Plot function with changing variable

Question

Eric 2015-5-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/218326-plot-function-with-changing-variable

评论： Eric 2015-5-24

So I have a function that approximately calculates the length of the space curve 2*sin(s),3*cos(s)*exp(s),s.^2/10.

function leng_straight = spacecurvelength(curve,a,b,n)
% Generating n x-points from a to b
syms s
xi= a:(b-a)/n:b;
yi=subs(curve,s,xi);% generating the y-values of the function
% assuming that between consecutive data points, the
% curve can be approximated by linear splines.
leng_straight=0;
m=length(xi);
% there are m-1 splines for m points
for i=1:1:m-1
dx=xi(i+1)-xi(i);
dy= yi(i+1)-yi(i);
leneach=sqrt(dx^2+dy^2);
leng_straight=leng_straight+leneach;
end
end

So I'm now trying to plot this function as I change the 'n' value which is the number of subintervals. Right now I'm getting a blank plot when I call it like this: I've tried changing the names of each of the 'length's to make it unique but no difference.

length = spacecurvelength(@(s) [2*sin(s),3*cos(s)*exp(s),s.^2/10],-4*pi,4*pi,2);
length = spacecurvelength(@(s) [2*sin(s),3*cos(s)*exp(s),s.^2/10],-4*pi,4*pi,5);
length = spacecurvelength(@(s) [2*sin(s),3*cos(s)*exp(s),s.^2/10],-4*pi,4*pi,10);
length = spacecurvelength(@(s) [2*sin(s),3*cos(s)*exp(s),s.^2/10],-4*pi,4*pi,20);
x=[2, 5, 10, 20];
y=length;
plot(x,y)
xlabel('n')
ylabel('length')
title(['Length of Space Curve as a function of n'])

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Answer 1

Walter Roberson 2015-5-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/218326-plot-function-with-changing-variable#answer_180210

在 MATLAB Online 中打开

curvelen = arrayfun(@(n) spacecurvelength(@(s) [2*sin(s), 3*cos(s)*exp(s), s.^2/10],-4*pi,4*pi,n), x);
plot(x, curvelen);

Note that using a variable named "length" is likely to cause trouble with using the MATLAB library function named "length".

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Walter Roberson 2015-5-24

在 MATLAB Online 中打开

No, arrayfun() causes the given anonymous function to be run once per array value (value in x in this case), returning back a vector of outputs. curvelen would be a vector the same length as x.

Are you allowed to use "for" ?

x = [2, 5, 10, 20];
numx = length(x);   %this is the MATLAB function, not your variable
curvelen = zeros(1,numx);
for K = 1 : numx
  curvelen(K) = spacecurvelength(@(s) [2*sin(s), 3*cos(s)*exp(s), s.^2/10],-4*pi,4*pi, x(K));
 end
 plot(x, curvelen)