Last else if graph not plotting

if i==3600

hold on;

plot(x,C);

elseif i==10800

plot(x,C);

elseif i==36000

plot(x,C);

elseif i==36300

plot(x,C);

hold off;

end

In the above program, if i varies from 0 to 36300 or more then last plot is not happening and if I add one more else if i ==36600 then that will not get plot.

Answer 1

[S. Walter]

I tried the following:

% Clear the command window and close the figures
clc
close all

% Make a color vector for plots
col_vec = lines(4);
% Start a counter for the legend entry
cnt = 1;
% Loop over the entries
for i = 1 : 1 : 36300
    % Make random data
    x = 0:1:10;
    C = rand(size(x));
    % If/else statements - added some extra code for plotting clarity
    if i==3600
    	hold on;
    	plot(x,C,'-o','MarkerFaceColor',col_vec(cnt,:))
    	leg{cnt} = 'first';
    	cnt = cnt+1;
    elseif i==10800
    	plot(x,C,'-d','MarkerFaceColor',col_vec(cnt,:))
    	leg{cnt} = 'second';
    	cnt = cnt+1;
    elseif i==36000
    	plot(x,C,'-s','MarkerFaceColor',col_vec(cnt,:))
    	leg{cnt} = 'third';
    	cnt = cnt+1;
    elseif i==36300
    	plot(x,C,'-^','MarkerFaceColor',col_vec(cnt,:))
    	leg{cnt} = 'fourth';
    	hold off;
    end          
end
% Annotate the plot
legend(leg,'location','northeast')
axis([0 10 0 1.1])
grid on
xlabel('x')
ylabel('C')

When I do that, I get four lines with markers

However, as the other commenter said, placing hold on and hold off ahead of the if/else statements is more robust, because if the condition is not met, you might not get the right hold conditions.

[ANMOL BAKSHI]

clc; clear;

lengt = input('The calculation length domain (in m) = ');

points = input('No. of grid points = ');

deltaX = (lengt./(points-1));

deltat = input('Time step (in seconds) = ');

diff = input('Diffusivity = ');

t = input('Time (in s) upto which calculation is to be done = ');

x = 0:deltaX*100:lengt*100;

for i = 0:deltat:t

if i==0

for m = 1:points

cp0(m) = 0.15;

end

else

for n = 1:points

cp0(n) = C(n);

end

end

for j = 2:points-1

diffe(j) = diff;

diffw(j) = diff;

ae(j) = (diffe(j)./deltaX);

aw(j) = (diffw(j)./deltaX);

ap0(j) = (deltaX./deltat);

ap(j) = (ae(j) + aw(j) + ap0(j));

P(j) = -aw(j);

Q(j) = ap(j);

R(j) = -ae(j);

d(j) = (cp0(j).*ap0(j));

end

P(1) = 0;

Q(1) = 1;

R(1) = 0;

d(1) = 1;

P(points) = 0;

Q(points) = 1;

R(points) = 0;

d(points) = 0.2;

Rp(1) = R(1)/Q(1);

dp(1) = d(1)/Q(1);

for k=2:points-1

Rp(k) = R(k)./(Q(k)-(P(k).*Rp(k-1)));

dp(k) = (d(k)-(P(k).*dp(k-1)))./(Q(k)-(P(k).*Rp(k-1)));

end

dp(points) = (d(points)-(P(points).*dp(points-1)))./(Q(points)-(P(points).*Rp(points-1)));

C(points) = dp(points);

for k=points-1:-1:1

C(k) = dp(k)-(Rp(k).*C(k+1));

end

if i==3600

hold on

plot(x,C);

C3600 = C;

elseif i==10800

plot(x,C);

C10800 = C;

elseif i==36000

plot(x,C);

C36000 = C;

elseif i==36300

plot(x,C);

C36300 = C;

hold off

end

end

Not working here.

[S. Walter]

I tried to run your code, setting the following variables:

% The calculation length domain (in m)
lengt = 1;
% No. of grid points 
points = 100;
deltaX = (lengt./(points-1));
% Time step (in seconds)
deltat = 1;
% Diffusivity
diff = 1e-5;
% Maximum time (in seconds)
t = 370000;

And I get the following result

It's a bit difficult to see but the result of i = 36000 and i = 36300 are basically on top of each other. Below is a picture of the result zoomed in to axis([49 51 0.57 0.59]):

Just as a note, you may want to evaluate how you are doing your time step. You currently have your and iteration in time as the same thing but your iterations must be integers. This line:

i = 0:deltat:t

[ANMOL BAKSHI]

Yes, got it.

Really thanks for the suggestions and help.

Answer 2

[KSSV]

Try like this:

hold on 
if i==3600
               
                plot(x,C);
                
              elseif i==10800
                plot(x,C);
                
              elseif i==36000
                plot(x,C);
               
              elseif i==36300
                plot(x,C);
              
                     
end          
hold off

Though I don't understand..what is the purpose to plot like this.

[ANMOL BAKSHI]

hold on

if i==3600

plot(x,C);

C3600 = C;

elseif i==10800

plot(x,C);

C10800 = C;

elseif i==36000

plot(x,C);

C36000 = C;

elseif i==36300

plot(x,C);

C36300 = C;

end

hold off

Tried that. Now, here C36300 is getting stored but plot for last else if is not happening.