# error when i want to run it

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lulu on 5 Feb 2020
Answered: Walter Roberson on 5 Feb 2020
clear % Clears workspace
clc % Clears history
clf % Clears figure window
N = 101; % Defines the no. of grid points
p = 0; % Sets the lower end of the domain
q = 100; % Sets the upper end of the domain
v = 0.8; % defines velocity of the water (meters)
dx = (q - p)/(N - 1); % Calculates the grid spacing
dt = 0.8; % Defines the time step.
t = 0; % Sets the initial time to zero
runtime = 100; % Defines time program runs for
s = 0.4; % Sets safety factor to 0.4 (Advised not to change this)
pausetime = 0.1; % Pause time between animation
L=1;
x = p : dx : q; % Calculates the value of x using the domain and the grid spacing
for i = 1 : N
U(i) = exp(-(x(i)-L/2).^2); % Defines the initial pollutant using initial conditions
end
% fprintf('Select a scheme from the list and enter the corrosponding number \n\nFirst Order Upwind - 1\nLax-Friedrichs - 2 \nLax-Wendroff - 3\nAdams Average (Lax-Friedrichs) - 4\nAdams Average (Lax-Wendroff) - 5\n\n')
Choice = input('Scheme no. = '); % Select a scheme to run
if Choice == 2
A = 1;
B = 1; % <<<< DO NOT CHANGE THESE VALUES
C = 0;
elseif Choice == 3
A = 1;
B = 1; % <<<< DO NOT CHANGE THESE VALUES
C = 0;
elseif Choice == 4
A = 2;
B = 2; % <<<<< Values can be changed
C = 1;
elseif Choice == 5
A = 1;
B = 1; % <<<<<<<< Values can be changed
C = 5;
end
% ===================== Runs similation ===================================
while t < runtime % Halts program when inequality is violated
% Applies periodic boundary conditions depending on choice
if Choice == 1
boundary1 = un(N);
boundary2 = un(1);
un = [boundary1 un(1:N-1) boundary2];
elseif Choice == 2
boundary1 = un(N);
boundary2 = un(1);
un = [boundary1 un boundary2];
elseif Choice == 3
boundary1 = un(N);
boundary2 = un(1);
un = [boundary1 un boundary2];
elseif Choice == 4
boundary1 = un(N);
boundary2 = un(1);
un = [boundary1 un boundary2];
elseif Choice == 5
boundary1 = un(N);
boundary2 = un(1);
un = [boundary1 un boundary2];
end
dt = s * dx / v; % Calculates time step
t = t + dt; % Adds time step to time
c = dt/dx; % Calculates this here to provide efficiency
if Choice == 1 % Applies First Order Upwind scheme
for j = 2 : N + 1
u(j) = un(j) - v * c * (un(j) - un(j-1));
end
elseif Choice == 2 % Applies either Lax-Friedrichs or Adams Average (Lax-Friedrichs) scheme
for j = 2 : N + 1
u(j) = (A*un(j+1) + B*un(j-1) + C*un(j))/(A+B+C) - v * c * (un(j+1) - un(j-1));
end
elseif Choice == 4 % Applies either Lax-Friedrichs or Adams Average (Lax-Friedrichs) scheme
for j = 2 : N + 1
u(j) = (A*un(j+1) + B*un(j-1) + C*un(j))/(A+B+C) - v * c * (un(j+1) - un(j-1));
end
elseif Choice == 3 % Applies either Lax-Wendroff or Adams Average (Lax-Wendroff) scheme
for j = 2 : N + 1
u(j) = (A*un(j+1) + B*un(j-1) + C*un(j))/(A+B+C) - v * c * (un(j+1) - un(j-1))/2 + c^2 * v^2 * 0.5 * (un(j+1) - 2*un(j) + un(j-1));
end
elseif Choice == 5 % Applies either Lax-Wendroff or Adams Average (Lax-Wendroff) scheme
for j = 2 : N + 1
u(j) = (A*un(j+1) + B*un(j-1) + C*un(j))/(A+B+C) - v * c * (un(j+1) - un(j-1))/2 + c^2 * v^2 * 0.5 * (un(j+1) - 2*un(j) + un(j-1));
end
end
plot(x,u(2:N+1),'b') % Plots pollutant (dependent variable)
xlabel('x [m]') % Adds appropiate labels
ylabel('U [m]')
% Adds title depending on scheme choice
if Choice == 1
title('Solves 1D Advection Equation using FOU Scheme','Fontsize',10)
elseif Choice == 2
title('Solves 1D Advection Equation using Lax-Friedrichs Scheme','Fontsize',10)
elseif Choice == 3
title('Solves 1D Advection Equation using Lax-Wendroff Scheme','Fontsize',10)
elseif Choice == 4
elseif Choice == 5
end
axis([p q min(u) 1.1*max(u)]) % Sets axis
pause(pausetime) % Shows results for each timestep
un = u(2:N+1); % Ommits boundary conditions ready for the next time step
end
end
function initial = U(x) % Defines initial conditions.
pollutantstart1 = 20; % Start of pollutant concentration 1
pollutantend1 = 40; % End of pollutant concentration 1
pollutantstart2 = 60; % Start of pollutant concentration 2
pollutantend2 = 80; % End of pollutant concentration 2
pollutantfloor = 0.1; % Defines bottom of pollutant
pollutantpeak = 0.8; % Defines top of pollutant
initial = pollutantfloor; % Sets up bottom of pollutant
% ============ Determins the areas in which the pollutant peaks ==========
if x >= pollutantstart1 && x <= pollutantend1
initial = pollutantpeak;
elseif x >= pollutantstart2 && x <= pollutantend2
initial = pollutantpeak;
end
end
lulu on 5 Feb 2020
and no graph coming
i just write the equation on initial condition

Walter Roberson on 5 Feb 2020
You should not typically use clear in a function, except sometimes to clear a large variable that takes up a significant fraction of memory.
Your code should be written to check the value of Choice when you get it from the user, to verify that it is 1, 2, 3, 4, or 5. You might want to consider using menu() instead of input() there.
Your current code does not create u unless Choice is one of those 5 numbers.