G'day Dr Manhattan,

Saying "is not very useful" is not very useful to us. What format would you find useful? sprintf may do what you want; its documentation has copious examples. Here's a few possibilities:

$ perl -Mstrict -Mwarnings -le '
    my $x = 9**-10;
    print "$x";
    print sprintf "%f" => $x;
    print sprintf "%.15f" => $x;
    print sprintf "%.31f" => $x;
    print sprintf "%g" => $x;
    print sprintf "%.15g" => $x;
    print sprintf "%.31g" => $x;
'
2.86797199079244e-10
0.000000
0.000000000286797
0.0000000002867971990792441349306
2.86797e-10
2.86797199079244e-10
2.86797199079244134930566254989e-10
[download]

print sprintf "%.15f" => $x;

While this works perfectly, this is a bit verbose IMHO. Why not simply:

printf "%.15f" => $x;

or even:

printf "%.15f", $x;

The sprintf documentation contains details of the formats; these are not shown in the printf documentation. So, as it was the sprintf formatting that I was explaining, it seemed appropriate to use sprintf in my example code.

More importantly though, I made a conscious decision to not use printf in my examples. That function has a number of gotchas which aren't specifically related to the actual formatting and which I didn't want to have to explain.

The printf documentation is short: just three paragraphs. The first gotcha, noted in the very first sentence, has tripped you up!

Here's what happens if I substitute my 'print sprintf' with your ("Why not simply") 'printf':

$ perl -Mstrict -Mwarnings -le '
    my $x = 9**-10;
    print "$x";
    printf "%f" => $x;
    printf "%.15f" => $x;
    printf "%.31f" => $x;
    printf "%g" => $x;
    printf "%.15g" => $x;
    printf "%.31g" => $x;
'
2.86797199079244e-10
0.0000000.0000000002867970.00000000028679719907924413493062.86797e-102
+.86797199079244e-102.86797199079244134930566254989e-10$
[download]

I'll also draw your attention to the last printf paragraph:

"Don't fall into the trap of using a printf when a simple print would do. The print is more efficient and less error prone."

Finally, I use "=>" insted of "," for clarity; particularly when separating different types of arguments. If you look at my posts, you'll find many examples of this kind of thing:

sprintf FORMAT => LIST
split PATTERN => STRING
join STRING => LIST
pack TEMPLATE => LIST
[download]

I find it makes the code easier to read and, when necessary, easier to debug. There's a clear delineation between the first argument, which affects how the function operates, and the remaining arguments, which specify what the function operates on.

-- Ken

Have a look at printf.

UPDATE:

my $a = 0.0000000000007;
printf("%.15f\n", $a);
[download]

McA

$ 9 ** -10
2.86797199079244131e-10

$ sprintf '%0.94f', 9 ** -10
0.00000000028679719907924413134598230166468724084040268010320495761789
+06136192381381988525390625
[download]

For a precise output:

use Math::BigRat;
my $num = Math::BigRat->new("9e-10");
print $num->as_float(), "\n";
[download]

Output: 0.0000000009

For a precise output:

That works if your small number starts out as a string, but if it comes from a calculation done using normal Perl math, the 'damage' has already been done.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

How can I print a very small snumber, like 9^-10 or something?

What number is that supposed to be?

At the moment I'm just getting output like '3e-009', which is not very usefull

That is http://en.wikipedia.org/wiki/Scientific_notation#E_notation, its useful it means 3 times ten to the power of negative nine

$ perl -le " my $num = 3e-009; $num =~ s{[eE]([+-])?0+}{ * 10**$1}; pr
+int $num; "
3 * 10**-9

$ perl -le " print 3 * 10**-9 "
3e-009
[download]