Few recommendations to your code:
- try to define all the variable before loop
- don't use clear all in your code. Calling clear all decreases code performance, and is usually unnecessary. To clear one or more specific variables from the current workspace use clearvars instead
clc
xs=8;
v=3;
L=20;
ta=0.2;
t=1:1:3;
n=1:1:40;
x=0:0.1:20;
us = zeros(1,length(x));
omga = zeros(1,length(n));
F = zeros(1,length(n));
u = zeros(length(n),length(x));
for k=1:length(t)
for j=1:length(n)
F(j)=exp(-((n(j)*pi*v*ta/L).^2)/4);
u(j,:)=sin(n(j)*pi*xs/L)*F(j)*sin(n(j)*pi*x/L)*cos(n(j)*pi*v/L*t(k)); % no loop needed
end
us(k,:) = sum(u);
end
plot(x,us);
