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Re: I need help debugging my code

From: Ben Abbott
Subject: Re: I need help debugging my code
Date: Tue, 28 Apr 2009 21:34:13 -0400

On Apr 28, 2009, at 9:09 PM, address@hidden wrote:

I’ve been working on this code for a week now, but I still keep getting error messages. Can anyone help? Thanks!
Here is my outside function file:

function f = yprime(t,y)
y(1) = 0.5;
t(1) = 0;
 f = y .- t.^2 .+ 1;

Here is my first code and error message:
function [t,y] = tryrk4(ydot, a, b, y0, h);
t(1) = a;
y(1) = y0;
N = 10;

for i=1:N
   k1 = feval(ydot, t(i)    , y(i)       );
   k2 = feval(ydot, t(i)+h/2, y(i)+k1*h/2);
   k3 = feval(ydot, t(i)+h/2, y(i)+k2*h/2);
   k4 = feval(ydot, t(i)+h  , y(i)+k3*h  );
   i = i+1;
   t(i) = a+i*h;
   y(i) = y(i-1) + (k1 + 2*k2 + 2*k3 + k4)*h/6;
for i=1:N
    [t,y] = [t(i),y(i)];
octave-3.0.2.exe:4:C:\Program Files\Octave\3.0.2_gcc-4.3.0\bin
> tryrk4(yprime, 0,2,.5,.2)
f =  1.5000

Here’s the error message:
error: octave_base_value::function_value(): wrong type argument `scalar'
error: evaluating assignment expression near line 18, column 7
error: evaluating for command near line 17, column 1
error: called from `tryrk4' in file `C:\Program Files\Octave \3.0.2_gcc-4.3.0\bin\tryrk4.m'

Thank you for your help! I'm still pretty new to octave, but I'm getting there!

No need for ".-" or ".+", just use "-" and "+".

Regarding the loop

for i=1:N
    [t,y] = [t(i),y(i)];

I don't know what you intend to do here. It appears to be unneeded, so I just deleted it.

You'll also need to pass the function either a handle or as a text string. In the context of your example, I choose to use a text string.

See the attached.

To run it place it in the appropriate directory and from Octave type ...

        [t, y] = tryrk4 ("yprime", 0, 2, 0.5, 0.2)

or just type

        demo tryrk4


Attachment: tryrk4.m
Description: Binary data

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