,1);

then you can just populate it w/o causing the reallocation.

But, in Matlab, look to vectorize and eliminate loops entirely -- in your case just write

a=['a':'z' 'A':'Z'].';

and be done with it.