Edit report at http://bugs.php.net/bug.php?id=53918&edit=1
ID: 53918
Updated by: cataphr...@php.net
Reported by: exploringbinary at gmail dot com
Summary: printf of floating point variable prints maximum of
53 decimal places
-Status: Bogus
+Status: Open
Type: Bug
Package: Math related
Operating System: Windows (32-bit system)
PHP Version: 5.3.5
Block user comment: N
Private report: N
New Comment:
Well, can't you just print the numbers in exponential format?
A representation with more than 50 leading digits in the fractional part
is not particularly useful.
This seems a better option than showing > 53 digits of decimal places
when the machine precision is of only 15.9546 decimal digits.
I imagine the magic arbitrary 53 digits limits was because the precision
of the machine precision of 53 binary digits, though this is of course
irrelevant for decimal representations.
Previous Comments:
------------------------------------------------------------------------
[2011-02-03 14:23:24] il...@php.net
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php
53 digits is the maximum limit enforced by most programming languages.
http://www.exploringbinary.com/print-precision-of-dyadic-fractions-varies-by-
language/
------------------------------------------------------------------------
[2011-02-03 12:56:28] exploringbinary at gmail dot com
Description:
------------
Bug #47168, "printf of floating point variable prints maximum of 40
decimal places", was not fixed as expected. Instead of the previous
arbitrary limit of 40 digits, there is now an arbitrary limit of 53
digits.
These three examples all print powers of two, which have exact
representations in double-precision floating-point:
<?php
printf("%1.176f\n",1.044048714879763924273647057481047608912186281291034647641381832875155719135597796355663380296618925058282911777496337890625e-53);
/* 2^-176 */?>
<?php
printf("%1.177f\n",5.220243574398819621368235287405238044560931406455173238206909164375778595677988981778316901483094625291414558887481689453125e-54);
/* 2^-177 */?>
<?php
printf("%1.178f\n",2.6101217871994098106841176437026190222804657032275866191034545821878892978389944908891584507415473126457072794437408447265625e-54);
/* 2^-178 */?>
The output is:
0.00000000000000000000000000000000000000000000000000001
0.00000000000000000000000000000000000000000000000000001
0.00000000000000000000000000000000000000000000000000000
(The first prints 1 significant digit, the second rounds to 1
significant digit, and the third prints no significant digits.)
Compare this to gcc C on Linux:
printf("%1.176f\n",1.044048714879763924273647057481047608912186281291034647641381832875155719135597796355663380296618925058282911777496337890625e-53);
/* 2^-176 */
printf("%1.177f\n",5.220243574398819621368235287405238044560931406455173238206909164375778595677988981778316901483094625291414558887481689453125e-54);
/* 2^-177 */
printf("%1.178f\n",2.6101217871994098106841176437026190222804657032275866191034545821878892978389944908891584507415473126457072794437408447265625e-54);
/* 2^-178 */
The output (which is correct) is:
0.00000000000000000000000000000000000000000000000000001044048714879763924273647057481047608912186281291034647641381832875155719135597796355663380296618925058282911777496337890625
0.000000000000000000000000000000000000000000000000000005220243574398819621368235287405238044560931406455173238206909164375778595677988981778316901483094625291414558887481689453125
0.0000000000000000000000000000000000000000000000000000026101217871994098106841176437026190222804657032275866191034545821878892978389944908891584507415473126457072794437408447265625
(I see that in formatted_print.c,
#define MAX_FLOAT_PRECISION 40
was changed to
#define MAX_FLOAT_PRECISION 53)
Test script:
---------------
<?php
printf("%1.178f\n",2.6101217871994098106841176437026190222804657032275866191034545821878892978389944908891584507415473126457072794437408447265625e-54);
/* 2^-178 */?>
Expected result:
----------------
All requested decimal places are printed.
Actual result:
--------------
Only 53 decimal places are printed.
------------------------------------------------------------------------
--
Edit this bug report at http://bugs.php.net/bug.php?id=53918&edit=1
