#!/usr/bin/perl

use warnings;
use strict;
use Math::SigFigs;

my $num = 123.456;

print FormatSigFigs($num,1), "\n";
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Thank you very much for your advice. I have tried your suggestion and it does indeed work very well. However, it has made me realise that my question was ill considered. My problem is actually as follows: I have an 8 character wide field into which I must round a number (possibly with a decimal point, possibly not). I need to be able to say, "Take this number, and round it so that, including it's decimal point, if it has one, and any zeros, is only 8 characters long". I have tried using $num = sprintf "%8g", $num; but this seems to give me a 7 character long number with a space at the front. Thank you once again for your advice and any further help you can offer.

$num = sprintf "%8g", $num;

Almost. shell$> perldoc -f sprintf reveals this little snippet:

  # Format number with up to 8 leading zeroes
  $result = sprintf("%08d", $number);
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Change the "d" to a "g" (which you had), and you'll be there. You'll probably find you want to do the SigFig rounding first, then format it for display with the sprintf.
cheers
davis
Is this going out live?
No, Homer, very few cartoons are broadcast live - it's a terrible strain on the animator's wrist

In that case, to a first approximation you could do something like the following:

#! /usr/bin/perl -w

use strict;

while( <DATA> ) {
    chomp;

    my $nr = squeeze($_);
    print "[$nr] $_\n";
}

sub squeeze {
  my $n = sprintf( '%0.8f', $_[0] );
  $n =~ s/^0//;
  substr( $n, 0, 8 );
}

__DATA__
12345678901234567890
1234567890
1234567.0
1234567.9
123456.789012345
12345
123.4567
123.4
.1
.1234567890
0.00000999999
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Then again, the few test cases here reveal a certain number of bugs. I contend that the results to squeeze() contain a bug, in that 1234567.0 and 1234567.9 return the same results, but in truth the latter should return 1234568. I don't know whether you will hit these borderline cases or not.

Maybe there's yet another module that deals with your problem.

<update>In response to gjb's remark about whether this code is a good idea or not, I was trying to point out the folly of storing numbers in fixed-width fields. The idea being that when you see what this outputs, maybe you better start thinking about overflow conditions. As I don't know the domain. I can't really deal with in a satisfactory matter (at least to my standards). I prefer to let the code stand as it is.</update>


print@_{sort keys %_},$/if%_=split//,'= & *a?b:e\f/h^h!j+n,o@o;r$s-t%t#u'

Heh. Try doing something like:

my $num = 12345.678; # a total of 9 characters
if (length $num > 8)
{
    if ($num =~ /\./) # If there's a decimal point
    {
        # Put it to seven significant figures
    }
    else
    {
        # Put it to eight significant figures
    }
}
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Another way to test for the decimal point, if you wanted to be purely math-based, could be if ($num == round($num)).

------
We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

The number of significant digits depends on the calculations you're doing with a set of input numbers. Essentially you'll have to keep track of the errors (relative and/or absolute) that accumulate during a calculation.

Two simple examples:

• 5.02 (+/- 0.01) + 3.1 (+/- 0.1) = 5.1 (+/- 0.2)
• 5.02 (+/- 0.01, 0.2%) * 3.1 (+/- 0.1, 4%) = 15.6 (+/- 0.8, 5%)

• The absolute error of the sum of two numbers is the sum of the absolute errors of the terms.
• The relative error of the product of two numbers is the sum of the relative errors of the factors.

You'll find a treatment of these concepts in any good book on numerical methods, Numerical Recipes in C is available online and not too bad. (Specifically, check out this chapter

Hope this helps, -gjb-

Update: Excellent pointer by davis in the node below.

Have a look at Does Perl Have A round() function? in the Perl FAQ Number 4 perlfaq4

E.G. The POSIX module (part of the standard Perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric functions.

    use POSIX;
    $ceil   = ceil(3.5);                        # 4
    $floor  = floor(3.5);                       # 3
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my $number = 0.9999;
printf "number = %.2f \n", $number;
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my $number = 0.9999;
$rounded = sprintf "%.2f", $number;
print "rounded = $number\n";
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Update:

A.A. is having a bad day and begs your indulgence for his stupidity

Drop the zero in the format.

Update: Urm... that will replace the leading zero with a space, which is only part of your problem... Apparently, the default precision offered by "%g" is smaller than the precision you desire.

OK, try this:

$_ = 1298.844667;
my $clem = sprintf("%.7g", $_); 
print "$clem\n";
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Result:

1298.845
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OK?