Why the plot figure is empty

Question

Zhiying Wang 2019-11-1

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/488736-why-the-plot-figure-is-empty

回答： Walter Roberson 2019-11-2

y = ones(1,11,'sym');
syms a;
n=0;
for x = -a/2:a/10:a/2;
    n=n+1;
y(n)=(15/8*a)^(1/2)*(1-4*x^2/a^2);
end
fplot(-a/2:a/10:a/2,y)

I want to plot y=(15/8*a)^(1/2)*(1-4*x^2/a^2), x is from -a/2 to a/2.

Why the figure is empty? There is Warning: Error updating ParameterizedFunctionLine. The following error was reported evaluating the function in FunctionLine update: Non-scalar in Uniform output, at index 1, output 1.

Set 'UniformOutput' to false.

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Walter Roberson 2019-11-1

You cannot do that.

Steven Lord 2019-11-1

在 MATLAB Online 中打开

I want the final plot contains a, instead of setting a value for a

You can't. Let's say a was 1. What does your y function look like?

>> syms a x
>> y = (15/8*a)^(1/2)*(1-4*x^2/a^2);
>> vpa(subs(y, a, 1))
ans =
1.369306393762915283642424457002 - 5.477225575051661134569697828008*x^2

What if a was -1 instead?

>> vpa(subs(y, a, -1))
ans =
- x^2*5.477225575051661134569697828008i + 1.369306393762915283642424457002i

In general you can't even say whether or not y is a real-valued or complex-valued function! How would you expect MATLAB to plot the function under those circumstances?

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Answer 1

Walter Roberson 2019-11-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/488736-why-the-plot-figure-is-empty#answer_399439

在 MATLAB Online 中打开

How about this:

for a = linspace(0,5,20)
    x = -a/2:a/10:a/2;
    y = -((4*x.^2)/a^2 - 1)*((15*a)/8).^(1/2);
    plot3(x,y,repmat(a,1,length(x)),'displayname', num2str(a));
    hold on;
end;
hold off
view(3)
legend show