In chatterbox, I (belatedly) noticed Adam wonder why his code failed for round(-4.35,0.1). I think the reason helps to illustrate why I didn't worry about rounding to the nearest even. I can sum up the situation pretty easily with this: 

 % perl -le "print 4.35*100-435"
-5.6843418860808e-014

[download]

A floating point number can never be exactly 0.05. Floating point numbers are represented in binary and have a limited number of bits. 0.05 in binary requires an infinite number of bits (forming a repeating pattern, similar to how 1/7 is 0.142857142857... in decimal). So the extra code to round to the nearest even will probably never trigger when rounding to 1 or more places after the decimal point (I say "probably" because I don't claim to be able to predict the interactions of limited floating point precision on the calculations being done and it is possible that things would cancel out and trigger the extra code, perhaps in cases where it really shouldn't have).

Okay, here is my version of Adam's code: 

sub round {
    my( $num, $prec )= @_;
    $num= int( my $frac= $num/$prec + 0.5 - ($num<0) );
    $num -= ($num<=>0)   if  $num == $frac  &&  $num % 2;
    return $num*$prec;
}

[download]
        - tye (but my friends call me "Tye")

In reply to RE: RE: Round a Number to Any Place by tye
in thread Round a Number to Any Place by reptile

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