Help with loop error

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amateurintraining 2017-10-13

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评论： James Tursa 2017-10-14

My function is as follows but keeps coming up with the error that the output argument "approx" is not assigned. How do I change it to produce the desired outputs?

function [ exact, approx, abs_error, terms ] = calcTaylorTerms( x, dx, tol, max_terms, exp_or_sin )
%CALCTAYLORTERMS
%   x: is x
%   dx: delta x
%   tol: the error tolerance to be met
%   max_terms: the maximum number of terms to include before giving up
%   exp_or_sin: a flag that determines whether f(x)=e^x or f(x)=sin(x)
%   exact: analytical solution to f(x+deltax)
%   approx: the Taylor series approximation of f(x+deltax)
%   abs_error: the absolute value of the difference between the exact
%   solution and the Taylor series approximation
%   terms: the number of terms ultimately used in the Taylor series
if isequal(exp_or_sin,"exp")||isequal(exp_or_sin,"sin")
    if isequal(exp_or_sin,"exp")
        exp_or_sin=exp(x);
    elseif isequal(exp_or_sin,"sin")
        exp_or_sin=sin(x);
    end
    abs_error=0;
    n=1;
    count=0;
    terms=1;
    while abs_error>=tol && n>1 && terms>max_terms
        count=count+1;
        terms(count)=exp_or_sin+(1)^(count+1)*(dx^n).*diff(exp_or_sin,x,n)/factorial(n);
        approx(count)=sum(terms);
        n=n+1;
        exact=exp_or_sin(x+dx);
        abs_error=abs((exact-approx)/exact);
    end
    exact=exp_or_sin(x+dx);
    approx=approx;
    abs_error=abs_error
    X=terms;
    X(:)=1;
    terms=sum(X)
else 
    exact=NaN;
    approx=NaN;
    abs_error=NaN;
    terms=NaN;
end
end

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Answer 1

James Tursa 2017-10-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/361252-help-with-loop-error#answer_285740

编辑：James Tursa 2017-10-13

在 MATLAB Online 中打开

Maybe move all of the default stuff prior to your if test. E.g.,

    exact = NaN;
    approx = NaN;
    abs_error = NaN;
    terms = NaN;
    if isequal(exp_or_sin,"exp")||isequal(exp_or_sin,"sin")
        :
       etc

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amateurintraining 2017-10-13

Unfortunately, if I do that it just produces approx=NaN, which means that the variable is still not defined. How do I call a variable out of a while-loop?

James Tursa 2017-10-14

If it is coming back as NaN, that gives you a clue. Namely, that these values were not set inside the while loop. Use the debugger to step through your code and figure out why you are not getting inside that while loop.

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