I'm trying to write a simple anonymous function which returns the length of an array
>> a=[1 2 3];
>> f = @() length(a);
>> f()
3
>> a = [1 2 3 4];
>> f()
3
Is it possible to write a function that returns the length of the updated array every time it's called?
An ugly method to accomplish what you want
global a;
a = [1 2 3];
f = @() eval('global a; length(a)')
f()
a = [1 2 3 4];
f()
I'm compelled to say that I strongly recommend against this type of code as it relies on both globals and calls to eval, both of which should be avoided when possible.
A better method would be to pass in a as an argument to the function
a = [1 2 3];
f = @(x) length(x)
f(a)
a = [1 2 3 4];
f(a)
or, because in this case calling f is identical to calling length, then there's really no reason to use anonymous functions at all.
a = [1 2 3];
length(a)
a = [1 2 3 4];
length(a)
eval method, it only encourages bad practise, don't use eval when avoidable! Defining the function to take an argument is the correct answer, aside from explaining why it was previously failing (because a was static within f when f was defined in terms of a)
a must not be an argument then using eval may not be avoidable. If passing in a as an argument is possible then eval should not be used.a must not be an argument (especially in something this simple), then you've structured your code badly.Defining an anonymous function in terms of a variable makes that variable static within the function. i.e.
% this...
a = [1 2 3];
f = @() length(a);
% ... is equivalent to this...
f = @() length([1 2 3]);
You want to create an anonymous function which can take an argument
f = @(x) length(x);
a = [1 2 3];
f(a); % >> ans = 3
a = [1 2 3 4];
f(a); % >> ans = 4
Although at that point, just use length(a) and don't define some pointer-function!
apassed in as an argument?lengthin your function, so why not just uselength? If you need a function handle, usef = @length;.