First, minor things:
- I would recommend you put a space (preceded by a colon maybe) at the end of your input strings so that what the user types is a bit separated from your prompt.
- I would also modify the function prompts to say that the functions must be expressed in term of x.
So, something like:
f1 = input('Please input your first function of x: ');
As your code is currently designed, it never sees the actual function. I feel your variable names f1 and f2 are a bit misleading as f1 and f2 are not functions, they're the values of the functions evaluated at the values of x. input does this evaluation for you (which incidentally makes input a dangerous function. If the user enters a command to format the hard disk, input will happily do so).
So, if you want to capture the function that was entered, you would need to stop input evaluating the function and return what was typed instead. You can then do the evaluation yourself:
f1 = input('Please input your first function of x: ', 's');
plot(x, eval(f1), 'c-', 'LineWidth', 2);
legend(f1, f2, 'location', 'bestoutside');
Note that I'm using eval to perform the same evaluation that input did. eval is a very dangerous function which typically encourages bad coding patterns and thus we strongly recommend against using it. In this particular context however it is the simplest way to achieve what you want without going into more advanced coding patterns.