http://www.perlmonks.org?node_id=11123182


in reply to Re^7: int() function
in thread int() function

"%.17g" formatting

I was surprised to see 16 digits after the decimal point in a "%.17" notation. Per perl's sprintf docs:

%g    a floating-point number, in %e or %f notation

But that's not what actually happens in Perl:

C:>perl -MConfig -le "print +5.3130431399669280e-34; printf qq($_\n), ++5.3130431399669280e-34 for qw/%.17f %.17e %.17g/; print qq($^V\t|\t$ +Config{myuname})" 5.31304313996693e-34 0.00000000000000000 5.31304313996692843e-34 5.3130431399669284e-34 v5.30.0 | Win32 strawberry-perl 5.30.0.1 #1 Thu May 23 12:20:46 +2019 x64

... In this instance, "%.17g" is neither equivalent to "%.17f" (17 digits after the fixed decimal point) nor to "%.17e" (17 digits after the floating decimal point).

I tried on an ancient linux machine with v5.8.5, with similar results:

% perl -MConfig -le 'print +5.3130431399669280e-34; printf qq($_\n), + +5.3130431399669280e-34 for qw/%.17f %.17e %.17g/; print qq($]\t|\t$Co +nfig{myuname})' 5.31304313996693e-34 0.00000000000000000 5.31304313996692843e-34 5.3130431399669284e-34 5.008005 | linux builder4.centos.org 2.6.9-55.0.9.elsmp # +1 smp thu sep 27 18:27:41 edt 2007 i686 i686 i386 gnulinux

The sprintf docs have used that phrasing at least as far back as v5.005 . Am I misunderstanding what I'm seeing? or has perl documentation about %g been wrong for that long?