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Plot Error Please help

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I'm trying to plot a piecewise function that I've created and it keeps giving me an error saying "vectors must be same length." this is what I have. Please help.
theta = linspace(0,2*pi,500);
for i=1:numel(theta)
if (theta(i) >= 0) && (theta(i) <= (pi/2))
y(i) = 6*(2*theta(i) - 0.5*sin(theta(i)));
elseif (theta(i) >= (pi/2)) && (theta(i) <= (2*pi/3))
y(i) = 6;
elseif (theta(i) >= (2*pi/3)) && (theta(i) <= (4*pi/3))
y(i) = 6 - 3*(1 - 0.5*cos(3*theta(i) - 2*pi));
elseif (theta(i) >= (4*pi/3)) && (theta(i) <= (3*pi/2))
y(i) = 3;
elseif (theta(i) >= (3*pi/2)) && (theta(i) <= (7*pi/4))
y(i) = 3 - 1.5*((theta(i) - (3*pi/2))/(pi/4))^2;
elseif (theta(i) >= (2*pi/3)) && (theta(i) <= (4*pi/3))
y(i) = 0.75 - 0.75*(1 - (theta(i) -(7*pi/4))/(pi/4))^2;
end
end
plot(theta,y,'r','linewidth',2)

Accepted Answer

Star Strider
Star Strider on 14 Mar 2021
It’s necessary to account for absolutely all possibilities.
Try this:
theta = linspace(0,2*pi,500);
for i=1:numel(theta)
if (theta(i) >= 0) && (theta(i) < (pi/2))
y(i) = 6*(2*theta(i) - 0.5*sin(theta(i)));
elseif (theta(i) >= (pi/2)) && (theta(i) < (2*pi/3))
y(i) = 6;
elseif (theta(i) >= (2*pi/3)) && (theta(i) < (4*pi/3))
y(i) = 6 - 3*(1 - 0.5*cos(3*theta(i) - 2*pi));
elseif (theta(i) >= (4*pi/3)) && (theta(i) < (3*pi/2))
y(i) = 3;
elseif (theta(i) >= (3*pi/2)) && (theta(i) < (7*pi/4))
y(i) = 3 - 1.5*((theta(i) - (3*pi/2))/(pi/4))^2;
elseif (theta(i) >= (2*pi/3)) && (theta(i) <= (4*pi/3))
y(i) = 0.75 - 0.75*(1 - (theta(i) -(7*pi/4))/(pi/4))^2;
else % Added Condition
y(i) = 0; % Added Assignment
end
end
figure
plot(theta,y,'r','linewidth',2)
That ran without error when I tested it.
  2 Comments
Star Strider
Star Strider on 14 Mar 2021
As always, my pleasure!

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More Answers (1)

KALYAN ACHARJYA
KALYAN ACHARJYA on 14 Mar 2021
Edited: KALYAN ACHARJYA on 14 Mar 2021
There must be one else condition (to avoid any mismatch between vectors length). If none of the conditions is true, then such condition y will be the "else" condition. Sure, some cases theta(i) maynot be satisfy any of those conditions.
Pre-allocation also helps to avoid a mismatch between the lengths of the vectors, but whether this is true in the case of your condition, if a theta (i) if-and, is not part of the conditions, then y should be zero .
Or assign last one as an "else" condition (Check the coditions)
theta = linspace(0,2*pi,500);
y=zeros(1,numel(theta)); % Thise also helps to avoid mismatch between vectors length
for i=1:numel(theta)
if (theta(i) >= 0) && (theta(i) <= (pi/2))
y(i) = 6*(2*theta(i) - 0.5*sin(theta(i)));
elseif (theta(i) >= (pi/2)) && (theta(i) <= (2*pi/3))
y(i) = 6;
elseif (theta(i) >= (2*pi/3)) && (theta(i) <= (4*pi/3))
y(i) = 6 - 3*(1 - 0.5*cos(3*theta(i) - 2*pi));
elseif (theta(i) >= (4*pi/3)) && (theta(i) <= (3*pi/2))
y(i) = 3;
elseif (theta(i) >= (3*pi/2)) && (theta(i) <= (7*pi/4))
y(i) = 3 - 1.5*((theta(i) - (3*pi/2))/(pi/4))^2;
else
%(theta(i) >= (2*pi/3)) && (theta(i) <= (4*pi/3))
y(i) = 0.75 - 0.75*(1 - (theta(i) -(7*pi/4))/(pi/4))^2;
end
end
plot(theta,y,'r','linewidth',2)

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