Array indices must be positive integers or logical values Error

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Abigail Mullen
Abigail Mullen 2018-9-4
回答： Ashutosh Prasad 2018-9-7
The question I am trying to answer is: Write a program to test the two finite difference approximations in section 1.6 (the forward and centered difference formulas). Test your program on the function arctan(x) at x=sqrt(2).Start with h = 1/2, and generate a sequence of approximations by reducing h by a factor of 2 each time, until h = 2−40. Observe how the error changes with h.
So far I have;
f(x)=atan(x);
x=sqrt(2) ;
h=1:.5:2^(-40);
%Forward Euler
F(x)=((f(x+h)-f(x))/h);
I keep getting the array indices must be positive integers or logical values error.
  2 个评论
Adam
Adam 2018-9-4
f(x)=atan(x);
What are you looking to do with this line? f will be an array here and x the index into it. You haven't defined x before this line though immediately afterwards you define it to sqrt(2) which clearly is not an integer to use as an index.
Are you expecting the above line to define a function? If so you would need it to be more like
f = @(x) atan( x );
Abigail Mullen
Abigail Mullen 2018-9-4
I suppose I am just not sure how to do this problem, thank you for your help - I am new to matlab.

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回答（1 个）

Ashutosh Prasad
Ashutosh Prasad 2018-9-7
The error that you are getting is because of the way you are defining the h vector. Your definition creates an empty vector with no elements. But I understand you want h to be halved on each iteration, that's when loops come handy.
Try execution the following script
f = @(x) atan(x);
x = sqrt(2) ;
h = 0.5;
%Forward Euler
while(h >= 2^(-40))
F = ((f(x+h)-f(x))/h);
h = h/2;
end
Let me know if this helps

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