Code-Golf: Friendly Number Abbreviator

Answer 1

Ruby - 79 77 75 83 chars

n,d=ARGV
l=n.to_s.length
printf"%.#{l>3?d:0}f%s",n.to_f/10**(l-l%3)," kMGTPE"[l/3]

Reads from command line arguments.

74 72 80 chars, prints output within double quotes

n,d=ARGV
l=n.to_s.length
p"%.#{l>3?d:0}f%s"%[n.to_f/10**(l-l%3)," kMGTPE"[l/3]]

66 74 chars, prints extra zeroes

n,d=ARGV
l=n.to_s.length
p"%.#{d}f%s"%[n.to_f/10**(l-l%3)," kMGTPE"[l/3]]

Based on this solution, and the sample code.

Answer 2

Haskell, 126 (without import, it's a function that takes two arguments):

f n p|l>3=showFFloat (Just p) (c n/c 10^(l-w)) [" kMGTPE"!!f]|True=show n where(f,w)=divMod l 3;c=fromIntegral;l=length$show n

Expanded:

import Numeric

doit :: Integer -> Int -> String
doit n p
    | l > 3 = showFFloat (Just p) d [" kMGTPE" !! f]
    | otherwise = show n
    where
    d = (fromIntegral n) / fromIntegral (10^(l-w))
    (f,w) = divMod l 3
    l = length $ show n

Answer 3

Perl 114 111 104 chars

My first ever code-golf entry!

Arguments provided from standard input: perl fna.pl 918395 1

($n,$d)=@ARGV;
@n=$n=~/./g;
@s=' kMGTPE'=~/./g;
printf"%.".(@n>3?$d:0)."f%s",$n/(10**($#n-$#n%3)),$s[@n/3];

Output:

918.4k

---

De-golfed version (with explanation):

( $number, $dp ) = @ARGV;      # Read in arguments from standard input

@digits = split //, $number;   # Populate array of digits, use this to count
                               # how many digits are present

@suffix = split //, ' kMGTPE'; # Generate suffix array

$number/(10**($#n-$#n%3));     # Divide number by highest multiple of 3

$precision = @n>3 ? $dp : 0;   # Determine number of decimal points to print

sprintf "%.".$precision."f%s", # "%.2f" prints to 2 dp, "%.0f" prints integer
        $number, $suffix[@n/3];# Select appropriate suffix