[Tutor] base n fractional

[Dave Angel]

Chris Castillo wrote:

> Message: 1
> Date: Sun, 5 Apr 2009 15:36:09 -0500
> From: Chris Castillo <ctcast at gmail.com>
> Subject: [Tutor] base n fractional
> To: tutor at python.org
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> I need some help converting the fractional (right side of the decimal) to
> base 10.
> I have this but it doesn't work
>
> mynum = raw_input("Please enter a number: ")
> myint, myfrac = mynum.split(".")
> base = raw_input("Please enter the base you would like to convert to: ")
> base = int(base)
> mydecfrac = 0
> fraclen = len(myfrac) - 1
>
> for digit in range(len(myfrac) -1, -1, -1):
>     mydecfrac = mydecfrac + int(myfrac[digit])
>     mydecfrac = mydecfrac / base
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>   
First we need a clear statement (with examples) of your goal.  Your 
prompts to the user indicate you want to convert from decimal to some 
other base.  But your comments here and elsewhere on the thread indicate 
the exact opposite.  The two problems are related, but mixing them will 
just confuse everybody.

> so when I say that to myself i see:
> number = 234
> mysum = 0
> for digit in range(len(number) -1, -1, -1):
>    mysum = (mysum) * (1/base) + int(number[digit])

This just isn't valid python.  You can't subscript an integer.  You probably need a string here.  That is what raw_input() would produce.

So let's get a specific example, and try to work with it.


Perhaps you want
    base = 5
   myfrac = "234"

and you want to figure the value that .234 would mean if it's 
interpreted as base 5.  First, let's do it by hand.
   The two is in the first digit to the right of the decimal place, and 
therefore represents 2/5
   The three is in the next place, and represents 3/25
   And the four is in the next place and represents 4/125
Result is 0.552 decimal

There are two ways to work a base conversion.  One is to do the 
arithmetic in the source base, and do successive multiplies of the 
destination base.  That would mean working in base 5 in this case, which 
is probably more work in Python.  The other is to work in the result 
base, and do the multiplies of the source base.  That's the approach you 
were taking, and it works great if the precision of a float is acceptable.


Your code is fine, although a bit convoluted.  Only problem is that 
you're working in integers, when you need float.  So just change 
mydecfrac to 0.0  and it'll work.