in reply to Round a Number to Any Place

As nuance pointed out above, your routine is not mathematically correct. Here is my attempt: 
sub round 
{
    my( $num, $place )= @_;
    $num = $num/$place;
    my $calc = int( $num + 0.5 * ( $num <=> 0 ) );
    $calc += (0 <=> $num) if( $calc % 2 and 0.5 == abs( int($num) - $n
+um ) );
    return $calc * $place
}

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Oh, and don't bother debating the validity of rounding to the nearest even. I wasted several hours trying to do that the last time around.

NOTE: I think there is a bug in that code, but I'm tired and I couldn't place it.

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RE: RE: Round a Number to Any Place
by tye (Sage) on Aug 30, 2000 at 09:31 UTC

    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

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    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;
}

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            - tye (but my friends call me "Tye")
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      That makes sense, I don't know why I didn't think of that. Thanks!
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        I didn't think of it either. But I did immediately jump into the Perl debugger and found that sufficient so I didn't have to "resort to" adding print statments. ;)

                - tye (but my friends call me "Tye")
        (Sneaky "persistent preview" for Coming soon: Algorithm::Loops finally removed from this out-of-the-way node after all of this time)
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In Section Cool Uses for Perl