Why am I getting this Error Message?

I am trying to take the second order of central difference derivative of a set data that was provided, why did I get an error message like this? I checked the formula many many times and it is correct. Don't know why, since the first order derivative works perfectly.

[dpb]

What is height(j-1) when j==1?

Answer 1

[Monalisa Pal]

Becasue the loop starts from j = 1 and when you are trying to access height(j-1) on line 6 during slope1 evaluation height(0) becomes undefined. You may follow this link for more description: https://www.mathworks.com/matlabcentral/answers/494553-first-and-second-order-central-difference

In short your for loop can start from 2 and you calculate slope(1) separately. Or you may skip slope1 calculation when j = 1 as follows:

for j = 1:length(height)-1
	slope(j) = (height(j+1) - height(j))/h;
	if j ~=1
		slope1(j) = (height(j+1) - 2*height(j) + height(j-1))/h^2;
	end
end

[Zifeng Qiu]

Thank you, this is super helpful