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Why is there a jagged line when I plot this function?

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econogist
econogist 2020-3-16
评论： econogist 2020-3-19
Hi guys,
I am trying to plot an implicit function and am getting strange results. I understand the smooth part of the function, but not the jagged part. Why is the jagged part there at all? Also, what can I do to eliminate it?
Here's the code:
f1=(@(x,y) (5.*y.^(1./2) -3)./(2.5.*y.^(1./2)+2.5.*x.^(1./2)-3)-y);
fimplicit(f1,[0 4 0 4],'black')

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回答（2 个）

tmarske
tmarske 2020-3-16
It's because fimplicit is using too few evaluation points, resulting in too coarse an interpolation (by default it uses 151). You can override this using the 'MeshDensity' argument:
f1=(@(x,y) (5.*y.^(1./2) -3)./(2.5.*y.^(1./2)+2.5.*x.^(1./2)-3)-y);
fimplicit(f1,[0 4 0 4],'black', 'MeshDensity', 5000)
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econogist
econogist 2020-3-17
Thanks for that! I see what you mean. So is there anyway I can get rid of that part of the plot completely? I tried limiting the range, but that gets rid of the part I want as well.
Walter Roberson
Walter Roberson 2020-3-17
编辑：Walter Roberson 2020-3-17
If you have the symbolic toolbox, you can solve f1 for x in term of y to get a single x for each y. you can then linspace() for y and put those through the equation to derive x, and then plot(x,y)

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econogist
econogist 2020-3-17
I do have the symbolic toolbox. I'm trying to do what you said but still having some trouble as I'm new to matlab. I think I'm on the right track, but it's coming up with an error message.
Here is what I have so far:
syms x y
eqn=(5.*y.^(1/2) -3)./(2.5.*x.^(1./2)+2.5.*y.^(1/2)-3)==y;
S=solve(eqn, x);
y=linspace(0,100);
plot(S,y)

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