Hex To Int In Python No Int Constructor

Question

I'm trying to turn a string representing a Hexidecimal number into an int in python without using the int constructor.

For example if I was given

    hexstring = "802"

How would I get that to be

    output = 2050

Without doing

    int("802",16)

How would I go about this?

@chepner: common homework problem, with a constraint to prevent the trivial answer. — Matthew Cole, Sep 08 '17 at 19:14
You could do `int()`'s work yourself, i.e. parse "802" into its component parts of `(2 * 1) + (0 * 16) + (8 * 256)`. — John Gordon, Sep 08 '17 at 19:15
`for i in range(0,): if hex(i)[2:] == hexstring: return i` -- just kidding, don't do this. — jedwards, Sep 08 '17 at 19:17
One way would be to use the values of this list: `weird_numbers = [ord(x) for x in hexdigits]`, perhaps in relation to the values of `ord('0')` and `ord('a')` (and/or `ord('A')`)... — jedwards, Sep 08 '17 at 20:22

score 2 · Answer 1 · edited Oct 22 '17 at 07:23

hexstring = "802"
L=len(hexstring)

def val(h_char):
    # Note you need to extend this to make sure the lowercase hex digits are processed properly
    return ord(h_char)- (55 if ord(h_char)>64 else 48)

def sumup(sum,idx):
    global hexstring # global variables are not recommended
    L=len(hexstring)
    return sum + 16**idx*val(hexstring[L-idx-1])

output = reduce(lambda a,b:sumup(a,b),range(L),0))

Below is just an explanation of the above and doesn't add any value
Processes on a list of [0,1,2] produced by range(L).

For each idx from above list a function call is made as sumup(sum, idx)=sum+16^idx*h_digit_at_idx.(^ is ** is exp in above)

h_digit_at_idx = ord(h_char)- (55 if ord(h_char)>64 else 48)

ord(h_char) produces 48,49...57,65,66,67,68,69,70 for hex characters 0,1...10,A,B,C,D,E,F

ord(h_char)-(55 if ord(h_char)>64 else 48 produces 0,1...10,11,12,13,14,15 for respective chars.

Finally the last argument of the reduce function is 0(which is the initial sum to start with)

Nice, yeah my solution used a "reducer" like 16*a + b (after turning the string into a list of integers in [0,16] with a similar `ord` trick. — jedwards, Sep 08 '17 at 20:28
Also this will produce unreliable results if lower case letter are input as hex digits. I hope OP can edit the statement to add that extra condition for lowercase hex digits. — kaza, Sep 08 '17 at 20:34

Hex To Int In Python No Int Constructor

1 Answers1