Preallocation of data

2011 4 26
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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

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Matt Fig
Matt Fig 2011-4-26

1 个投票

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!!

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MrPuzzled Marc
MrPuzzled Marc 2011-4-26
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
Matt Fig 2011-4-26
And what are the values for looptimes and last?
MrPuzzled Marc
MrPuzzled Marc 2011-4-26
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
MrPuzzled Marc 2011-4-26
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
MrPuzzled Marc 2011-4-26
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
MrPuzzled Marc 2011-4-26
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
MrPuzzled Marc 2011-4-27
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
MrPuzzled Marc 2011-4-27
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
MrPuzzled Marc 2011-4-27
Hi mr matt, I think replace (M-last) with (5000*looptimes) will yield a better results. Thanks for your help.

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