Need help with my bisection code

Question

Laurence Guevremont 2016-10-8

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https://ww2.mathworks.cn/matlabcentral/answers/306362-need-help-with-my-bisection-code

回答： Nahid Hasan 2022-2-2

Basically, I use Newton-Raphson method to find roots but if this one fails, my program calls bisection method which is not working. Something is wrong and I can't seem to figure it out. I need to use bisection method to find the roots and my output will be x3, nrfail. The interval is from 0 to 100. If someone could please help. I think I am missing naming the variables and there is an error before the break.

function [ x3, nrfail ] = bisection( fun,x1,y1,x2,y2,tol )
    check=1;
    dyold=1e12;
    nrfail=0;
    tol=1e-12;
      while check > tol
          x3=(x1+x2)/2;
          y3=feval(fun,x3); 
          if y1*y3 < 0
              x2=x3;
              y2=y3;
          else 
              x1=x3;
              y1=y3;
          end % ERROR
          dy=abs(yl-yr);
          if dy>dyold
              nrfail=1;
              break
          end
          check= abs(1-x1/x3);
      end
  end

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Answer 1

Marc Jakobi 2016-10-8

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1. You are "missing" the variable "yr". You have to define it somewhere before using this line:

dy=abs(yl-yr);

2. After the if statement

if dy>dyold
      nrfail=1;
      break
end

you should add the line

dyold = dy;

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Marc Jakobi 2016-10-8

not enough arguments (input or output) means you are using the wrong syntax when calling the function. How do you call the function?

Walter Roberson 2016-10-8

在 MATLAB Online 中打开

Often "not enough arguments" is caused by trying to pass a function into another function, such as if were to call

plot_root_performance(bisection)

intending that bisection be received as a function that could then be called inside plot_root_performance . This syntax instead calls bisection with no arguments and passes whatever it returns as the first argument to plot_root_performance . If you are trying to do something like that then you should use @ to signal that you want a handle to the function passed in, like

plot_root_performance(@bisection)

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