Trying to do central difference using for loop, getting error

Question

Ismail khalil 2016-12-6

0
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https://ww2.mathworks.cn/matlabcentral/answers/315680-trying-to-do-central-difference-using-for-loop-getting-error

评论： Ismail khalil 2016-12-6

m=1;
k=1;
c=1;
x(2)=0;
dx=1;
d2x=-1;
h=0.5;
for i=2:0.1:20
    x(i-1)=x(i)-h*dx+(h/2)*d2x;
    x(i+1)=(1/(m/h^2)+(c/2*h))*((2*m/h^2-k).*x(i)+(c/2*h-m/h^2).*x(i-1));
end
plot(x)

what is wrong with my code, I am getting an error Subscript indices must either be real positive integers or logicals. As you can see I am trying to get a plot of x(i+1) which depends on x(i) and x(i-1), thanks

appreciate all the help

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Jan 2016-12-6

I've formatzted your code using the "{} Code" button. You can do this by your own also.

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Answer 1

David Barry 2016-12-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/315680-trying-to-do-central-difference-using-for-loop-getting-error#answer_246196

You must index into an array/matrix with integer values. In your case you are trying to index into variable x with the variable i but you have asked for i to range from 2 to 20 in 0.1 steps. What are you expecting x(2.1) to give for example?