# Trouble making my for loop work

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Isabelle Davies on 10 Apr 2022
Edited: Isabelle Davies on 10 Apr 2022
I'm trying to simulate the movement of a pendulum by plotting a line and a marker, and then using a for loop:
L = 2 ;
g = 9.81 ;
Theta_0 = pi/3
t = 2*pi*sqrt(L/g)
cos(sqrt(g/L) * t);
Theta = Theta_0 * cos(sqrt(g/L) * t)
N_Theta = -1 * Theta
x = L * sin(Theta) ;
y = L * (1 - cos(Theta)) ;
figure(1)
hold on
axis equal
axis([-2,2,-0.5,3.5]);
for i = 1:length(t)
plot([0 x(i)], [2 y(i)], '-', 'LineWidth', 6, 'Color', 'k');
plot(x(i), y(i), 'b.', 'Markersize', 70)
drawnow
pause(0.1)
end
hold off
Ok so I've got the i thing figured out (thanks to those in the comments for your help), but now I'm trying to figure out how to make it so that I can actually see the different plotted pendulums without them stacking on top of each other. I'm pretty certain it's got to do with my placement of hold on and hold off, but I can't figure out what the right position is.
Sorry if this is really basic stuff or I've made an obvious mistake, I'm a beginner and just trying my best.
Thanks!
##### 2 CommentsShowHide 1 older comment
Isabelle Davies on 10 Apr 2022
@VBBV yes you're right sorry, I can't even find what I did to show that. Now what I have is just the first frame showing as a figure (essentially the first plot in my question). I'll fix this now

Atsushi Ueno on 10 Apr 2022
>so I'm thinking maybe I used the wrong variable for i?
Kindly. The direction of the iterator is from negative (smaller one) to positive (larger one).
L = 2 ;
g = 9.81 ;
Theta_0 = pi/3;
t = 2*pi*sqrt(L/g);
Theta = Theta_0 * cos(sqrt(g/L) * t)
Theta = 1.0472
N_Theta = -1 * Theta
N_Theta = -1.0472
for i = Theta:N_Theta
i
end
% Nothing hapen here
for i = N_Theta:Theta
i
end
i = -1.0472
i = -0.0472
i = 0.9528
##### 1 CommentShowHide None
Atsushi Ueno on 10 Apr 2022
Because your iterator omits writing increments. The omitted increment becames "+1".
Theta = 1.0472;
N_Theta = -Theta;
for i = Theta:1:N_Theta
i % Nothing hapen
end
for i = Theta:-1:N_Theta
i
end
i = 1.0472
i = 0.0472
i = -0.9528
for i = N_Theta:-1:Theta
i % Nothing hapen
end
for i = N_Theta:1:Theta
i
end
i = -1.0472
i = -0.0472
i = 0.9528

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