Preallocation of data

Hi, I have a code which stated it suggest to preallocate to consider speed. I have a incoming data and I stored as received. Here is the code:

received=fgetl(s);

file= [file received];

How can I preallocate the file when I m receiving? The formation of data should look like file =01010101010101010101010101010000.........

How can i do that?

Thanks

Answer 1

[Matt Fig]

Is this just the same question you have asked twice before? If not, you need to include more code. For example, pre-allocation works when used in a loop, yet I see no loop in what you have posted above. Post the code of the loop where you are doing the concatenation.

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EDIT In response to your comments below...

Yes, I built the array by column, but then reshaped it to a row vector. You get what you want, and this is faster than some complex indexing scheme into C. Just try it before you think it won't end up a row vector!

Idx = 1+5000*looptimes:last+5000*looptimes;
C = zeros(16,last);  % Start out this way, fix in last line...

for ii = 1:length(Idx)
    C(:,ii) = F((-15+16*(Idx(ii))):16*(Idx(ii)));
end
C = reshape(C,1,16*last);  % Make into a row vector!!

[MrPuzzled Marc]

Yes, but I m still blur on it.

C=[];

for i=1+5000*looptimes: last+5000*looptimes

C=[C F((-15+16*(i)):16*(i))];

end

[Matt Fig]

And what are the values for looptimes and last?

[MrPuzzled Marc]

The answer you gave to me is to fill each column but now i want to fill it horizontally. Something like zeros (1,M)....

[MrPuzzled Marc]

Is the size of the file. Last is the remainder after division if the file size cannot be fully divisible by 5000.

[M N]=size(A);

looptimes=fix(M/5000);

last=rem(M, 5000);

[MrPuzzled Marc]

Ok, i finally got what you mean. Thanks. Anyway, preallocation could not be assigned when there is no for loop? Like for example my code earlier on?

received=fgetl(s);

file= [file received];

The received is the received bits from the serial port and then 16bits is stored back onto file. I cannot preallocate it?

[MrPuzzled Marc]

Is this FILE still can be preallocate it for certain values so that when it keep receiving it the data will save the execution time?

[MrPuzzled Marc]

How to preallocate for this one?

[M N]=size(A);

looptimes=fix(M/5000);

last=rem(M,5000);

send_count=1;

C=[];

for i=1+5000*(send_count-1):5000*(send_count)

C=[C F((-15+16*(i)):16*(i))];

end

send_count=1+send_count;

??????

[MrPuzzled Marc]

Hi Mr Matt, I have written this to replace it. However, I m not sure if the minus will minus the data correctly. For example, I have a file size of 6353. My data is divided to send 5000, and 1353(Remainder). However, if i wrote in this manner will it gives the 5000 that i wanted?

Idk=(1+5000*(send_count-1)):(5000*send_count);

C=zeros(16,(M-last));

for i=1:length(Idk)

C(:,i)=F((-15+16*(Idk(i))):16*(Idk(i)));

end

C=char(C);

C=reshape(C,1,16*(M-last));

send_count=1+send_count;

Is the (M-last) will gives the correct front 5000bits that I wanted?

[MrPuzzled Marc]

Hi mr matt, I think replace (M-last) with (5000*looptimes) will yield a better results. Thanks for your help.