"number-in-base" ``oneliner''

[Alex Martelli]

I hesitate a bit to post this, but... on the Italian Python NG, somebody
was asking whether there was any way to convert an integer number x into
a string which represents it in an arbitrary base N (given a sequence
with a len of N that gives the digits to use) as "a single expression".

I haven't found a really general way, much less a clear one, but, the
best I have so far is...:

def number_in_base( x, N, digits, maxlen=99):
return '-'[x>=0:] + (
(x and ''.join([digits[k%N] for i in range(maxlen)
for k in [abs(x)//N**i] if k>0])[::-1]
) or digits[0])

Besides the lack of clarity, the obvious defect of this approach is that
darned 'maxlen' parameter -- but then, since I can have only a 'for',
not a 'while', in a list comprehension or generator expression, and I
don't think recursion qualifies as 'a single expression'...:-(

Anyway, improvements and suggestions welcome, thanks!
Alex

[Irmen de Jong]

Andrea Griffini wrote:

roman = lambda x: "".join(["".join(map(lam bda c: "IVXLCDM"[int(c)
+2*N],"/0/00/000/01/1/10/100/1000/02".split("/")[x//10**N % 10]))
for N in (3, 2, 1, 0)])

This rocks!! I'm still trying to understand how it works though :)
--Irmen

[Andrea Griffini]

On Mon, 01 Nov 2004 11:06:24 +0100, Irmen de Jong
<[e-mail]> wrote:

Andrea Griffini wrote:

roman = lambda x: "".join(["".join(map(lam bda c: "IVXLCDM"[int(c)
+2*N],"/0/00/000/01/1/10/100/1000/02".split("/")[x//10**N % 10]))
for N in (3, 2, 1, 0)])

This rocks!! I'm still trying to understand how it works though :)

The idea is to use the pattern

"", "0", "00", "000", "01", "1", "10", "100", "1000", "02"

where every character listed above is an offest in

"IVX", "XLC", "CDM", "M??"

For example applying the offsets to the first group you get:

"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"

i.e. the roman numbers for 0..9; and applying it to the second

"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC",

i.e. 0, 10, 20, ... 90.

I just used "split" and string indexing instead of regular
lists because the code becomes smaller (and more readable
than a mess of quotes and commas).

The rest is a loop on the thousands, the hundreds, the tens
and the units of the number and the joins needed to glue
all the pieces into a resulting string.

Andrea