Evaluating a function with three variables

2018 9 26
2 个回答
回答已采纳
更新时间：2018 9 28
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0 个投票

Dear all, Since I am new in MatLab, I want to evaluate the following equation y=(1./((x.^2)+1))-(p./((x+w).^2+1))-(p./((x-w).^2+1)); I want to sweep the variables x,p and w as follows x from -5 to 5 with 100 steps ,p from 0 to 1 with 100 steps ,w from 0 to 3 with 100 steps and then I want to normalized the functiony and then I want to extract the specific values of x (lets name new variable r) which give me specific value of y (lets say y=0.5) for each combination of variables x,w,p. After that I want to make a surface graph of ( r,w and p)
thanks

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 采纳的回答

KSSV
KSSV 2018-9-26
编辑：KSSV 2018-9-26

2 个投票

N = 100 ;
x = linspace(-5,5,N) ;
p = linspace(0,1,N) ;
w = linspace(0,3,N) ;
[x,p,w] = meshgrid(x,p,w) ;
y=(1./((x.^2)+1))-(p./((x+w).^2+1))-(p./((x-w).^2+1));
figure
hold on
for i = 1:N
surf(x(:,:,i),p(:,:,i),w(:,:,i),y(:,:,i))
end
% GEt manually y = 0.5
idx = abs(y-0.5)<=10^-3 ;
xr = x(idx) ;
pr = p(idx) ;
wr = w(idx) ;
figure
scatter3(xr,pr,wr,10,wr)
% use isosurface
p = patch(isosurface(x,p,w,y,0.5));
p.FaceColor = 'red';
p.EdgeColor = 'none';
daspect([1 1 1])
view(3);
axis tight
camlight
lighting gouraud

6 个评论
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Alin Brad
Alin Brad 2018-9-26
thank you for answer.
now, I to want to extract values of x which give me certain values of y. lets say r= value of x which gives me the certainvalues of y for each combination, I define for example this required value of y as follows yabs=abs(min(y)); Ycertain=(max(y)-yabs)/2; and make a surface graph (r,w,p)
thanks
KSSV
KSSV 2018-9-26
I have edited the answer...
Alin Brad
Alin Brad 2018-9-26
编辑：Alin Brad 2018-9-26
kindly. I want now to not get the value of y as 0.5, but I want it as Ycertain=(max abs(y)-min(abs(y))/2 and then extract the value of corresponding x thanks
KSSV
KSSV 2018-9-26
You find the value and and place the value inplace of 0.5....it is simple.
val = max(y(:)-abs(y(:)/2)) ;
Alin Brad
Alin Brad 2018-9-26
I have made the modification and change the value of y to idx = (max(y(:))-(abs(min((y(:)))))/2) however, I got one value of idx and now values of xr and got the following message #Array indices must be positive integers or logical values#.
Walter Roberson
Walter Roberson 2018-9-26
You would not use val as idx. You would use
idx = abs(y-val)<=10^-3 ;

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更多回答（1 个）

Alin Brad
Alin Brad 2018-9-26

0 个投票

I have made this modification to make surface and make xr as Z dimension , but it doesnot work properly
N = 100 ;
x = linspace(-5,5,N) ;
p = linspace(0,1,N) ;
w = linspace(0,3,N) ;
[x,p,w] = meshgrid(x,p,w) ;
y=(1./((x.^2)+1))-(p./((x+w).^2+1))-(p./((x-w).^2+1));
figure
hold on
for i = 1:N
surf(x(:,:,i),p(:,:,i),w(:,:,i),y(:,:,i))
end
% GEt manually y
val=(max(y(:))-(abs(min((y(:)))))/2)
idx = abs(y-val)>=10^-3 ;
xr = x(idx) ;
pr = p(idx) ;
wr = w(idx) ;
figure
scatter3(xr,pr,wr,10,wr)
% use isosurface
p = patch(isosurface(xr,pr,wr,y,val));
p.FaceColor = 'red';
p.EdgeColor = 'none';
daspect([1 1 1])
view(3);
axis tight
camlight
lighting gouraud

4 个评论
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Walter Roberson
Walter Roberson 2018-9-26
You cannot use isosurface() for scattered points.
idx = abs(y-val)>=10^-3 ;
is creating a logical mask that talks about individual points inside the grid y . When you take x(idx) and so on you are extracting only those points -- scattered points.
Consider
yt = y;
yt(~idx) = nan;
p = patch(isosurface(x, y, w, yt, val));
You will probably want to do that in a different plot, or erase what you have drawn first: the values form a thin irregular sheet that is effectively hidden by the heavy point density of the scatter3()
Alin Brad
Alin Brad 2018-9-26
Thank you The bellow program returns a message The size of X must match the size of V or the number of columns of V. and show a plot of p as z axis which should be xr as z axis
N = 100 ;
x = linspace(-5,5,N) ;
p = linspace(0,1,N) ;
w = linspace(0,3,N) ;
[x,p,w] = meshgrid(x,p,w) ;
y=(1./((x.^2)+1))-(p./((x+w).^2+1))-(p./((x-w).^2+1));
figure
hold on
for i = 1:N
surf(x(:,:,i),p(:,:,i),w(:,:,i),y(:,:,i))
end
% GEt y
val=(max(y(:))-(abs(min((y(:)))))/2)
idx = abs(y-val)>=10^-3 ;
yt = y;
yt(~idx) = nan;
xr = x(idx) ;
pr = p(idx) ;
wr = w(idx) ;
figure
scatter3(xr,pr,wr,10,wr)
% use isosurface
p= patch(isosurface(xr, pr, wr, yt, val));
p.FaceColor = 'red';
p.EdgeColor = 'none';
daspect([1 1 1])
view(3);
axis tight
camlight
lighting gouraud
Walter Roberson
Walter Roberson 2018-9-28
Notice in my call I did not select subsets of x and so on for the isosurface call: I left them the original size but assigned nan to locations in the grid that were not of interest.

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