Plot Error Please help

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)

Answer 1

[Star Strider]

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.

[Ryan Williams]

Thank you!

[Star Strider]

As always, my pleasure!

Answer 2

[KALYAN ACHARJYA]

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)