EZplot takes very long time to plot using, symbolic expression

Question

Wilhelm Dahlin 2022-2-14

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1649735-ezplot-takes-very-long-time-to-plot-using-symbolic-expression

回答： Sourav Karmakar 2022-2-17

Hi all,

I'm having some difficult plotting graphs using EZplot. I dont know why but it has something to do with the matrix Ab. Before changing one element from "0" to "k1" in Ab, everything worked as it should and the plot appeard after 2 seconds - but after that small change it takes roughly 5 minutes for EZplot to plot the graphs. The script runs smoothly all the way to the start of the ezplot, I checked using "steps".

Below is the code. Am i missing something obvious or do I need to rewrite some of it?

m1=9;
m2=9;
m3=1;
k1=1;
k2=10;
k3=6;
syms t
Ab=[0 1 0 0 0 0; -k1/m1 0 k1/m1 0 0 0; 0 0 0 1 0 0; k1/m2 0 -(k1+k2)/m2 0 k2/m2 0; 0 0 0 0 0 1; 0 0 k2/m3 0 -(k2+k3)/m3 0];
x0=[1; 0; 0; 0; 0; 0];
eta=expm(t.*Ab);
ut=eta*x0;
u1=ut(1);
u2=ut(2);
v1=ut(3);
v2=ut(4);
w1=ut(5);
w2=ut(6);
%x1=u1, x2=v1, x3=w1
%Plot
figure(2)
ezplot(u1, [0, 50]);
ylim([-1.1, 1.1]);
hold on
ezplot(v1, [0, 50]);
ylim([-1.1, 1.1]);
ezplot(w1, [0, 50]);
ylim([-1.1, 1.1]);
legend('x(1)', 'x(2)', 'x(3)', 'Location', 'southeast')

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Answer 1

Sourav Karmakar 2022-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/1649735-ezplot-takes-very-long-time-to-plot-using-symbolic-expression#answer_898300

Hey Wilhelm ,

When you are using ‘0’ instead of ‘k1’ in Ab matrix then your ‘u1’,’v1’,’w1’ will contain simpler symbolic expressions in comparison to other value of ‘k1’. That’s why, using “EZplot”, you will get your plot figures within few seconds.

While using ‘k1’, the symbolic expressions will be complex for those variables. Then, it will take longer time to plot the respective figures. “ezplot” is not recommended, use fplot or fimplicit instead .

Hope this helps!