"Index esceeds the number of array elements (1)". When clearly there is capacity for 201 spaces in the array, what is happening!?

clear all
clc 
clf

f = @(x) ((x.^4+x.^2)/(x.^4+1));   % for this, f'' = f

hstep = 0.1;
x = -10:hstep:10;   
n = length(x);

y = f(x);          % values of the function on the mesh

ypp = zeros(n,1);  % initialise the vector for the approximations

for i=2:n-1;
    ypp(i) = (y(i+1)-2*y(i)+y(i-1))/(hstep^2);
end

figure;
plot(x,y);
xlabel('x')
axis square
hold on;
plot(x(2:end-1), ypp(2:end-1), 'or');
legend('y', 'ypp','Location', 'north')

Think it might be a simple error but I just cant crack it.

[Stephen23]

"Think it might be a simple error but I just cant crack it."

To use MATLAB you have to learn the difference between array and matrix operations, otherwise your code will produce nonsense and you won't know why.

https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

[Nathan Barnes]

• alright bro chill, im learning, thanks for the link tho :)

Answer 1

[Adam Danz]

"Index esceeds the number of array elements (1)". When clearly there is capacity for 201 spaces in the array, what is happening!?

Not so clear to me. Running the code from your question, y equal 1.0126. It's a scalar value, not a vector.

The first line of the loop tries to pull the 3rd value from y but there is only 1 value.

for i=2:n-1;
    ypp(i) = (y(i+1)-2*y(i)+y(i-1))/(hstep^2);
    %         ^^^^^^   ^^^^
end

To get a vector output from f(), repalce / with ./

f = @(x) ((x.^4+x.^2)./(x.^4+1));  
%                    ^