how do i solve this coding problem?

Question

GISEOK LEE 2016-10-27

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/309490-how-do-i-solve-this-coding-problem

回答： Nick Counts 2016-11-5

clear all, close, clc, 
syms f(x),
syms a, 
syms b;
f(x) = x-2*sin(x);
x1=1.0; x2=2.0; iternum = 0;
if(f(x1)*f(x2)<0)
    x3=(x1*f(x2)-x2*f(x1))/(f(x2)-f(x1));
else
    printf('false'\n);
end;    
while(1)       
     A = [x1^2 x1 1; x2^2 x2 1; x^3 x3 1];
     B = [f(x1); f(x2); f(x3)];
     X_co = inv(A)*B;
     a = X_co(1,1); b = X_co(2,1); c = X_co(3,1);
     X = [-b+sqrt(b^2-4*a*c)/(2*a); (-b-sqrt(b^2-4*a*c))/(2*a)];
     x4 = X(1,1); x5 = X(2,1);
      iternum = iternum +1;
     if((x4>x1) && (x4 < x2))
        x1 = x4;
     else
        x1 = x5;
     end;    
     if(abs(f(x1))<0.01)
         break;
     end;      
end;

the resualt is conversion to logical from sym is not possible

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Answer 1

Nick Counts 2016-11-5

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/309490-how-do-i-solve-this-coding-problem#answer_242053

when you write an expression like (x4 < x1) when both x4 and x1 are sym objects, Matlab builds a new symbolic expression. It does not evaluate that expression.

It's just like you wrote "2sin(t) < 1"

Is that true or false?

There's no way to answer that without more information.

To evaluate a symbolic expression you need to use something like solve

Also look at your while loop. You are changing iternum each iteration, but that value is never used, so each time through the loop the conditions are identical. If the loop didn't break out the first time, it would never break out!

Definitely check out the documentation for sym / syms. You could also run these checks by stepping through a bunch of x values, which is kind of brute force, but sometimes brute force is the fastest/easiest approach

Good luck!