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

4 views (last 30 days)
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)

Accepted Answer

Jos (10584)
Jos (10584) on 7 Jan 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 % ??

More Answers (1)

Jacob Halbrooks
Jacob Halbrooks on 7 Jan 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 Comment
Matt J
Matt J on 7 Jan 2014
Edited: Matt J on 7 Jan 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

Sign in to comment.

Categories

Find more on Startup and Shutdown in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!