[Tutor] Input to python executable code and design question

[jfouhy at paradise.net.nz]

Quoting Ismael Garrido <ismaelgf at adinet.com.uy>:

> I am trying to make a program that will plot functions. For that, I need
> to be able to get an input (the function to be plotted) and execute it.
> So, my question is, how do I use the input? I have found no way to 
> convert the string to some kind of executable code.

So you want the user to be able to type something like "f(x) = sin(2*x)" and
then your program will plot it --- is that correct?

Maybe you could parse it yourself?  I have found SimpleParse quite easy to use
--- http://simpleparse.sourceforge.net/ .  You will need to write your own
grammar to describe functions --- something like this, I guess:

eqn := fname, '(', varlist, ')=', expr
varlist := var, (',', var)*

expr := atom, (binop, atom)?
atom := var / (fun, '(', expr, ')') / num
fun := 'sin' / 'cos' / 'tan' / ...

var := char
fname := char+
num := digit+
binop := '+' / '*' / '/' / '-'

char := [a-zA-Z]
digit := [0-9]

although you will need to work on that to get the precedence right and avoid
undesired recursion and the like.  SimpleParse will give you a tree (made of
nested tuples) representing your function.  You can then have a go at converting
the tree to a function.  

I guess the standard way to do this would be something like:

def convert(node):
    functionName = node[0]
    children = node[1]
    if functionName == '*':
        return convert(children[0]) * convert(children[1])
    elif functionName == '+':
        ...

But you may be able to come up with something more clever.

Hope this helps.

-- 
John.