my while statement wont work

I am attempting to create a temperature field with one set boundary condition and a wire in one corner of my matrix. To populate the matrix i have been using while loops but only the first one is working. In need of some advice.

clc;clearvars;close all;
H=0.5; %Height in mm
W=2; %Width in mm
Nh=50; %number of elements in y axis
Nw=200; %number of elements in x axis
dx=W/Nw; %increment size in x
dy=H/Nh; %increment size in y
%discretizing
x=0:dx:W;
y=0:dy:H;
%
Err=1;      %initial error
Tol=1e-3;   %Error tolerance
T=zeros([50,200]);
%Boundary Conditions
Tt=20;  %This is correct
T(1,1:Nw)=Tt;
C=35;
T(26:end,1)=C;
T(27:end,2)=C;
T(28:end,3)=C;
T(29:end,4)=C;
T(30:end,5)=C;
T(31:end,6)=C;
T(32:end,7)=C;
T(33:end,8)=C;
T(34:end,9)=C;
T(35:end,10)=C;
T(36:end,11)=C;
T(37:end,12)=C;
T(38:end,13)=C;
T(39:end,14)=C;
T(40:end,15)=C;
T(41:end,16)=C;
T(42:end,17)=C;
T(43:end,18)=C;
T(44:end,19)=C;
T(45:end,20)=C;
T(46:end,21)=C;
T(47:end,22)=C;
T(48:end,23)=C;
T(49:end,24)=C;
T(50:end,25)=C;


while Err>Tol
    Told=T;
    for ii= 2:Nh-25
        for jj= 1
            T(ii,jj)=(T(ii-1,jj)+(2*T(ii+1,jj))+T(ii,jj+1))/4;
        end
    end
    Err=max(max(abs(T-Told)./T))*100;
    %mesh(T),colorbar;colormap;drawnow;
end

while Err>Tol
    Told=T;
    for q= 50
        for r= 26:Nw-1
            T(q,r)=((2*T(q,r-1))+T(q-1,r)+T(q,r+1))/4;
        end
    end
    Err=max(max(abs(T-Told)./T))*100;
    %mesh(T),colorbar;colormap;drawnow;
end

% below is not working for some reason, should be the same values as the first
% while loop
while Err>Tol
    Told=T;
    for l= 2:Nh-1
        for p= Nw
            T(l,p)=((2*T(l,p-1))+T(l-1,p)+T(l+1,p))/4;
        end
    end
    Err=max(max(abs(T-Told)./T))*100;
    %mesh(T),colorbar;colormap;drawnow;
end

while Err>Tol
    Told=T;
    for k= 2:Nh-1
        for u= 2:Nw-1
            T(k,u)=(T(k+1,u)+T(k,u+1)+T(k-1,u)+T(k,u-1))/4;
        end
    end
    Err=max(max(abs(T-Told)./T))*100;
    %mesh(T),colorbar;colormap;drawnow;
end

Answer 1

[Chris]

It looks like you need to reset Err to 1 after a loop completes.