I did bump into this question while searching for this topic, but this one seems to be outdated.

Reading https://blogs.mathworks.com/loren/2016/10/24/matlab-arithmetic-expands-in-r2016b , implicit expansion was introduced in 2016b, but I can still find the reference codes in the papers using bsxfun for arithmetic expansion. So I assume that there are some circumstances that make bsxfun preferable to other methods.

I did compare the speeds between bsxfun, repmat, and implicit expansion (I used the code of Jonas from the link)

The below shows the comparisson in calculation time using tic toc:

which shows that implicit expansion is clearly faster than bsxfun or repmat. Is there any reason to use bsxfun nowadays?

Here is the code I used to compare the speed:

n = 300;
k=100; %# k=100 for the second graph
a = ones(10,1);
rr = zeros(n,1);
bb = zeros(n,1);
ntt = 100;
tt = zeros(ntt,1);
for i=1:n;
   r = rand(1,i*k);
   for it=1:ntt;
      tic,
      x = bsxfun(@plus,a,r);
      tt(it) = toc;
   end;
   bb(i) = median(tt);
   for it=1:ntt;
      tic,
      y = repmat(a,1,i*k)   repmat(r,10,1);
      tt(it) = toc;
   end;
   rr(i) = median(tt);
   for it=1:ntt;
      tic,
      z = a   r;
      tt(it) = toc;
   end;
   gg(i) = median(tt);
end
figure;
plot(bb,'b')
hold on
plot(rr,'r')
plot(gg,'g')
legend(["bsxfun","repmat","implicit"])

CodePudding user response：

All bsxfun does is Binary Singleton eXpansion. It's more typing than the, now usual, implicit expansion. I'd guess The MathWorks kept bsxfun around for backwards compatibility, but no longer works on it; it might even internally just map to implicit expansion.

The documentation on bsxfun states:

It is recommended that you replace most uses of bsxfun with direct calls to the functions and operators that support implicit expansion. Compared to using bsxfun, implicit expansion offers faster speed of execution, better memory usage, and improved readability of code. For more information, see Compatible Array Sizes for Basic Operations.

More helpful links can be found in this answer by nirvana-msu, amongst others to blogs by MathWorks employees discussing this.

So I'd say that the only reason to use bsxfun instead of implicit expansion would be if you'd run the code on a pre-2016b version of MATLAB.