If/ElseIf Statement Inequality Not Resolving Correctly

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Sarah Ten Eyck
Sarah Ten Eyck 2019-4-2
评论： Sarah Ten Eyck 2019-4-3
I'm trying to plot the angular position of a robotic arm whose equation of motion changes over time. Ideally the plot of the position should have this shape:
but instead looks like this:
I have an if/elseif/else statement to handle the change in equation. The problem is that "else" part is being tripped for all values of t, which is why the plot is linear. I cannot seem to figure out why this is, but I can only assume it's because I've made some mistake with my if conditions. Below is a copy of my code. Any help would be appreciated. Thank you!
vm1 = 0.41415;
theta01 = -0.57982;
thetaf1 = 0.57982;
hold on
t = linspace(0,3);
if (t < 0.5)
theta1 = theta01 + (vm1 .* (t .^ 2));
elseif (t > 2.5)
theta1 = thetaf1 - (vm1 .* (9 - (6 .* t) + (t .^ 2)));
else
theta1 = vm1 .* (t - 1.5);
end
plot(t, theta1)

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采纳的回答

Cris LaPierre
Cris LaPierre 2019-4-3
If statements only work on one value at a time. You can't have it check the entire vector all at the same time. You can put the if statement inside a for loop and loop through each value of t one by one:
for i = 1:length(t)
if (t(i) < 0.5)
theta1(i) = theta01 + (vm1 .* (t(i) .^ 2));
elseif (t(i) > 2.5)
theta1(i) = thetaf1 - (vm1 .* (9 - (6 .* t(i)) + (t(i) .^ 2)));
else
theta1(i) = vm1 .* (t(i) - 1.5);
end
end
or create a piecewise function:
theta1 = (theta01 + (vm1 .* (t .^ 2))).*(t<0.5) + ...
(thetaf1 - (vm1 .* (9 - (6 .* t) + (t .^ 2)))).*(t>2.5) + ...
(vm1 .* (t - 1.5)).*(t>=0.5).*(t<=2.5);
The actual functions may not be quite right yet, but at least now you can visualize it.
piecewiseFxn.png
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Cris LaPierre
Cris LaPierre 2019-4-3
Nice! Love when the math works!
I must have been writing my last comment the same time you were writing yours. What I said was in reference to decreasing the point interval spacing.
Sarah Ten Eyck
Sarah Ten Eyck 2019-4-3
You were right on that front, there was still the weird discontinuity when I decreased the interval. Cheers!

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