I might not be understanding your question properly, but it looks like your code works just fine. Here is an example:
a = linspace(0,2*pi,1000); % Create a vector of dimension N = 1000 with values increasing
% linearly from 0 to 2*pi
fun = @(x) sin(a*x); % Define your function
plot(a, fun(1)); % plot 1 period of the sine wave (from 0 to 2*pi);
hold
plot(a, fun(2)); % x = 2
plot(a, fun(4)); % x = 4
I made a a 1000 elements vector, then used your exact code to define fun, and used the new function with three different inputs. Works perfectly as you wrote it.
