如果在Octave / Matlab上不起作用

Question

I have the code divided on three functions:

fm (This function works )

function y=f(x)
y = x^2;
endfunction

df.m (The function that return the derivate of y=x^2 . This function works )

function y=df(i)
  syms x,diff(f(x),x);
  subs(ans,x,i);
  y = ans;
endfunction

newton.m ( maxIt = max of tries)

function [x1,nIt] = newton (f,df,x0,tol,maxIt)
   k=0;
   x=x0;
   nIt = 0;
   while (k<maxIt)
     if(df(x)== 0)
       fprintf("La derivada, da resultado 0");
       k=maxIt;
     else
       i = x - (f(x)/df(x))
       x = i;
       nIt = nIt + 1
       k=k+1;
     endif
  end
endfunction

Ok, I executed the command newton(@f,@df,0,10,2) . Two-thirds of the program works OK, but if I do df(0) , the result is 0 but the if doesn't work. I verified that df(0) returns 0, and it's OK.

I´m new in MATLAB/Octave.

Answer 1

You have several mistakes in your code. Not all of them are directly relevant, but fixing all of them will make your code more readable and robust.

The function f if fine.

The function df assumes that f is known somehow, but it's not a good practice. you better pass f too to this function. Also, don't use ans as a variable, store your intermediate result in different variables. This is how df should look:

function y = df(f,a)
syms x
d = diff(f(x),x);
y = subs(d,x,a);
end

Now you have to change all the calls to df in your newton function to df(f,x) . I'm not sure what is your final goal, but right now, the variables x1 and tol are not in use in any way.

Hope it solves the problem ;)