Undefined function or variable "new_m" - matlab

Question

I am new to Matlab. I have three functions. When I call co.m and pol_det.m they both work fine. However, when I call minor.m which in itself calls pol_det which in its turn calls co.m I get an error referring to the co.m: Undefined function or variable "new_m".
I am working with R2007b version. The three functions are below. Originally, they are written each in a separate .m document.
function [ k ] = pol_det(a)
%calculates the determinant of a general matrix (not just consisting of
%numbers)
dim=size(a); %dimensions of a matrix
if dim(1)~= dim(2)
disp('Only Square Matrices, please')
end
m=length(a);
k=0;
if(m==2)
k=sum_p(conv(a(1,1),a(2,2)),- conv(a(2,1),a(1,2))); %calc. the determinant of a 2x2 m.
else
for i=1:m
k=k+((-1)^(1+i))*conv(a(1,i),co(a,1,i)); %calc. the determinant using cofactor expansion
end
end
if (k==0)
disp('Matrix non-invertible')
end
end
function [ out ] = co( a,i,j )
%cofactor expansion,
%http://people.math.carleton.ca/~kcheung /math/notes/MATH1107/wk07/07_cofactor_expansion.html
[m,n]=size(a);
%create a new matrix by eliminating the row and column in which the %element is present
%new_m=zeros(m,n)
row=1;
col=1;
for i1=1:m
for j1=1:n
if(i1~=i && j1~=j)
new_m(row,col)=a(i1,j1);
col=col+1;
end
end
if(col~=1)
row=row+1;
end
col=1;
end
%new_m
out=pol_det(new_m);
end
function [ m ] = minor(a)
dim=size(a); %dimensions of a matrix
if dim(1)~= dim(2)
disp('Only Square Matrices, please')
end
a=a.';
for i=1:dim(1)
for j=1:dim(1)
a(i,:)=[];
a(:,j)=[];
m(i,j)= pol_det(a);
end
end
end

Answer 1

Your problem is that, given certain values of a, i, and j, you may never enter the conditional statement inside your loops that initializes new_m. In such a case, the variable won't exist when you then get to the following line out=pol_det(new_m);.
You should set a default value for newm before your loops and conditional statements, such as [], so that the variable will always have a value. You should also make sure pol_det can appropriately handle this default value. Best practice is to make use of preallocation, both to improve performance and avoid conditional existence of necessary variables.