Why is this line so inefficient

3 次查看（过去 30 天）
John Gilmore
John Gilmore 2022-7-26
评论： David Hill 2022-7-27
I have a very simple equation that I'm coding in MATLAB that I wouldn't think should be taking as much time as it is. The initial line is in a for loop so I vectorized the problem and that turned out to be even slower (which I was pretty surprised by). Does anyone see anything obvious that I'm not considering for why this is so slow? Typically values for n and m are between 0-10
% Rnm = zeros(size(rho));
% MaximumExponent = (n-m)/2;
% s = 0:MaximumExponent;
% numerator = (-1) .^ s .* factorial(n-s);
% denominator = factorial(s) .* factorial((n+m) / 2-s) .* factorial((n-m) / 2-s);
% Rnm = sum((numerator ./ denominator) .* rho .^ (n-2 .* s), 2);
Rnm2 = zeros(size(rho));
for s=0:(n-m)/2
numerator = (-1)^s * factorial(n-s);
denominator = factorial(s)*factorial((n+m)/2-s)*factorial((n-m)/2-s);
Rnm2 = Rnm2 + (numerator / denominator) * rho.^(n-2*s);
end % for s statement
Here is a profile for the runtime...
EDIT: I should probably also add a couple more details on some things. rho will be a column vector of ~500,000 elements. I imagine the size of rho is alone the reason that most of this takes so long, but it still seems a bit abnormally long to me.
EDIT 2: Should probably include my system details...running Windows 10 Pro, 11th Gen Intel Core i5-1135G7 @ 2.4GHz, 16GB RAM. Nothing to fantastic, but not the worst PC in the world. That's why I'd think this should be running faster...

请先登录，再回答此问题。

回答（1 个）

David Hill
David Hill 2022-7-26
Ran in:
tic;
rho=rand(500000,1);n=9;m=3;
s=0:(n-m)/2;
numerator = (-1).^s .* factorial(n-s);
denominator = factorial(s).*factorial((n+m)/2-s).*factorial((n-m)/2-s);
Rnm=sum((numerator./denominator).*rho.^(n-2*s),2);
toc;
Elapsed time is 0.109334 seconds.
  4 个评论
显示 2更早的评论
John Gilmore
John Gilmore 2022-7-27
While the script isn't running? Running that command shows nothing.
David Hill
David Hill 2022-7-27
Ran in:
tic;
rho=rand(500000,1);n=9;m=3;
f=factorial(0:n);%could do lookup table for factorials (might help speed slightly)
s=0:(n-m)/2;
numerator = (-1).^s .* f(n-s+1);
denominator = f(s+1).*f((n+m)/2-s+1).*f((n-m)/2-s+1);
Rnm=sum((numerator./denominator).*rho.^(n-2*s),2);
toc;
Elapsed time is 0.102554 seconds.

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Memory Usage 的更多信息

标签

产品


版本

R2022a

Community Treasure Hunt

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

Start Hunting!

Translated by