large file slow running

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Joydeb Saha
Joydeb Saha 2021-9-20
回答： Jan 2021-9-20
latmin=-90; %chooce your location
latmax=90;
longmin=-180; a=1;
longmax=180;
data=zeros(90,90);
for b=1:length(x(:,1))
if (x(b,8)>=longmin && x(b,8)<longmax && x(b,7)>=latmin && x(b,7)<latmax);
for col=1:13
data(a,col)=x(b,col);
end
a=a+1;
end
end
theres a file called x.mat (601760x13). What might be the reason it runs too slow
  1 个评论
KSSV
KSSV 2021-9-20
What you are trying to achieve? It can be achived without loop. You need to give an explanation of your problem. What does x.mat have?

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回答（1 个）

Jan
Jan 2021-9-20
I guess that the iterative growing of the array data slows down the processing.
Remember that the growing of an array required to create a new array and to copy the foermer contents.
x = [];
for k = 1:1e6
x(k) = rand;
end
This creates a new array in each iteration. Although the final array needs 8MB only (8 bytes per double), Matlab has to allocate and to copy sum(1:1e6)*8 bytes, which are more than 4 TB ! The solution is easy: Allocate with the final size:
x = zeros(1, 1e6); % Pre-allocation
for k = 1:1e6
x(k) = rand;
end
In your case you do not know the needed size in advance. You tried to pre-allocate with data=zeros(90,90), but this is not the final size. You could determin the final size:
latmin = -90;
latmax = 90;
longmin = -180;
longmax = 180;
index = (x(:,8) >= longmin & x(:, 8) < longmax & x(:, 7) >= latmin && x(:,7) < latmax);
data = zeros(sum(index), 13);
a = 1;
for b = 1:size(x, 1) % nicer and faster than: length(x(:,1))
if index(b)
for col = 1:13
data(a, col) = x(b, col);
end
a = a + 1;
end
end
But it is easier, nicer and faster to perform the copy directly without a loop:
latmin = -90;
latmax = 90;
longmin = -180;
longmax = 180;
index = (x(:,8) >= longmin & x(:, 8) < longmax & x(:, 7) >= latmin && x(:,7) < latmax);
data = x(index, 1:13);

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