help with matlab error

2019 4 25
1 个回答
更新时间：2019 5 11
140 次查看（30 天）

0 个投票

Just need help with the problem
1st part of code is in the attahcment and heres the second part
I am getting an error that says "variable reshas an incorrect value" and "output arguement xroot and maybe others not assigned to during call to PA_secant" and "not enough output arguements"

8 个评论
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madhan ravi
madhan ravi 2019-4-25
Attaching a picture is never useful than attaching code/code file.
Walter Roberson
Walter Roberson 2019-4-25
If i==imax the first iteration of the loop, then the break will be taken before anything is assigned to xroot or residual.
Tony Vo
Tony Vo 2019-4-25
function [xroot,residual,ea,iterCount] = PA3_secant(fun,xi,es,max_it,varargin)
%copy/paste the commands for your function here. You can change the names,
%but not the order, of the input and/or output variables in the function
%command to match your code. Do not change the name of the function,
%however, as this exact name (i.e. "PA3_secant") is required for
%the test suite to run.
f=@(x) fun(x);
if (nargin<3)
e = 1e-5
else e=es;
end
x0=xi(1);
x1=xi(2);
i=0;
if (nargin<4)
imax=30;
else imax=max_it;
end
x2=x1+1;
flag=1
while (abs(f(x2))>e)
if(i~=0)
x1=x2;
end
x2=x1-((f(x1)/(f(x1)-f(x0)))*(x1-x0));
x0=x1;
i=i+1;
if (i==imax)
flag=0;
break;
end
if (flag==1)
xroot=x2
else xroot=[];
end
iterCount=i;
residual=f(xroot);
ea=abs((x2-x1)/(x2));
end
Tony Vo
Tony Vo 2019-4-25
So what do you reccomend that I do?
Walter Roberson
Walter Roberson 2019-4-25
What are the input values you are testing with?
Tony Vo
Tony Vo 2019-4-25
used for the first three tests
f = @(x) sin(2*x) - tan(x/4);
[xr,res,ea,iter] = PA3_secant(f, [1 1.1], 1e-3, 50);
Tony Vo
Tony Vo 2019-4-25
this is the problem statement
Write a function m-file that implements the secant method. Your function should accept the following inputs (in order):
  1. A function defining the roots problem
  2. A vector of two initial guesses (first element is the first guess, second element is second guess)
  3. A stopping criterion ( es ) for the numerical solution with a default value of 1e-5
  4. A maximum iteration count for the numerical solution with a default value of 30
  5. An arbitrary number of parameter values associated with the roots problem
Your function should output the following four scalar outputs (in order):
  1. The root estimate
  2. The residual in the numerical solution
  3. The approximate relative error in the numerical solution
  4. The number of iterations required for convergence
Walter Roberson
Walter Roberson 2019-4-25
When I run your code with those parameters, I do not get any error message.
"Variable res has an incorrect value" looks like it might be from the automatic grading system.

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回答（1 个）

Cris LaPierre
Cris LaPierre 2019-5-11
编辑：Cris LaPierre 2019-5-11

0 个投票

It looks like your teacher is using MATLAB Grader for this problem. Because of variable scope in a function, MATLAB Grader is calling your function with inputs your teacher defines, and assessing the outputs. It can not check any other variables. The outputs are captured in variables your teacher has named, which is why you see an error message about a variable your code does not create.
It would appear the output variable they named res is not being assigned the expected value by your function. The inference, then, is that your function has not been properly implemented.
I think it's safe to assume res = residual. Check how you are calculating your residual. Compare to your notes from class. If there is any feedback provided in this assessment test, use that to help find your mistake.

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