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Looking for an example showing short-cirkuit (&&) superior to normal (&) operation

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Basically i am just trying to figure out how much of an issue this is, speed increase-wise. But i havent been able to make an example showing why one should bother at all.
My example shows about even performance:
clear all;
clc;
%Number of random numbers generated
T = 5000000;
%Number of times experiment is repeated
N = 20;
%Timer variables for tic/toc
timer1 = zeros(N,1);
timer2 = zeros(N,1);
for j=1:N
y1 = zeros(T,1);
y2 = zeros(T,1);
x1 = normrnd(0,1,T,1);
x2 = normrnd(0,1,T,1);
x3 = normrnd(0,1,T,1);
x4 = normrnd(0,1,T,1);
%Short cirkuited loop
tic
for i = 1:T
if (x1(i)>0 && x2(i)>0 && x3(i)>0 && x4(i)>0)
y1(i) = 1;
end
end
timer1(j) = toc;
%standard loop
tic
for i = 1:T
if (x1(i)>0 & x2(i)>0 & x3(i)>0 & x4(i)>0)
y2(i) = 1;
end
end
timer2(j) = toc;
end
mean(timer1)
mean(timer2)

Risposta accettata

Jos (10584)
Jos (10584) il 7 Gen 2014
In my view, the short-circuit behaviour of && and is not primarily about speed, but more about code readability and ease of code maintenance:
  • The expression A && B forces you to express A and B in full, so that both are convertible to logical scalar values.
  • The expression A & B allows for many misinterpretations, especially when they are vectors.
Example:
x = [true false] ; y = [ true true]
tf = any(x) && all(x) % easy to interpret
tf = x & y % ??

Più risposte (1)

Jacob Halbrooks
Jacob Halbrooks il 7 Gen 2014
The short-circuiting behavior of && is occasionally useful for performance optimization. If you use more intensive conditions, you'll likely see some effect. Here's an example:
T = 100;
q = zeros(1000,1);
tic
for i = 1:T
if (all(q == 1) && (numel(q) > 100))
end
end
toc;
tic;
for i = 1:T
if (all(q == 1) & (numel(q) > 100))
end
end
toc;
It's sometimes valuable to optimize code by structuring the order of conditions according to likelihood and expense. For example, a quick check that often returns false should be evaluated first in a && expression.
Another aspect of short-circuiting that makes it valuable is that it allows you to write defensive code to check for the presence of a valid variable to test. For example, let's assume you have a variable v that is intialized to empty but may be set to an object at some point. You can use short-circuiting to write conditional checks like:
if ~isempty(v) && v.DoSomethingFlag
  1 Commento
Matt J
Matt J il 7 Gen 2014
Modificato: Matt J il 7 Gen 2014
Another aspect of short-circuiting that makes it valuable is that it allows you to write defensive code to check for the presence of a valid variable to test.
You don't really need explicit short-circuiting for this. IF statements always reinterpret & as && as necessary. For example, these commands give no errors,
v=[];
if ~isempty(v) & v.DoSomethingFlag,
disp 'never reached',
end

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