Are there any other better ways to access a single bit of string of digits?

[Denis McMahon]

On Sun, 31 May 2015 11:36:35 -0700, fl wrote:

> I am new to Python. I would manipulate a string of hex numbers. If the
> first digit is bigger than 7, the first two digits are required to add
> 4.

What happens if the first two digits are ff, does it become 103 or 03.

If you have ffff_ffff_ffff

Do you want to create 103ff_103ff_103ff

or

03ff_03ff_03ff

or

0400_0400_03ff

or

10400_0400_03ff ......

> For example, '8022_3345' will be changed to '8422_3345'. The underscore
> between two 4-digit's was generated previously (i.e.
> it is already in the .txt file).
> 
> I have not tried to read the .txt file to a list yet. I just try the
> following:
> 
> tmp ['12345678', '23456789', '3456789a', '456789ab']
> 
> Each 8-digit hex number is assigned to a variable, such as:
> 
> digit8=tmp[0]
> 
> I can compare digit8[0] with 7, and so on...
> 
> The underscore, I think, can be removed by first a string replace.
> 
> My question here is:
> 
> Do you have better ways than my tmp, digit8 operation?

Yes, if these are numbers, manipulate them as numbers, not strings.

def bing(n):
    n = int(n.replace("_", ""), base=16)  # convert to numbers
    if n > 0x7fffffff:                    # if 0x80000000 or more
        n = n + 0x04000000                # add 0x04000000
    return n                              # and return result

newnums = [ bing(x) for x in oldnums ]

It could probably be done as a single list comprehension, but it might 
get a bit messy.

-- 
Denis McMahon, denismfmcmahon at gmail.com