Matlab anonymous function with array inputs

Question

I have an anonymous function:

a = [1, 2];
b = [1, 1; 3, 2];
c = [4, 2];
ff = @(x) (exp(a .* x) .* c) * b;

The problem is that, when I have an array, say x = [1,2,3,4] , Matlab thinks that I'm using the array and multiplying that in the exponential, and not each element. The error is

Error using .* Matrix dimensions must agree.

Error in @(x)(exp(a.*x).*c)*b

I just need something like c1 * exp(a1 *x) * b11 + c2 * exp(a2 * x) * b21 + ...

I can use a for loop if I want to evaluate the function for each x element and it gives me the answer that I want, but I think there may be an easiest way, like when we can use simply f(x) and get an array with each element evaluated in the function. I tried using arrayfun but I get the same error. I want to skip for loops since they are slow for bigger matrices.

You surely mean c1 * exp(a1 *x) * b11 + c2 * exp(a2 * x) * b21 + ...? — Luis Mendo
– Luis Mendo, Commented Nov 28, 2013 at 23:37
Which size should your result be (for the given example values)? — Luis Mendo
– Luis Mendo, Commented Nov 28, 2013 at 23:40
For a particular x , a 1x2 array. If I use a Nx1 array, it should give a Nx2 array with each column for different x — PerroNoob
– PerroNoob, Commented Nov 28, 2013 at 23:45

Marcin · Accepted Answer · 2013-11-28 23:32:00Z

3

What about this:

results = arrayfun(ff, x ,  'UniformOutput', false);
results{:}

answered Nov 28, 2013 at 23:32

Marcin

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9 Comments

PerroNoob Over a year ago

This seems to work perfectly, thank you. What does the {:} do? Since with the first command I get something like [1x2 double]

Marcin Over a year ago

@DavidWinchester arrayfun returns a cell. results{:} will just output the results.

Luis Mendo Over a year ago

@DavidWinchester Yes, use results = cell2mat(results.'); to get an Nx2 matrix

Luis Mendo Over a year ago

@DavidWinchester .' is matrix or array transposition. It turns the 1x4 array of 1x2-vectors into a 4x1 array of the same 1x2-vectors; and then cellmat puts that into matrix (4x2) form

Luis Mendo Over a year ago

@DavidWinchester Without the dot it also applies complex conjugation. Careful with that if you apply it for example to a matrix of complex numbers.

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lennon310 · Accepted Answer · 2013-11-28 23:47:24Z

1

Use arrayfun(ff,x,'UniformOutput',false) to return the 4 cells corresponding to your four outputs. I think it should be c1 * exp(a1 *x) * b11 + c2 * exp(a2 * x) * b21 + ... otherwise you need to transpose your b matrix before the multiplication.

edited Nov 28, 2013 at 23:47

answered Nov 28, 2013 at 23:31

lennon310

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1 Comment

PerroNoob Over a year ago

Yes, someone pointed that out before, I corrected my question. Thanks.

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