poisson

% Program 8.5 Finite difference solver for 2D Poisson equation
%   with Dirichlet boundary conditions on a rectangle
% Input: rectangle domain [xl,xr]x[yb,yt] with MxN space steps
% Output: matrix w holding solution values
% Example usage: w=poisson(0,1,1,2,4,4)
%
% changes to boundary made higlighted by ***
function w=poisson_p6(xl,xr,yb,yt,M,N)
f=@(x,y) -5/3.85; % define input function data ***
g1=@(x) 30; % define boundary values ***
g2=@(x) 30; % ***
g3=@(y) 30; % ***
g4=@(y) 30; % ***
m=M+1;n=N+1; mn=m*n;
h=(xr-xl)/M;h2=h^2;k=(yt-yb)/N;k2=k^2;
x=xl+(0:M)*h; % set mesh values
y=yb+(0:N)*k;
A=zeros(mn,mn);b=zeros(mn,1);
for i=2:m-1 % interior points
  for j=2:n-1
    A(i+(j-1)*m,i-1+(j-1)*m)=1/h2;A(i+(j-1)*m,i+1+(j-1)*m)=1/h2;
    A(i+(j-1)*m,i+(j-1)*m)=-2/h2-2/k2;
    A(i+(j-1)*m,i+(j-2)*m)=1/k2;A(i+(j-1)*m,i+j*m)=1/k2;
    b(i+(j-1)*m)=f(x(i),y(j));
  end
end
for i=1:m % bottom and top boundary points
  j=1;A(i+(j-1)*m,i+(j-1)*m)=1;b(i+(j-1)*m)=g1(x(i));
  j=n;A(i+(j-1)*m,i+(j-1)*m)=1;b(i+(j-1)*m)=g2(x(i));
end
for j=2:n-1 % left and right boundary points
  i=1;A(i+(j-1)*m,i+(j-1)*m)=1;b(i+(j-1)*m)=g3(y(j));
  i=m;A(i+(j-1)*m,i+(j-1)*m)=1;b(i+(j-1)*m)=g4(y(j));
end
v=A\b; % solve for solution in v labeling
w=reshape(v(1:mn),m,n); %translate from v to w
mesh(x,y,w')

Not enough input arguments.

Error in poisson_p6 (line 14)
m=M+1;n=N+1; mn=m*n;