The best part is that *2 and /2 are lossless operations on floats. 

>perl -le"$,=' '; $x=.1; print map /(.)(.{11})(.*)/, unpack 'B*', reve
+rse pack 'd', $x/=2 for 1..5;"
0 01111111010 1001100110011001100110011001100110011001100110011010
0 01111111001 1001100110011001100110011001100110011001100110011010
0 01111111000 1001100110011001100110011001100110011001100110011010
0 01111110111 1001100110011001100110011001100110011001100110011010
0 01111110110 1001100110011001100110011001100110011001100110011010
- ----------- ----------------------------------------------------
S Exponent    Mantissa

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Precision already loss is not recuperated, of course. 

my $n; $n += 0.1 for 1..10;          # $n = 0.1 * 10 = 1  ...ish
my $r = int($n*2)/2;

print("round($n) = $r\n");           # round(1) = 0.5
print("cause it's really doing\n");  # cause it's really doing
printf("round(%.16e)\n", $n);        # round(9.9999999999999989e-001)

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In reply to Re^2: Round down to '.5' or '.0' by ikegami
in thread Round down to '.5' or '.0' by awohld

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