How to plot this function?

Question

Michaël 2011-3-14

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/3203-how-to-plot-this-function

采纳的回答： Matt Fig

Hello,

Could you please help me plot this function with Matlab ?

<http://img638.imageshack.us/img638/3205/fonction.png >

I'm a beginner. Your help will be valuable to me.

Many thanks

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Answer 1

Matt Fig 2011-3-15

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https://ww2.mathworks.cn/matlabcentral/answers/3203-how-to-plot-this-function#answer_4821

在 MATLAB Online 中打开

If you want to do it manually, try this:

f = @(z) quadv(@(x) (1/(3*sqrt(2*pi)))*exp(-(x-30).^2/18),z,100);
Z = 0:.25:50;
G = zeros(1,length(Z));
for ii = 1:length(Z)
   G(ii) = f(Z(ii)); 
end
plot(Z,G)
xlabel('z')
ylabel('(1/\surd(2*pi))\inte^{-(x-30)^2/18}')

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Answer 2

Matt Tearle 2011-3-14

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Take a look at erfc. A simple change of coordinates should get you there.

Then plotting can be done by

x = linspace(xmin,xmax,npoints);
y = constant*erfc(x);
plot(x,y)

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Michaël 2011-3-14

Thank you for your answer.

But can't I put my function directly (without going through a pre-determined function)?

Thanks

Matt Tearle 2011-3-15

Why would you want to? This function is defined via an improper integral, which means you'll have to do an infinite-domain integral for each value of z. This will be slow and will introduce all sorts of numerical errors. On the other hand, the error function is a well-known function -- its implementation in MATLAB is stable and tested. Why reinvent the wheel?

But if you really want to, you can use something like quad to do the integral.

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