User-defined monthly loan payment function issues

Hello! I am trying to create a very simple user-defined function to calculate monthly payments on a loan, but I am unsure why my function is returning "0". I am sure it must be a silly formatting error, but I would appreciate any help pointing out my mistake! Thank you!

% amort returns monthly payment of a loan given the loan amount, annual interest
% rate, and length of loan
% P=loan amount
% r=annual interest rate (in percent)
% N=length of loan in years
function [M]=amort(P,r,N)
M=P*((r/1200)/(1-(1+(r/1200)))^((-12)*N))

[Walter Roberson]

What are some example parameters to the call?

[Sydney]

One example I ran used P=260000, r=6.75, N=15

Is this what you were asking by parameters?

I called it using:

amort(260000,6.75,15)

Answer 1

[Walter Roberson]

M=P*((r/1200)/(1-(1+(r/1200)))^((-12)*N))
    12      1 2  3  4      321 23   2  10

That has a sub-expression (1-(1+(r/1200))) . That sub-expression simplifies to -(r/1200) . So you effectively have

   P * ( (r/1200)/(-r/1200)^(-12*N) )

which is

   P * (-r/1200)^(12*N + 1)

(or something similar)

It seems unlikely that is what you wanted.

[Sydney]

That formula came directly from my homework assignment:

Did I enter it wrong into MATLAB?

[Walter Roberson]

You entered it wrong in MATLAB.

M = P * (r/1200) ./ (1 - (1+r/1200).^(-12*N))

[Sydney]

Thank you for your help!!