How to define a new function from a matrix function?

Question

It is a very simple problem, though I do not know how to deal with it.

I defined a function f(x,y) which will return a, say, 2*2 matrix. I want to define a new function g whose value is the (1,1) element of f(x,y). I naively tried g=@(x,y)(f(x,y))(1,1), which of course failed . Please help me!

The easiest way to get the (1,1) element of a matrix is just to call `f(1,1)`. A function isn't really needed. — eigenchris, Dec 05 '14 at 14:56
Sorry, I realize you called your function `f`. If you have a matrix `M`, you can get its `(1,1)` element using `M(1,1)`. — eigenchris, Dec 05 '14 at 15:02
Have a look at [this question](http://stackoverflow.com/questions/3627107/how-can-i-index-a-matlab-array-returned-by-a-function-without-first-assigning-it). There several possible solutions are described. — hbaderts, Dec 05 '14 at 15:06

score 1 · Answer 1 · answered Dec 05 '14 at 15:07

1

You can use :)

g = @(x,y)([1 0] * f(x,y) * [1 0]')

answered Dec 05 '14 at 15:07

Kostya

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Beautiful solution, I'll keep that one in mind! Unfortunately not very practical for large matrices. – hbaderts Dec 05 '14 at 15:15
Of course, getfield is a proper one :) – Kostya Dec 05 '14 at 15:16

score 1 · Answer 2 · edited May 23 '17 at 11:57

You can use the function getfield

Let's define f(x,y) to return a 2x2 matrix

f = @(x,y) [1*x 2*y;3*x 4*y].^2 ;

Then let's define a function g11(x,y) which return the element {1,1} of f(x,y). (and an extra function g21 which return the {2,1} element)

g11 = @(x,y) getfield( f(x,y) , {1,1} ) ;
g21 = @(x,y) getfield( f(x,y) , {2,1} ) ;

And now:

>> f(3,4)
ans =
     9    64
    81   256

>> g11(3,4)
ans =
     9

>> g21(3,4)
ans =
    81

For more fancy usage of field assignment without temporary variable, read all the nice answers in this question

How to define a new function from a matrix function?

2 Answers2