Not getting the expected size matrix from evaluating a function handle that is equal to zero

h = @(x,y) 0

[X, Y] = meshgrid(linspace(0, 2, 10));

Z = h(X,Y);

X and Y are 10 x 10.

I am expecting Z to be zeros matrix of 10x10. But I am getting Z to be a 0 of 1x1.

Why?

Answer 1

[Leonardo]

h = @(x,y) zeros(size(x));
[X, Y] = meshgrid(linspace(0, 2, 10));
Z = h(X,Y)

Z = 10×10

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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surf(X,Y,Z)

Answer 2

[VBBV]

h = @(x,y) zeros(10)

h = function_handle with value:
    @(x,y)zeros(10)

[X, Y] = meshgrid(linspace(0, 2, 10));
Z = h(X,Y)

Z = 10×10

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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[Nilay Modi]

But I am not going to know ahead of time what my meshgrid output size will be...I am trying to have a general function.

[Leonardo]

Replace 10 by size(x):

h = @(x,y) zeros(size(x));
[X, Y] = meshgrid(linspace(0, 2, 10));
Z = h(X,Y)

Z = 10×10

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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[Nilay Modi]

something like this...

function graph_surface(h, NameValueArgs)

arguments

h function_handle

NameValueArgs.xy_limit

end

[X, Y] = meshgrid(linspace(0, NameValueArgs.xy_limit, 10));

Z = h(X,Y);

surf(X,Y,Z)

end

[Torsten]

Then you will have to pass this information as input to the function handle.

You could do

h = @(x,y) zeros(size(x))

or more general

h = @(x,y,m,n) zeros(m,n);
[X, Y] = meshgrid(linspace(0, 2, 10));
Z = h(X,Y,10,10)

Z = 10×10

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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[Nilay Modi]

Ok. I see your work arounds.

But why isn't Matlab not matching my intuition...for every input (x,y) I am expecting f(x,y) = 0...so I expect z to be zeros of the size of my input matrix and instead Matlab spits out one 0...and then we go about finguring out work arounds.

[Torsten]

And what would you expect MATLAB to return if x and y have different sizes ? Should it read your mind ?

You must explicitly tell MATLAB the size of the matrix of zeros it should return. In your case

h = @(x,y) 0

you told MATLAB to return the scalar 0 - independent of the sizes of the inputs.

[Bruno Luong]

Believe or not MATLAB is not yet smater than you. It does exacly what you tell it to do (returning scalar 0)

[Nilay Modi]

syms g(x,y)

g(x,y) = zeros(size(x));

graph_surface( ...

matlabFunction(g), ...

xy_limit=5, ...

FaceColor='interp')

function graph_surface(h, NameValueArgs)

arguments

h function_handle

NameValueArgs.xy_limit

NameValueArgs.FaceColor

end

[X, Y] = meshgrid(linspace(0,NameValueArgs.xy_limit,10));

Z = h(X,Y);

surf(X,Y,Z, FaceColor=NameValueArgs.FaceColor)

xlabel('x')

ylabel('y')

zlabel('z')

box on

ax = gca;

ax.BoxStyle = 'full';

axis tight

axis equal

axis vis3d

end

[Nilay Modi]

inside my graph_surface function, h is just 0 and not zeros(size(x)). Why?

[Torsten]

Define

g = @(x,y,m,n) zeros(m,n)

in the script part

and use it as

Z = g(X,Y,size(X,1),size(X,2))

in your function.

Don't overcomplicate things - your code looks complicated enough.

[Nilay Modi]

it works now, after changing to

g = @(x,y) zeros(size(x));

graph_surface( ...

g, ...

xy_limit=2.5, ...

FaceColor='interp')

Answer 3

[Steven Lord]

I am expecting Z to be zeros matrix of 10x10. But I am getting Z to be a 0 of 1x1.

Why?

Because that's what you told your function to return.

Based on your later comment, you don't want your function to return a 1-by-1 or even a fixed 10-by-10. You want it to return a zeros matrix the same size as the input. Correct?

f = @(X, Y) zeros(size(X));
X = ones(4)

X = 4×4

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

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f(X, X)

ans = 4×4

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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Y = ones(5, 8)

Y = 5×8

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

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f(Y, Y)

ans = 5×8

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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Z = 42

Z = 42

f(Z, Z)

ans = 0