ndgrid

Arguments

x, y, z, ...

vectors

X, Y, Z, ...

matrices in case of 2 input arguments, or else hypermatrices

Description

This is an utility routine useful to create arrays for function evaluation on 2, 3, ..., n dimensional grids. For instance in 2d, a grid is defined by two vectors, x and y of length nx and ny, and you want to evaluate a function (says f) on all the grid points, that is on all the points of coordinates (x(i),y(j)) with i=1,..,nx and j=1,..,ny. In this case, this function can compute the two matrices X,Y of size nx x ny such that :

X(i,j) = x(i)   for all i in [1,nx]
Y(i,j) = y(j)       and j in [1,ny]

and the evaluation may be done with Z=f(X,Y) (at the condition that you have coded f for evaluation on vector arguments, which is done (in general) by using the element-wise operators .*, ./ and .^ in place of *, / and ^).

In the 3d case, considering 3 vectors x,y,z of length nx, ny and nz, X,Y,Z are 3 hypermatrices of size nx x ny x nz such that :

X(i,j,k) = x(i)  
Y(i,j,k) = y(j)   for all (i,j,k) in [1,nx]x[1,ny]x[1,nz]
Z(i,j,k) = z(k)

In the general case of m input arguments x1, x2, .., xm, then the m output arguments X1, X2, .., Xm are hypermatrices of size nx1 x nx2 x ... x nxm and :

Xj(i1,i2,...,ij,...,im) = xj(ij)   
for all (i1,i2,...,im) in [1,nx1]x[1,nx2]x...x[1,nxm]

Examples

// create a simple 2d grid
nx = 40; ny = 40;
x = linspace(-1,1,nx);
y = linspace(-1,1,ny);
[X,Y] = ndgrid(x,y);

// compute a function on the grid and plot it
//deff("z=f(x,y)","z=128*x.^2 .*(1-x).^2 .*y.^2 .*(1-y).^2");
deff("z=f(x,y)","z=x.^2 + y.^3")
Z = f(X,Y);
clf()
plot3d(x,y,Z, flag=[2 6 4]); show_window()

// create a simple 3d grid
nx = 10; ny = 6; nz = 4;
x = linspace(0,2,nx);
y = linspace(0,1,ny);
z = linspace(0,0.5,nz);
[X,Y,Z] = ndgrid(x,y,z);

// try to display this 3d grid ...
XF=[]; YF=[]; ZF=[];

for k=1:nz
   [xf,yf,zf] = nf3d(X(:,:,k),Y(:,:,k),Z(:,:,k));
   XF = [XF xf]; YF = [YF yf]; ZF = [ZF zf];
end

for j=1:ny
   [xf,yf,zf] = nf3d(matrix(X(:,j,:),[nx,nz]),...
                     matrix(Y(:,j,:),[nx,nz]),...
                     matrix(Z(:,j,:),[nx,nz]));
   XF = [XF xf]; YF = [YF yf]; ZF = [ZF zf];
end

clf()
plot3d(XF,YF,ZF, flag=[0 6 3], leg="X@Y@Z")
xtitle("A 3d grid !"); show_window()

See Also

• kron — Kronecker product (.*.)

Authors

B. Pincon