Piecewise 3D plotting

Hi, I need to do a 3D plot of the following functions using piecewise:

y1 = -0.00278018473212*x.^3 + 0.0764514697018*x.^2 + -0.481036946424*x + 4.37885609332;% both need to be in the interval of 4 < x < 14 as well
y2 = 0.0000772436429838*x^3+0.0146583319491*x^2-0.184551271403*x+4.99872818128;% both need to be in the interval of 4 < x < 14 as well
y3 = 4.5 %interval 2.5 < x < 4
y5 = 5.5 %interval 14 < x < 14.5
y6 = 5.0 %interval 14 < x < 14.5

Z = piecewise(2.5 < x < 4 ,y3,4 < x < 14,y2,14 < x < 14.5, y5);

Error using surf
Z must be a matrix, not a scalar or vector.

How can I:

1) plot the function correctly

and

2) add 'y1' into the piecewise as it occupies the same interval as y2 (4<x<14)?

Thanks in advance.

[Matt J]

Surface plots have 2 independent variables, but you appear to have only one, namely x. So, in what way is it a 3D plot?

[Moisés Abel]

I just edited it for it to look clearer, the other plot was supposed to be 2D, but I now need to make a 3D plot

[Matt J]

It's not clearer because Z still depends on only one independent variable, not two.

As a preliminary remark, though, ndgrid and surf are not doing anything meaningful here. Z is a symbolic variable, which surf() doesn't know how to deal with.

[Moisés Abel]

So what would Z need to be in this case?

[Matt J]

I see that you have edited your post a second time, but the revisions you've made are irrelevant to the questions I asked you before.You will have no hope of clarifying the post until you answer them.

So once again: Are you trying to make a surface plot? In a surface plot, Z is a function of two variables Z(x,y). But in your post, Z depends on only one one variable Z(x). So, in what way is Z() to be seen as a function of two variables?

Answer 1

[Matt J]

Perhaps this is what you meant:

syms Z(x,y)
y1 = -0.00278018473212*x.^3 + 0.0764514697018*x.^2 + -0.481036946424*x + 4.37885609332;% both need to be in the interval of 4 < x < 14 as well
y2 = 0.0000772436429838*x^3+0.0146583319491*x^2-0.184551271403*x+4.99872818128;% both need to be in the interval of 4 < x < 14 as well


Z1(x,y) = piecewise( 2.5 < x & x < 4 , 4.5,...
               4 < x   & x < 14, y1,...
              14 < x   & x < 14.5, 5.5 );


Z2(x,y) = piecewise( 2.5 < x & x < 4 , 4.5,...
               4 < x   & x < 14, y2,...
              14 < x   & x < 14.5, 5.5 );



fsurf(Z1); hold on; fsurf(Z2); hold off
xlim([2.5 14.5])
ylim([2.5 14.5])

[Moisés Abel]

Not quite, maybe I should rewrite my code and rephrase my post so that it is clearer what I want to do