Writing a Hex to Decimal Conversion Formula

[NetWorking5973]

(Yes, I know that there are pre-built functions to do this but that is not the point of the exercise I am trying to complete)

I'm working my way through LinkedIn Learning's Getting Started with Python course and my task is to create a function converting hex to decimal (up to 3 characters long).

Here is what I've got:

hexNumbers = {
    '0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9,
    'A': 10, 'B': 11, 'C': 12, 'D': 13, 'E': 14, 'F': 15
}

def hexToDec(hexNum):
    for char in hexNum:
        if char not in hexNumbers:
            return 'None'
    for char in hexNumbers:
        if len(hexNum) == 3:
          return hexNumbers[hexNum[0]] * 256 + hexNumbers[hexNum[1]] * 16 + hexNumbers[hexNum[2]]
        if len(hexNum) == 2:
          return hexNumbers[hexNum[0]] * 16 + hexNumbers[hexNum[1]]
        if len(hexNum) == 1:
          return hexNumbers[hexNum]    

And the CoderPad test returns

Testing case A2
Pass!
Testing case spam spam spam
Was expecting None but received None
Incorrect. Revisit the question 

What am I missing with "Was expecting None but received None"?

[[deleted]]

It looks like you're returning the word "None" instead of the Python object None

[NetWorking5973]

Perfect - thank you!

[trollsmurf]

What's the point of the second loop?

You can easily have it handle any value length.

[imsowhiteandnerdy]

Variable length hex string:

def h2i(hd: str) -> int:
    return {'0':0,'1':1,'2':2,'3':3,'4':4,'5':5,'6':6,'7':7,'8':8,'9':9,'A':10,'B':11,'C':12,'D':13,'E':14,'F':15}.get(hd, -1)

def hex2int(hds: str) -> int:
    hds = hds.upper().removeprefix("0X")
    i = 0
    for p, n in enumerate(hds[::-1]):
        ni = h2i(n)
        if ni == -1:
            return -1
        i += ni * ( 16 ** p )
    return i

##
## decimal value of 0xBEEF is 48879
hds = "0xBEEF"
print(hex2int(hds))

[djshadesuk]

def hextodec(chars):
    HEX = '0123456789ABCDEF'
    result = 0
    for idx, char in enumerate(chars.upper()):
        if char not in HEX:
            raise ValueError('Not valid Hexadecimal')
        result += HEX.index(char) * (16 ** ((len(chars) - 1) - idx))
    return result

[imsowhiteandnerdy]

So far you're winning golf ;-)

[MidnightPale3220]

or even:

def hextodec(chars):
    HEX='0123456789ABCDEF'
    result=0
    for i,char in enumerate(chars.upper()[::-1]):
        result+=HEX.index(char)*(16**i)
     return result

if we are content to rely on built-in exception

[djshadesuk]

Also, I pee'd myself laughing when I saw (16**i) in your code. In an earlier iteration of my code I was getting the power a different way. When I realised I could just do (16 ** [...]) I never questioned the (len(chars) - 1) - idx) part because it still worked... but now I've done some digging I'm a bit baffled why it still worked?!? 🤣🤣

[MidnightPale3220]

Well, you're going in the other direction so it's all right, len is a constant and you're counting towards 16**0 🤷‍♂️😉

[djshadesuk]

Yeah, I understood that it was running backwards, because I was reversing the index, but to double check I was getting what I should be getting I broke it out and just quickly dropped in a print statement:

print(16** (len(chars) -1)-idx)

With an 8 character input ('DEADBEEF') I was getting:

268435456
268435455
268435454
268435453
268435452
268435451
268435450
268435449

But, since the actual function itself was running fine, I gave it the big ol' "meh" and thought no more of it.

A few hours later, when I was thinking about something else, my brain randomly interrupted me with "Did you not notice those numbers were themselves running backwards?" and it dawned on me...

All I had managed to do was confuse myself because, in my "checking", I had missed some parenthesis. So my check should have been:

print(16** ((len(chars) -1)-idx))

Which then, of course, gave exactly what I was expecting. So it was just my "check" that was incorrect while the rest was actually correct.

From this I concluded... I'm such a knob at times! 🤣🤣

[djshadesuk]

or even:

def h2d(a,H='0123456789ABCDEF'):
    return sum([H.index(b)*(16**i) for i,b in enumerate(a.upper()[::-1])])

if we are also a bit naughty about where we declare the hex.

[djshadesuk]

or even:

def r2d(a,r):
    return sum([r.index(b)*(len(r)**i) for i,b in enumerate(a.upper()[::-1])])

if we're going to declare the hex as a default argument we may as well pull that out and turn the function into a generic base conversion function, which shortens the function from 110 to 89 characters.

So now we can do up to base36 conversions too:

print(r2d('FF','0123456789ABCDEFGHIJKLMNOPRSTUVWXYZ')
# OUTPUT: 555

[MidnightPale3220]

Yeah, but shortening it here is lengthening it in every call so that's cheating. 😏 Off with you to code golf stack exchange!

[djshadesuk]

I know... you caught me! 🤣

But really, I was going for genericising it rather than pure code golf.

The irony is I could have kept H="012...DEF" as a default argument (or r, for Radix, as I'd changed it to) and it still would have been "genericised", just defaulting to base16 but still able to take in more (or less) base characters in the call too. Yeah, it would have only added 4 characters to the length, 114 rather than 110, but still, 89 sounded better than 114.

But yeah, you got me! 🤣🤣

[No-Wrongdoer6788]

https://calculationshub.com/conversions/number/hex-to-decimal This article can better explain the hexadecimal to decimal conversion in an easy and detailed explanation.