Undefined function or variable

I am getting this error even though I have defined the "mat" function in my code. "Undefined function or variable 'mat'." I did some research but I can't find the reason. Because the code is too long, I do not share it all, but I share the relevant part below. other parts of the code are working.

Thanks in advance for your answers.

counter=0;
converg=1;
while converg>0.01;
    for j = 2:n-1
        min = 1000;
        for l = 2:m-1
            if number(j,l)~=0 && number(j,l) < min
                min = number(j,l);
            end
        end
        for i = 2:m-1
            if index(j,i)~=0
               
                n2 = number(j,i);
                n1 = n2 - min +1;
                if n1 == 1
                    mat(n1,n1) = C(j,i);
                    mat(n1,n1+1) = E(j,i);
                    d(n1,1) = (-1) * (P(j-1,i) * N(j,i)+P(j+1,i)*S(j,i))+Q(j,i);
                end
                if n1 == max(number(j,:)) - min +1
                    mat(n1,n1) = C(j,i);
                    mat(n1,n1-1) = W(j,i);
                    d(n1,1) = (-1) * (P(j-1,i) * N(j,i)+P(j+1,i)*S(j,i))+Q(j,i);
                end
                if n1~=1 && n1 ~= max(number(j,:))-min+1
                    mat(n1,n1) = C(j,i);
                    mat(n1,n1-1) = W(j,i);
                    mat(n1,n1+1) = E(j,i);
                    d(n1,1) = (-1) * (P(j-1,i) * N(j,i)+P(j+1,i)*S(j,i))+Q(j,i);
                end
            end
        end
        
    Pwf = thm(mat,d);  
       
        for i = 2:m-1
            if index(j,i)~=0
                n2 = number(j,i);
                n1 = n2 - min +1;
                P(j,i) = Pwf(n1,1);
            end
        end
        clear Ps
        clear mat
        clear d
    end
   
    Pdiff = abs(Pold - P);
    converg = max(max(Pdiff));
    Pold = P;
    counter = counter + 1
end

[Star Strider]

The error (that should have been part of the orignal post) is:

Unrecognized function or variable 'n'.

referring to:

for j = 2:n-1

The obvious solution is to define ‘n’ (as an integer greater than 2) prior to the while loop.

If the error refers to the ‘mat’ variable (I dio not see a ‘mat’ function defined anywhere), note that it is possible that it might not be assigned if the if blocks prevent that assignment because the relevant conditions are not met.

[muharrem çevik]

I have previously defined 'n' as an integer greater than 2. I redefined the mat function to see the error, but the code started giving other errors. I think I have a bigger problem :)

I'm trying to solve a 2D grid system. I had solved a system with this code before, but it failed in that. I think there is an error in my grid system.

Thank you very much for your reply.

[Star Strider]

My pleasure.

Supply the missing ingredients and you may get a useful response.

[Walter Roberson]

 min = number(j,l);

We recommend against using min or max as the names of variables, as it is very common to find that later in the same function one wants to use the min() or max() function. It also confuses readers, who would have to look carefully at the flow of code

[muharrem çevik]

Thank you. I will consider.