Vectorising a function array in Matlab

Question

My generic problem is illustrated by the following example:

f=@(x,y) cos(x.*y);
Yvalues = linspace(0,1,50);
W = @(x) f(x,Yvalues);

which works fine if I only want to evaluate W at one point at a time. For instance:

norm(W(pi/3)-f(pi/3,Yvalues))
ans =

      0

But how do I go about evaluating W at any number of points?

Thanks in advance.

in your second code snippet, I believe you meant Yvalues when you wrote x ? Also, the norm of a vector is by definition scalar (it's the "length" of the vector), so the fact that your second code snippet is returning an expected value. Note that W(pi/3)-f(pi/3, Yvalues) returns a row of 50 zeros. As long as the input to W is the same length as Yvalues, W returns a vector. — Dang Khoa
– Dang Khoa, Commented Sep 9, 2011 at 17:18

stardt · Accepted Answer · 2011-09-09 22:15:03Z

1

If you change

f=@(x,y) cos(x.*y);

to

f=@(x,y) cos(x'*y);

you can execute W([1 2 3])

For example,

>> f = @(x,y) cos(x'*y);
>> yv = linspace(0,1,5);
>> W = @(x) f(x,yv);
>> W(1)
ans =
    1.0000    0.9689    0.8776    0.7317    0.5403
>> W(2)
ans =
    1.0000    0.8776    0.5403    0.0707   -0.4161
>> W(3)
ans =
    1.0000    0.7317    0.0707   -0.6282   -0.9900
>> W([1 2 3])
ans =
    1.0000    0.9689    0.8776    0.7317    0.5403
    1.0000    0.8776    0.5403    0.0707   -0.4161
    1.0000    0.7317    0.0707   -0.6282   -0.9900

answered Sep 9, 2011 at 22:15

stardt

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1 Comment

Dang Khoa Over a year ago

cos(x'*y) gives you something completely different than cos(x.*y) though - depending on size(x'*y) you'll get a scalar, a vector, or a matrix; cos(x.*y) is an element-by-element multiplication of x and y, so this operation returns a vector of length x (which is, incidentally, the same length as y). So I guess it depends on what the OP wanted; just saying that f(x,y) as defined here will return variable-sized matrices based on the input sizes.

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