您正在关注此问题

您将在关注的内容源中看到更新。
您可能会收到电子邮件，具体取决于您的通信预设项。

Two for loops taking very long to get the complete solutions

1 次查看（过去 30 天）

显示更早的评论

AtoZ 2018-11-6

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions

⋮

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions

评论： AtoZ 2018-11-7

采纳的回答： madhan ravi

在 MATLAB Online 中打开

The two for loops in the code take very long to complete

clear all
syms a
for k=0:0.1:pi
  for x=0:0.1:pi
    f = sin(k+a) - x*cos(x);
    y=vpasolve(f,a,[0 pi])
  end
end

How to fix this?

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633439

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633439

"How to fix this?"

Do not use vpasolve in a loop.

Seriously, symbolic maths is slow. If you want fast code, find the solution numerically.

AtoZ 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633446

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633446

@Stephen Thanks. I thought I was doing the solution numerically already. :-)

Stephen23 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633450

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633450

编辑：Stephen23 2018-11-6

在 MATLAB Online 中打开

"I thought I was doing the solution numerically already. :-)"

Kind of, but you are finding a numeric solution using symbolic variables:

syms a

Symbolic maths is slow. Why not just write fast numeric code without symbolic variables?

AtoZ 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633453

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633453

Oh got it now. Thanks.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

madhan ravi 2018-11-6

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#answer_345441

⋮

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#answer_345441

编辑：madhan ravi 2018-11-6

在 MATLAB Online 中打开

EDITED

syms k a x
f = sin(k+a) - x*cos(x);
solution=solve(f,a)
x=0:0.1:pi;
k=x;
solutions=vpa(subs(solution),3)

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633416

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633416

编辑：AtoZ 2018-11-6

在 MATLAB Online 中打开

@madhan thanks. Could you please also say something about `cell(1)` and ctr=1; what are these? Secondly,

if isempty(y)==0
      break

Is it excluding the regions with no solutions? Actually there should be some regions where there's no solution, so I want to also know those regions explicitly. and the output format is very long, I tried to put

format short

at different places in your code but it didn't work.

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633420

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633420

在 MATLAB Online 中打开

`cell(1)` and ctr=1;

its called as pre-allocation for increasing speed and efficiency of the loop car is just a basic counter, see edited answer

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633421

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633421

of course it will take time because your loop is huge

Torsten 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633429

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633429

Solve your equation once outside the loop and only substitute k and x values into the solution inside the loop.

Stephen23 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633430

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633430

编辑：Stephen23 2018-11-6

"its called as pre-allocation for increasing speed and efficiency ..."

Nope. Note that cell(1) only creates a 1x1 cell array, so does not preallocate the cell array to its final size, or anything close to it.

https://www.mathworks.com/help/matlab/matlab_prog/preallocate-memory-for-a-cell-array.html

Also note that isempty(y)==0 will always be true, so it serves no useful purpose.

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633432

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633432

在 MATLAB Online 中打开

syms k a x
f = sin(k+a) - x*cos(x);
solution=solve(f,a)
x=0:0.1:pi;
k=x;
solutions=vpa(subs(solution),3)

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633433

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633433

Thank you @Torsten

AtoZ 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633438

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633438

在 MATLAB Online 中打开

@madhan Thanks. This last one was quicker. What does 3 in

solutions=vpa(subs(solution),3)

mean? Sorry but i am very new to Matlab :-)

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633440

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633440

3 represents numbers to be displayed after the decimal point

AtoZ 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633441

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633441

编辑：AtoZ 2018-11-6

在 MATLAB Online 中打开

How can we set a range on `a`? `a` has to be between 0<=a<=pi. just by

solution=solve(f,a,[0 pi])?

Torsten 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633462

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633462

I don't understand why you don't take the code I suggested under

https://de.mathworks.com/matlabcentral/answers/428022-numerical-solution-of-a-trigonometric-equation

It's efficient and takes into account the constraint on a.

AtoZ 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633472

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633472

I actually didn't understand why did we need to use all those conditions and why not just a straightforward solution like this one. I am actually trying to get the full solutions in the form of a table(x,k). I will try to do it if I can. Or if you know how, you could post there and I will accept. Thanks

Torsten 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633481

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633481

编辑：Torsten 2018-11-6

But the thing here is just the same.

You have to manually exclude complex solutions and real solutions that are not in the interval [0:pi]. Or did I get something wrong ?

Stephen23 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633493

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633493

编辑：Stephen23 2018-11-6

"I actually didn't understand why did we need to use all those conditions and why not just a straightforward solution like this one."

Torsten's code is a straightforward solution. It is very simple!

You are failing to take into account the complexity of symbolic maths. Just because it is hidden inside two commands (e.g. syms and vpasolve) does not mean that symbolic maths is simple or fast. You are comparing a few simple lines of numeric code with a huge symbolic black-box, which is about as apples and oranges as you can get. Just because you are not looking at the black box does not mean that you can disregard its effects.

Bruno Luong 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633529

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633529

编辑：Bruno Luong 2018-11-6

'I actually didn't understand why did we need to use all those conditions and why not just a straightforward solution like this one.' Thanks

The apparent complication because that fact that you deliberately restrict the solution a in [0,pi] as show in pour VPA code.

But Torsten code is the most efficient instead of using those horrible vpasove for something that can be computed from formula and few tests.

Torsten solution is fatest, simplest and mots reliable.

madhan ravi 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633531

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633531

I second Stephen and Brunos comment

Bruno Luong 2018-11-6

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633539

⋮

链接

此评论的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/428266-two-for-loops-taking-very-long-to-get-the-complete-solutions#comment_633539

编辑：Bruno Luong 2018-11-6

在 MATLAB Online 中打开

If you need a vectorized Torsen code, here it is

x=0:0.1:pi;
k=(0:0.1:pi)';
b=asin(x.*cos(x));
b(imag(b)~=0)=NaN;
a1 = mod(b-k,2*pi);
a2 = mod(pi-b-k,2*pi);
a = a1;
outside = a1>pi & a2<=pi;
a(outside) = a2(outside);
a(a>pi) =NaN;
close all
surf(x,k,a);
xlabel('x');
ylabel('k');
zlabel('a');