what's wrong with the code?

Question

B 2015-3-20

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https://ww2.mathworks.cn/matlabcentral/answers/184121-what-s-wrong-with-the-code

评论： B 2015-3-20

Hi, what's wrong with the following code?

=========================================================================================================x=pi/4; n=2; sum=2; while(1) m=2*n-1; sumnew=sum+(-1)^(n-1)*x^m/factorial(m); error=(sumnew-sum)/sumnew; n=n+1; sum=sumnew; if error<0.001; break end end

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Answer 1

Andrei Bobrov 2015-3-20

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https://ww2.mathworks.cn/matlabcentral/answers/184121-what-s-wrong-with-the-code#answer_172016

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Try it

x= pi/4;
n = 2;
sum1 = 2;
er = 1;
while  abs(er) > 0.001
    m = 2*n-1;
    sumnew = sum1+(-1)^(n-1)*x^m/factorial(m);
    er = (sumnew-sum1)/sumnew;
    n = n+1;
    sum1 = sumnew;
end

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Answer 2

Roger Stafford 2015-3-20

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https://ww2.mathworks.cn/matlabcentral/answers/184121-what-s-wrong-with-the-code#answer_172018

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If you were trying to calculate sin(pi/4), you started with the wrong values for n and sum1. For sin(pi/4) it should be the following. (As yours stands, it computes 2+sin(pi/4)-pi/4.)

   x=pi/4; 
   n=1;    % <-- Start with n = 1
   sum1=0; % <-- Start with sum1 = 0
   while(1)
     m=2*n-1;
     sumnew=sum1+(-1)^(n-1)*x^m/factorial(m);
     error=abs((sumnew-sum1)/sumnew);  % <-- Also this should be absolute value
     n=n+1;
     sum1=sumnew;
     if error<0.001; break end 
   end