how to preallocate a variable
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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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dpb
2014-3-16
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.
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