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What is it better?

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gianluca
gianluca 2012-3-16
From the computational point of view, is it better one cycle for with following if statements, or several cycles for for each if statement? I think it's better this:
for i = 1:length(GR)
if tool == 1
Bmud(i,1) = B_3_c(1,1) .* (rho(i,1) .* 8.345) + B_3_c(2,1);
elseif tool == 2
Bmud(i,1) = B_3_e(1,1) .* (rho(i,1) .* 8.345) + B_3_e(2,1);
end
end
than
if tool == 1
for i = 1:length(GR)
Bmud(i,1) = B_3_c(1,1) .* (rho(i,1) .* 8.345) + B_3_c(2,1);
end
elseif tool == 2
for i = 1:length(GR)
Bmud(i,1) = B_3_e(1,1) .* (rho(i,1) .* 8.345) + B_3_e(2,1);
end
end

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采纳的回答

Sean de Wolski
Sean de Wolski 2012-3-16
The second one. Every computation you can pull outside fo a for-loop is a good thing!
Also note this can be easily vectorized, for example with tool==2
Bmud(1:length(GR),1) = B_3_e(1,1) .* (rho(1:length(GR),1) .* 8.345) + B_3_e(2,1);
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Jan
Jan 2012-3-16
No, Matlab is not smart for the ordering: it multiplies from left to right. Therefore:
A = rand(2000); b = rand; c = rand;
tic; for i=1:100; B = A * b * c; clear('B'); end; toc
tic; for i=1:100; B = A * (b * c); clear('B'); end; toc
tic; for i=1:100; B = (A * b) * c; clear('B'); end; toc
Sorry, I cannot measure it currently, because the virus-scanner has hijacked my machine.
Daniel Shub
Daniel Shub 2012-3-16
@Jan, I did a similar test. It appears to go left to right.

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更多回答（1 个）

Daniel Shub
Daniel Shub 2012-3-16
I hope this is not homework ...
With only a single for loop you make the tool==1 comparison length(GR) times, while in the dual loop case you only make it once. So there is a slight computational edge. As there are only two multiplications and an addition within the loop (assuming B_3_* are matrices and not functions) that comparison may be a non-trival portion of your compute time ...
Ideally you want to get as much work outside the loop as possible, so a better approach would be:
if tool == 1
m = B_3_c(1, 1).*8.345;
b = B_3_c(2, 1);
N = length(GR);
elseif tool == 2
m = B_3_e(1, 1).*8.345;
b = B_3_e(2, 1);
N = length(GR);
else
N = 0;
end
Bmud = nan(length(GR), 1); % You may have already done the intialization ...
for i = 1:N
Bmud(i, 1) = m*rho(i, 1)+b;
end
So the loop only has 1 multiplication and 1 addition instead of 2 multiplications.
The MATLAB answer of course is to get rid of the loop completely.
if tool == 1
Bmud = (8.345.*B_3_c(1,1).*rho(1:length(GR),1))+B_3_c(2,1);
elseif tool == 2
Bmud = (8.345.*B_3_e(1,1).*rho(1:length(GR),1))+B_3_e(2,1);
end

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