why while loop repeats so long?

Question

arian hoseini 2022-11-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1867848-why-while-loop-repeats-so-long

评论： arian hoseini 2022-12-1

i have this code that works correctly but i wrote another one that looks like loop repeats for so long...

clc
clear all
format longG %changes the output display format in the Command Window to the format specified by style.
alpha = [510; 310; 78; 120; 350; 490; 53; 150; 170; 149];    
B = [7.20; 7.85; 7.98; 7.10; 7.30; 6.90; 8.01; 7.30; 7.42; 7.16];
C = [0.00142; 0.00194; 0.00482; 0.00321; 0.00210; 0.00178; 0.00212; 0.00350; 0.00267; 0.00390];
D = [0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001];
Pmin =[150; 100; 50; 50; 100; 100; 100; 100; 100; 100];
Pmax =[600; 400; 200; 200; 350; 500; 300; 300; 300; 300];
n = 10;
Pr = 2500;
L = 8.6;
%-----------------------------program starts here--------------------------
ep=1;
count=0
while  ep>=0.95
     count = count+1;
    for i=1:n
        c=(B(i)-L);b=(2*C(i));a=(3*D(i));
        delta=(b^2-(4*a*c));
        P(i,:)=((-b+sqrt(delta))/(2*a));
        if P(i)< Pmin(i)
            P(i)=Pmin(i);
        elseif P(i)> Pmax(i)
            P(i)=Pmax(i);
        end
        Pt=sum(P);
        ep=Pr-Pt;
        M=sum(ones(length(D(i)),1)/(2*(D(i))));
    end
    DL=ep/M;
    if ep>0
        L=L+DL;
    elseif ep<0
        L=L-DL;
    end
end
disp(L); %show the final Lambda
vpa(P,6)

this one is not working correctly

clc
clear all
format longG
a = [510; 310; 78; 120; 350; 490; 53; 150; 170; 149];    
b = [7.20; 7.85; 7.98; 7.10; 7.30; 6.90; 8.01; 7.30; 7.42; 7.16];
c = [0.00142; 0.00194; 0.00482; 0.00321; 0.00210; 0.00178; 0.00212; 0.00350; 0.00267; 0.00390];
d = [0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001];
Pmin =[150; 100; 50; 50; 100; 100; 100; 100; 100; 100];
Pmax =[600; 400; 200; 200; 350; 500; 300; 300; 300; 300];
n = 10;
Pr = 2500;
L = 8.6;
syms p;
F=sym('F',[1 10]);
X=sym('X',[1 10]);
ep=1;
count=0;
while ep>=0.9
    count=count+1;
    for i=1:10
        F(i)=a(i)+b(i).*p+c(i).*p.^2+d(i).*p.^3;
        D=diff(F(i),p)-L==0;
        S=solve(D,p);
        X(i)=max(S);
        if X(i)<pmin(i)
            X(i)=pmin(i);
        elseif X(i)>pmax(i)
            X(i)=pmax(i);
        end
        pt=sum(X);
        ep=2500-(pt)
        M=sum(ones(length(d(i)),1)/(2*(d(i))));
    end
    DL=ep/M;
    if ep>0
        L=L+DL;
    elseif ep<0
        L=L-DL;
    end
end
disp(L)
vpa(X)

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Torsten 2022-11-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1867848-why-while-loop-repeats-so-long#answer_1116738

在 MATLAB Online 中打开

If you output "count" for the numerical solution, it's about 14000.

Now if you measure the time for one symbolic iteration in the while loop (approximately 1.6 seconds) and multiply this value by 14000 - I guess you will have to wait quite a long time for a result:

1.6*14000/3600
ans = 6.2222

6.2 hours. Wow! So stick to the numerical solution.

Your code would be much faster if you solved the nonlinear equation for L using "fzero", e.g.

L0 = 6.0;
L = fzero(@fun,L0)
L = 8.6749
function res = fun(L)
  B = [7.20; 7.85; 7.98; 7.10; 7.30; 6.90; 8.01; 7.30; 7.42; 7.16];
  C = [0.00142; 0.00194; 0.00482; 0.00321; 0.00210; 0.00178; 0.00212; 0.00350; 0.00267; 0.00390];
  D = [0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001; 0.0000002; 0.0000003; 0.0000001];
  Pmin =[150; 100; 50; 50; 100; 100; 100; 100; 100; 100];
  Pmax =[600; 400; 200; 200; 350; 500; 300; 300; 300; 300];
  Pr = 2500;
  c=B-L;
  b=2*C;
  a=3*D;
  delta=b.^2-(4*a.*c);
  P=(-b+sqrt(delta))./(2*a);
  P(P<Pmin) = Pmin(P<Pmin);
  P(P>Pmax) = Pmax(P>Pmax);
  Pt=sum(P);
  res=Pr-Pt;
end