I would not recommend using a string to define the input function.
A much more reliable way would be to write your code to accept a function handle (which is the correct variable type for holding functions):
For example, define your function something like this:
function out = myfun(fun)
assert(isa(fun,'function_handle'),'Input must be a function handle')
out = fun(pi); % do whatever you want with the function handle...
end
and then simply call your function using any function handle:
myfun(@sin)
myfun(@cos)
myfun(@sqrt)
myfun(@(n)3*(n.^2))
If you really want to limit the user to using strings (not designed for holding functions) then consider using str2func or switch inside your function.
