Error while writing code for bounding phase algorithm

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Satya Venkata Siddhardha Manchala
回答： ADITYA SHARMA 2022-2-24
x0=0;
step_size=input('Enter step size ');
a=x0-abs(step_size);
b=x0+abs(step_size);
F_x0=feval(objF,x0);
F_a=feval(objF,a);
F_b=feval(objF,b);
figure; hold on;
if (F_a>=F_x0 && F_x0>=F_b)
step_size = 1*step_size;
else
step_size = -step_size;
end
k=0;
F_p = feval(objF,k);
F_q = feval(objF,k+1);
for k = 0:10
if F_q<F_p
x(0)=zeros(1, 1000000);
x(k+1)=x(k)+2^k*step_size;
else
xleft=x(k-1);
xright=x(k+1);
end
end
ObjF is
function y=objF(x)
y=x^5-5*x^3-20*x+5;
Now matlab states that there are errors at F_x0=feval(objF,x0); and at y=x^5-5*x^3-20*x+5;
I absolutely cannot fathom the reason for these error messages. Can someone help me?

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采纳的回答

Prem Kumar Tiwari
Prem Kumar Tiwari 2018-9-28
It would've been great if you could've stated the error that Matlab shows. However, the issue is most probably because in a call to feval, the first argument has to be either a string that is name of the function like 'objF' or handle of the function like '@objF' without quotes of course.
To learn more about feval, check out the documentation here at : feval - Documentation
  1 个评论
Walter Roberson
Walter Roberson 2018-9-28
Right.
These days if you are given a function handle, you should typically use function syntax instead of feval. Like if
F = @objF;
then instead of
F_x0=feval(objF,x0);
you should code
F_x0 = F(x0);
feval() should be reserved for one of two cases:
1) You are doing some wizardry involving the symbolic engine, in which case your command would start feval(symengine, ....)
2) You are specifically trying to design a program that has to be able to accept function names as character vectors instead of the better function handles. Using function names as character vectors has a bunch of limitations, and is not recommended, but some people are given mandates to program for compatibility with MATLAB 4 before function handles existed.

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更多回答（1 个）

ADITYA SHARMA
ADITYA SHARMA 2022-2-24
%Try this code.
clc;
clear;
global nf;
nf = 0;
k = 0;
syms x;
fx = input('Enter function in x: ');
b = 2; %Enter base for the exponent;
disp('f(x) = '); disp(fx);
while (1)
x0 = input('Enter initial guess value: ');
d = input('Enter a positive value for increment: ');
xi = [x0-d; x0; x0+d];
fx1 = eval(subs(fx,x,xi(1)));
fx2 = eval(subs(fx,x,xi(2)));
fx3 = eval(subs(fx,x,xi(3)));
fx_eval = [fx1; fx2; fx3];
disp('The column vector[x0-d; x0; x0+d] is xi = ');disp(xi);
disp('The column vector[f(x0-d); f(x0); f(x0+d)] is f(x) = '); disp(fx_eval);
if (fx1>=fx2 && fx2>=fx3)
break;
elseif (fx1<=fx2 && fx2<=fx3)
d = -1*d;
break;
else
disp('Enter new set of intial values: ');
end
end
xi(1) = x0-d;
fprintf('Intial value x0 = %.3f\n',x0);
fprintf('Increment d = %.3f\n',d);
fprintf('=====================================================================================================================================================|\n');
fprintf('\t|Iteration number| \t\t\t\t |x_values|\t\t\t\t\t\t\t\t |Function Evaluations at x_values|\t\t\t\t\t\t |Length|\n');
fprintf('=====================================================================================================================================================|\n');
while(1)
xi(3) = xi(2)+d*b^k;
fx1 = eval(subs(fx,x,xi(1)));
fx2 = eval(subs(fx,x,xi(2)));
fx3 = eval(subs(fx,x,xi(3)));
fx_eval = [fx1; fx2; fx3];
%fx3 = eval(subs(fx,x,xi(3)));
fprintf('\tk = %3d\t\t | \t',k);
fprintf('\t%f\t ',xi);
fprintf('|');
fprintf('\t\t%f',fx_eval); fprintf('\t|'); fprintf('\t\t%f',(xi(3)-xi(2)));
fprintf('\n');fprintf('-----------------------------------------------------------------------------------------------------------------------------------------------------|\n');
% disp('xi = ');disp(xi); disp('f(x) = '); disp(fx_eval);
if (fx3<fx2)
k = k+1;
xi(1) = xi(2); xi(2) = xi(3);
fx1 = fx2;fx2 = fx3;
nf = nf+1;
else
if d>0
fprintf('Minimum point lies in the interval (%.3f,%.3f)\n',xi(1),xi(3));
else
fprintf('Minimum point lies in the interval (%.3f,%.3f)\n',xi(3),xi(1));
end
fprintf('No. of iterations = %d \n',k);
fprintf('No. of function evaluations = %d \n',nf);
break;
end
end

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