Output argument <Variable> (and maybe others) not assigned during call to <Function>.

I have a function that currently works perfectly:

function [x,cv] = mybasic(A,b,M,x,TOL,nmax)
% Basic iteration x <- x+M\(b-A*x)
x = x(:); % make sure initial guess is column vector
r = b-A*x;
e = M\r;
err0 = norm(e);  % euclidean magnitude
rerr = 1;        % relative error
n = 0;
if nargout==2
    cv = zeros(nmax+1,2+length(x)); % preallocate space for cv
    cv(1,:) = [n,x.',rerr]; % iter. #, iterate (row vector), rel. error
end
while (rerr>TOL && n<nmax)
    x = x+e;
    r = r-A*e;
    e = M\r;
    rerr = norm(e)/err0;
    n = n+1;
    if nargout==2
        cv(n+1,:) = [n,x.',rerr];
    end
end
n
if nargout==2
    cv(n+2:end,:) = []; % delete unused space in cv
end
if rerr>TOL && n==nmax
    warning('MYBASIC:noCV','no cv within %i iterations',n)
end
end

However, if all I do is change the function output to [n,x,cv] instead and try and run the function with a script file, I end up getting the error Output argument "cv" (and maybe others) not assigned during call to "mybasic". Here is the script I am trying to use:

A = [1,.375,.375;.375,1,.375;.375,.375,1];
b = [1;1;1];
M = diag(diag(A));
TOL = 1e-5;
nmax = 100;
x0 = zeros(3,1);
[n,x,cv] = mybasic(A,b,M,x0,TOL,nmax);

Please help. Thanks!

Answer 1

[Guillaume]

The cv variable is only created iff nargout is 2. With 3 outputs, indeed cv will not exist.

Change

if nargout == 2

to

if nargout == 3

if you move cv to the 3rd ouptut.

[Kendell Crowley]

Wow, that was the issue. Thanks!