# How to plot a 3D surface with two vectors and one array?

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Bogdan MP on 21 Oct 2021
Commented: Walter Roberson on 22 Oct 2021
I want to plot a 3D surface , but I cannot apply the surf() command straightforwardly for some reason.
I have two vectors for x and y coordinates. Let us say
x = linspace(-10, 10, 100);
y = linspace(-0.5, 0.5, 100);
To calzulate I have to call some function which works only for fixed values of x and y. Thys, I calculate it as follows
for i = 1 : length(x)
for j = 1 : length(y)
func(i,j) = @MyFunction(__some_parameters__, x(i), y(j));
end
end
As a result I have two vectors for x and y and a matrix of the size length(x)*length(y) for the variable z. Could you tell me please, if there are any convenient methods to rewrite this data to a format that is appropriate for applying the surf() command?
P.S. I used plot3(x(i), y(j), func(i,j), 'b.') with two cycles, but it is a ridiculous "dirty hack" and not a surface.
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### Accepted Answer

Walter Roberson on 21 Oct 2021
Create full vectors of coordinates and arrayfun()
x = linspace(-10, 10, 100);
y = linspace(-0.5, 0.5, 100);
[X, Y] = meshgrid(x, y);
Z = arrayfun(@(xx,yy) sin(xx)+atan2(yy,xx), X, Y);
surf(x, y, Z, 'edgecolor', 'none')
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Walter Roberson on 22 Oct 2021
surf(x, y, z')
If that worked for your purposes, then chances are that when you constructed z, you varied x values down columns, and that you varied y values across rows -- that array z(J,K) is a function of x(J), y(K) . That is a very common data organization... but it is not the MATLAB data organization. In MATLAB, up/down (so, along columns) is y, and left/right (so, along rows) is x, and array location z(J,K) is z(y(J), x(K)) .
When you use meshgrid(xvector, yvector), then the coordinates are arranged in such a way that surf(x, y, z) works out.
When you use ndgrid(xvector, yvector) then the coordinates are arranged in such a way that surf(x, y, z) errors (unless xvector and yvector are the same length).
xvec = 1:2; yvec = 1:3
yvec = 1×3
1 2 3
[X, Y] = meshgrid(xvec, yvec)
X = 3×2
1 2 1 2 1 2
Y = 3×2
1 1 2 2 3 3
[X, Y] = ndgrid(xvec, yvec)
X = 2×3
1 1 1 2 2 2
Y = 2×3
1 2 3 1 2 3

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