[Tutor] A little math and Python: rpn was: Horner's rule

[Gregor Lingl]

Gregor Lingl schrieb:

>
> I tried to find a better solution:
>
> def rpn(inpstr, trace=0):
>    return rpnr(inpstr.split(), trace)
>
> def rpnr(lst, trace):
>    if trace: print lst
>    if len(lst) == 1: return lst[0]
>    if lst[-2] in ["+", "-", "*", "/"]:
>        return str( eval( lst[0] +lst[-1] + rpnr(lst[1:-1],trace)) )
>    else:
>        return str( eval( rpnr(lst[:-2],trace) + lst[-1] + lst[-2]) )
>

Maybe this one is clearer:

# evaluation of an rpn-string

def rpn(inpstr, trace=0):
    return rpnr(inpstr.split(), trace)

def rpnr(lst, trace):
    if trace: print lst
    if len(lst) == 1: return lst[0]
    if lst[-2] in ["+", "-", "*", "/"]:
        num1, num2, op = lst[0:1], lst[1:-1], lst[-1]
    else:
        num1, num2, op = lst[:-2], lst[-2:-1], lst[-1]
    return str(eval (rpnr(num1,trace) + op + rpnr(num2,trace)) )