CODEWITHSUNDEEP
perlnumbersrounding
Ryan You
Ryan You

Reputation: 73

How to round to the first left digit for decimal in Perl or Any Language?

I'm not sure if I explain my question clearly in title, basically I need a floor/ceil function like this:

sub ceil($num)

ceil(120) = 200
ceil(12) = 20
ceil(1.2) = 2
ceil(0.12) = 0.2
ceil(0.012) = 0.02
ceil(0.00000012) = 0.0000002

same apply to negative number (negative sign on both input and output)

thanks!

Upvotes: 2

Views: 173

Answers (2)

Hynek -Pichi- Vychodil
Hynek -Pichi- Vychodil

Reputation: 26121

sub floor {
    my $a = shift;
    $a =~ s/([1-9])(\d*)(?:\.\d*)?/$1.("0"x length($2))/e;
    0 + $a;
}

sub ceil {
    my $a = shift;
    my $f = floor($a);
    $f =~ s/([1-9])/1+$1/e if abs($a) > abs($f);
    0 + $f;
}

And test:

$ perl -E'sub floor {my $a = shift; $a=~s/([1-9])(\d*)(?:\.\d*)?/$1.("0"x length($2))/e; 0+$a} sub ceil {my $a = shift;my $f = floor($a); $f=~s/([1-9])/1+$1/e if abs($a) > abs($f); 0+$f} printf "%10g %10g %10g\n", $_, floor($_), ceil($_) for (120, 12, 1.2, 0.12, 0.012, 200, 20, 0.2, -120, -12, -0.12, -0.2,0, 1.2e12, -1.2e-12)'
       120        100        200
        12         10         20
       1.2          1          2
      0.12        0.1        0.2
     0.012       0.01       0.02
       200        200        200
        20         20         20
       0.2        0.2        0.2
      -120       -100       -200
       -12        -10        -20
     -0.12       -0.1       -0.2
      -0.2       -0.2       -0.2
         0          0          0
   1.2e+12      1e+12      2e+12
  -1.2e-12     -1e-12     -2e-12

Upvotes: 3

isick
isick

Reputation: 1487

This might work? I haven't tested it, but I think the logic is there.

Step 1 we find the nearest power of 10 (assign that to "$base")

Step 2 we determine if $num is a cleanly divisble by this base

Step 2.1 if it is, return that

Step 2.2 if it is not, subtract the modulo and add 1

$base = 10*floor(log($num,10));
return ($num % $base)?($num - ($num%$base) + 1):$num;

Math isn't my strongest thing so there could be a better way to go

Upvotes: 1

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