Application of the Newton-Raphson method of finding roots, program does not work

i've been given a task to use the newton-raphson method to find roots for an equation, getting all kinds of errors. This is the first time i'm using matlab, and would appreciate if any of you could help me debug this thing and go 1 step ahead in learning proper syntax by it...

the program is divided to main+3 functions, and here they are:

 function main()
    global h0 m k g
    h0=300;
    m=0.25;
    k=0.1;
    g=32.174;
    t0=3;
    while abs(6.0032-t0)>0.005
        t0=nraphson(t0, fun(t0), der(t0));
    end

function [ t0 ] = nraphson( t0, fun, der )
%This is the Newton-Raphson iterative method
tplus= t0-(fun(to)/der(t0);
t0=tplus;
end

function [ a ] = fun(t0)
%This is the mathematical function we are asked to work with
    global h0 g k m
        a=h0-(m*t0*g/k)+((g*m^2)/k^2)*(1-exp(-k*t0/m));
end

function [ b ] = der( t0)
%Derivative of the function we were given to find a root for 
global m g k 
b= (-m*g/k)+g*m*exp(-k*t0/m)/k;
end

It's fairly simple, and i probably botched it in feeding the arguments/output values properly into functions and whatnot. -the program has to be in 4 parts like this, this is what iv'e been asked. Any help would be tremendously appreciated, been cracking my head for a full day now with no progress....

[Star Strider]

‘...all kinds of errors.’

Care to elaborate?

[Rob Dunham]

Currently i'm getting these- " Attempted to access fun(3); index out of bounds because numel(fun)=1. "

" Error in nraphson (line 3) tplus= t0-fun(t0)/der(t0); "

" Error in main (line 10) t0=nraphson(t0, fun(t0), der(t0)) ;"

I edited the first post, minor changes were made.

[Walter Roberson]

The code you show is not the same as what you show in the error message.

[James Tursa]

This error text does not match your posted code. Please update your posted code to what you currently have.

The error means fun is being treated as a variable and its dimensions are 1x1. So at this point, the code cannot see the function fun (e.g., it is being shadowed by the variable fun).

[Rob Dunham]

Code updated, it is now identical to the source, as are the errors.

[Walter Roberson]

You are passing fun(t0) into nraphson, where t0 has a definite value. That means that fun(t0) is to be evaluated and the numeric result is to be passed in. You then try to index that numeric result at t0, since variable(t0) means indexing unless variable is a function handle.

Try

t0=nraphson(t0, @fun, @der)

[Rob Dunham]

This did the trick. You have my thanks Walter, both for helping solve this particular problem and for explaining so i won't repeat a similar mistake in the future.