Lopping error while doing 2D (x,t) finite difference method solution for a particle simulation

I tried loops of for and while with if inside but its showing only zeros in the n matrix , some how the loop is not getting incorporated. Please help me finding what is the error in this case particularly.

clear all;
clc
D = 1e-4 * 1e-4 ;                         % 1e-4 cm^2/sec
J=1e6 * 1e6;                              %input flux at half
format long;
%% Grid defination
del_x = 1e-9;
del_t=1e-6;
GridPoints_x=0:del_x:10e-6;
GridPoints_t=0:del_t:1e-5;
npoints_x=length(GridPoints_x);
npoints_t=length(GridPoints_t);
n=zeros(npoints_x,npoints_t);
Source = zeros(npoints_x, npoints_t);% Source Profile

flux_location_x = 5 * 1e-6;                  % Injection Location
flux_location_t = 1 * 1e-6;                  % Injection Time
f_index_x = find(GridPoints_x == flux_location_x);% Location Index
f_index_t = find(GridPoints_t == flux_location_t);% Time Index
i = 1;
Source(f_index_x, f_index_t) = J ;
tol = 1e-10;
dn=10;
while 1
n_old=n;
for m=2:npoints_t
    n_old=n;
    for i= 2:npoints_x-1
            n(i , m) = ((1/del_x^2) * (n(i + 1, m) + n(i - 1, m)) +n(i, m - 1)/(D * del_t) + Source(i, m) ...
    )/(2/del_x^2 + 1 / (D * del_t));
    end 
    n_new=n;
    if(abs(n_old-n_new)<=tol)
        break
    end
 end
end 

[Torsten]

D = 1e-4 * 1e-4 ;                         % 1e-4 cm^2/sec
J=1e6 * 1e6;                              %input flux at half
format long;
%% Grid defination
del_x = 1e-9;
del_t=1e-6;
GridPoints_x=0:del_x:10e-6;
GridPoints_t=0:del_t:1e-5;
npoints_x=length(GridPoints_x);
npoints_t=length(GridPoints_t);
n=zeros(npoints_x,npoints_t);
Source = zeros(npoints_x, npoints_t);% Source Profile

flux_location_x = 5 * 1e-6;                  % Injection Location
flux_location_t = 1 * 1e-6;                  % Injection Time
f_index_x = find(GridPoints_x == flux_location_x)% Location Index

f_index_x =

  1×0 empty double row vector

f_index_t = find(GridPoints_t == flux_location_t)% Time Index

f_index_t = 
     2

[Subham Mahanti]

Thank you atleast now i know where is the error but why is the error ?

[Torsten]

Because testing for equality of two floating point numbers always makes trouble:

https://de.mathworks.com/matlabcentral/answers/267964-while-comparing-the-equality-of-two-floating-numbers-a-b-we-use-abs-a-b-tolerance-could-anyo

[Subham Mahanti]

Okay understood ! Thanks a lot