Error: Index in position 1 is invalid. Array indices must be positive integers or logical values.

Question

Austin Lutterbach 2019-9-12

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480018-error-index-in-position-1-is-invalid-array-indices-must-be-positive-integers-or-logical-values

回答： Star Strider 2019-9-12

I am attempting to evaluate the nth derivative, at the ith x value, and store it. The motivation for this is to calculate the error that arises from approximating the Runge function using Chebyshev points.

My code:

syms x
func = 1 / (1 + 25*x^2);
Errs = zeros(17,200);
i = 0;
j = 0;
for n = 10:10:170
    for x1 = -1:0.001:1
        g = diff(func,n+1);
        Err = (1 / ((2^n)*(factorial(n+1))))*(vpa(subs(g,x,x1)));
        Errs(i,j) = Err;
        j = j + 1;
    end
    i = i + 1;
end
%n=10
plot(xx, Errs(0))

I get the error found in the title. I have very little experience with Matlab, but some experience with other programming languages. How are my indices negative? I define i and j to be equal to zero and increment by 1, so I am not sure where these negative values are coming from. Any help is appreciated, thank you.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Star Strider 2019-9-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480018-error-index-in-position-1-is-invalid-array-indices-must-be-positive-integers-or-logical-values#answer_391500

在 MATLAB Online 中打开

The problem is that ‘i’ and ‘j’ are initially both 0.

I also did what I could to speed up your code, although it is still extremely slow:

syms x
func(x) = 1 / (1 + 25*x^2);
Errs = zeros(17,200);
i = 1;
j = 1;
n = 10:10:170;
x1 = -1:0.001:1;
for k1 = 1:numel(n)
    for k2 = 1:numel(x1)
        g = diff(func,n(k1)+1);
        Err = (1 / ((2^n(k1))*(factorial(n(k1)+1))))*g(x1(k2));
        Errs(i,j) = Err;
        j = j + 1;
    end
    i = i + 1
end

My changes were to make ‘func’ an actual function, so that now ‘g’ is also a function, meaning that you can eliminate the subs call, and the vpa call, that is unnecessary (except possibly with respect to the output of your code at the end).