Matlab Derivative Approximation

[JefeNorte]

I am trying to write a program that estimates the derivative of a polynominal and determines the error. So far my code is

% The code for Problem 3.

a=5-4*x^2+3*x^3-2*x^4+x^5; % ask for a function to be differentiated

x=input('Enter the value x at which to find the derivative '); % ask for input
dh=.5;
for i=1:dh:100% do the loop
dh=.5*(1/2)^i; % step size
df=((5-4.*(x+dh).^2+3.*(x+dh).^3-2.*(x+dh).^4+(x+dh).^5)-(5-4*x^2+3*x^3-2*x^4+x^5))/dh; % centered difference approximation
if 1+dh==1
break
else
dh=dh/2;
end
end
error=(-8*x+9*x^2-8*x^3+5*x^4)-df;
disp(df)
disp(error)


The result is two numbers that are correct but the df is switched with error and vise versa. Does anyone know what I am doing wrong

 

[eys_physics]

I don't know if this has to with your problem. But in my opinion has your for-loop an infinite number of turns because dh will never be 0 (dh is a real number). So instead you should stop your loop then dh<e, where e is some limit.

 

[oswaler]

?

I would suggest checking your code for any errors or typos. It is also important to double check your equations and make sure they are correct. Additionally, it may be helpful to break down your code into smaller steps and test each step individually to pinpoint where the error is occurring. You could also try using a different method for estimating the derivative, such as the forward or backward difference method, to see if you get the same results. Lastly, it is always a good idea to consult with other experts or resources to get feedback and troubleshoot any issues you may be having.