Request for Comments: Rounding in PHP
- Version: 1.0
- Date: 2008-08-23
- Author: Christian Seiler firstname.lastname@example.org
- Status: Implemented
- First Published at: http://wiki.php.net/rfc/rounding
This RFC discusses the situation on floating point rounding in PHP, explains why it is currently borken and proposes a fix to solve these problems.
Parts of this proposal are based on a proposal by George Whiffen. Below there is an analysis the differences.
PHP offers a method round to round floating point numbers to a certain precision. This, however, does not always work as a user would expect. This is due to the fact that IEEE 754 floating point values are stored in the binary system which cannot represent every decimal value exactly.
Rounding is traditionally seen as round to integer so that no fractions are left over. If the number is already an integer, this is not a problem. But if the number is not an integer, it depends on the chosen algorithm on what is done depending on the fraction.
The following rounding methods exist:
Round to negative infinitiy
This method always rounds toward negative infinity, should be obvious. Some examples:
-0.5 -> -1 0.5 -> 0 2.4 -> 2 -3.6 -> -4 4.8 -> 4
The floor function does this.
Round to positive infinitiy
This method always rounds toward positive infinity, should be obvious. Some examples:
-0.5 -> 0 [actually, -0] 0.5 -> 1 2.4 -> 3 -3.6 -> -3 4.8 -> 5
The ceil function does this.
Round to zero
-0.5 -> 0 [actually, -0] 0.5 -> 0 2.4 -> 2 -3.6 -> -3 4.8 -> 4
Round away from zero
-0.5 -> -1 0.5 -> 1 2.4 -> 3 -3.6 -> -4 4.8 -> 5
Round to nearest
This method rounds fractions to the nearest integer. This is fine except when the fraction is exactly .5. Some examples not including the edge case .5:
2.4 -> 2 -3.2 -> -3 -3.6 -> -4 4.8 -> 5 2.501 -> 3 -2.501 -> -3 2.499 -> 2 -2.499 -> -2
There are four main variants of this algorithm when it comes to treating .5:
Round half up
This rounds .5 away from zero, some examples:
-1.5 -> -2 1.5 -> 2 -2.5 -> -3 2.5 -> 3
This is also called arithmetic rounding and is the traditional rounding method that is taught in school.
Round half down
This rounds .5 toward zero, some examples:
-1.5 -> -1 1.5 -> 1 -2.5 -> -2 2.5 -> 2
Round half even
This rouds .5 towards the next even integer, some examples:
-1.5 -> -2 1.5 -> 2 -2.5 -> -2 2.5 -> 2
This is also called banker's rounding.
Round half odd
This rounds .5 towards the next odd integer, some examples:
-1.5 -> -1 1.5 -> 1 -2.5 -> -3 2.5 -> 3
Where rounding occurs in PHP
Rounding of floating point values occurs in PHP in several different places:
There are several places where explicit rounding occurs, i.e. where the user specifies that a floating point number is to be rounded:
- (s|f)printf with %f as modifier
round() and number_format() use an explicit floating point algorithm in math.c, while the printf() functions do rounding while converting the float to a string using an algorithm that uses bigints (zend_strtod.c).
- float to string conversion (this is essentially sprintf(“%g”, $float))
- (s|f)printf with %g as modifier
Here, only the biginit algorithm is used.
History of round() in PHP
In the following section, I will outline the history of the round() function in PHP in order to provide thorough background information for the discussion.
First version of math.c in CVS
- Signature: round($float), rounds to integer, no precision argument
- Uses rint() for rounding.
- On systems without rint(), rint() is emulated with an algorithm that does arithmetic rounding.
ISO C specifies that rint() rounds according to the current rounding direction of the CPU, which is round-to-nearest round-half-even (banker's rounding) by default on any system that I know of but may be changed during runtime. IEEE 754 does not specify arithmetic rounding as a rounding method, but IEEE 754r will.
This first version already shows discrepancies on different systems. (arithmetic rounding on systems without rint(), banker's rounding on every system with sane defaults and rint())
Version 1.22 (May 17, 2000)
A second parameter is added to the function signature: round($float, $places) where $places specifies the precision. It now implements an algorithm that does arithmetic rounding only.
Version 1.104 (Aug 8, 2003)
Due to incorrect results on some systems (for reasons, see below) the algorithm is modified slightly using a “fuzz” (here too: see below).
Version 1.106 (Aug 9, 2003)
Fuzz is always disabled on Win32, a (useless) configure check is added on UNIX.
General information on floating point arithmetics
This section tries to gather general information on floating point arithmetics.
Representation of floating point values
IEEE 754 specifies that floating point values are represented through three different numbers: The sign of the number, the exponent and the fraction. A floating point number is to be interpreted as:
(-1)^sign * fraction * 2^exponent
IEEE 754 specifies several different floating point data types, of which two are relevant here:
- Double precision: The fraction is 52 bits, the exponent 11 bits and one sign bit.
- Extended precision: The fraction is 64 bits, the exponent 15 bits and one sign bit.
Representation of decimal numbers
Since IEEE 754 uses the binary (base 2) system rather than the decimal (base 10) system for storing floating point values, it is not always possible to exactly represent a decimal number as a floating point value. Take, for example, the number 0.1. It cannot be represented exactly as a binary floating point number with finite precision just as 1/3rd can't be represented as a decimal number with finite precision. The closest floating point number within double precision is
The closest floating point number within extended precision is
However, when rounded as integers or strings, the first 15 significant digits of a floating point number are always exact. So when converting the number 0.1 with a precision of 14 digits after the first significant digit to a string, it will still yield 0.1. So within 15 digits precision, floating point numbers can be used to exactly represent a decimal number.
Precision relevancy of arithmetics with FP numbers
Consider the following piece of C code:
// volatile to disable compile-time optimizations for this example volatile double v = 2877.0; double d = v / 1000000.0;
The question here is: What does the double variable d contain? The answer is compiler-dependent. The Microsoft C Compiler on any system will have d contain the closest double representation of 0.002877. The GNU C Compiler will do so on x86_64 systems. But on 32 bit x86 systems the GNU C Compiler will have d contain the closest extended precision representation of 0.002877 truncated to double which is NOT the same as the closest double representation. This is due to the fact that internal calculations are done using extended precision and results are truncated to double precision only when they are stored in memory.
This can be avoided, however. The trick is to force the FPU to always use double precision for calculations. The problem is that this is not possible in a platform-independent way. The GNU C library offers the _FPU_SETCW and _FPU_GETCW macros in fpu_control.h while on Windows Systems a function named _controlfp is available for this job. FreeBSD provides fpsetprec() and other Operating Systems that run on x86 require inline assembly.
Please note that zend_strtod() is also affected by this problem: On systems with the GCC and a 32 bit x86 processor, zend_strtod() will yield different results than strtod(). See below.
Analysis of the problems of the previous round() implementation
The previous round() implementations does not work properly on several cases.
First of all, it is nowhere clearly defined, which rounding method round() uses. Let's assume arithmetic rounding since the current algorithm tries to do that.
The problem with round() is not the rounding algorithm itself, that is very straight-forward (for brevity, only the version for positive values is included here):
double result = floor(value + 0.5);
This algorithm correctly rounds a floating point number to integer using arithmetic rounding.
However, the rounding algorithm in PHP supports arbitrary precision. Thus the algorithm actually looks like this:
In a world with infinite precision this is completely correct. But due to the finite precision of doubles, it introduces two areas which cause problems:
- The multiplication with 10^places.
- The division by 10^places.
In an attempt to solve these problems, the so-called “fuzz” was added. The fuzz simply means that instead of adding 0.5, a small bit more is added: 0.50000000001. This, however, does not solve the problem but introduces a new one.
Let us have a look at the problems introduced by those three steps:
Multiplication with 10^places
Multiplication with 10^places is problemtic because of the fact that if the previous floating point representation was not exact, after multiplying with 10^places the resulting floating point number may not be the exact representation of the intended number.
Take, for example, the number 0.285. Its floating point representation is 0.284999999999999975575093458246556110680103302001953125. If you multiply that with 100, the resulting number has the floating point representation 28.499999999999996447286321199499070644378662109375. This is not the exact representation of 28.5 - which is actually 28.5 in this case.
The same happens for 1.255: The representation is 1.25499999999999989341858963598497211933135986328125. Multiply that by 100 and get 125.4999999999999857891452847979962825775146484375. The exact representation of 125.5 however is 125.5.
If 0.5 is now added to that number and that number is then rounded with floor, the result will be 28 and not 29 which would be the naive expected value.
Division through 10^places
Division through 10^places is problematic because of two possible effects:
- If the internal calculation is done in extended precision, the truncated value may not be the exact double representation of the chosen value, see above.
- If places > 22 then 10^places itself cannot be represented as an exact floating point number and thus the division will be inaccurate. After dividing by 10^places, the result may deviate from the nearest floating point representation of the exact result - try var_dump(2e-23 - round(2e-23,23));. Thus, even if the rounding is exact, the result after the division is not the nearest representation as would be expected.
The round fuzz
The round fuzz tries to correct the multiplication problem but causes another one: round(0.9499999999999,1) will return 1.0 instead of the expected 0.9. Also, since the fuzz is not activated on all platforms (but these problems are platform-independent - with the exception of the extended precision then truncation problem during the division), this does not actually fix the issue.
Summary of the problem analysis
The addition of the places parameter of the round() function is the actual cause of the calculation errors. Furthermore, the fact that PHP does not clearly specify which rounding method is used (the manual only states that the number is “rounded”) has been cause for quite a bit of confusion.
Proposal and Patch
This proposal suggests how to fix the round function in order to make it work properly and reduce the confusing among users.
First of all, this proposal does not want to “fix” the printf() functions. printf() does round internally using the float-to-string converion algorithm in zend_strtod.c which uses bigints to do the conversion. So when it comes to 0.285 which is represented as 0.284999something, it will return 0.28 instead of 0.29 if used with %2f as format string. But the problem with fixing printf() is portability: Every other language supporting printf() or similar format strings do it wrong in the exact same way, PHP should not deviate from that (in my eyes). It is always possible to do printf(“%.2f”, round($float, 2)); if one really wants correct results, as long as round() works properly. Also, changing the printf() bigint algorithm will have adverse effects on the fact that printf is often used with very high precision (> 20) to debug floating point algorithms.
However, the PHP manual should contain a warning for printf() that rounding may not work as expected and that explicit rounding should be done prior to passing the value to printf() if chosen so.
Second, %g in printf() and implicit float-to-string conversion in PHP a la (string)$float shouldn't be fixed either. They only actually round at the 15th significant digit (if the precision ini setting is not touched). If somebody really operates with floating point values at the edge of decimal precision, other problems occur anyway, so one shouldn't bother. But also here, the manual for the precision ini setting should be changed that manually lowering the setting will not always result in correct rounding and that the round function should be used instead.
Building an abstraction layer for FP control register manipulation
See http://www.christian-seiler.de/projekte/fpmath/ for further information on how to ensure double precision on different architectures, operating systems and with different compilers.
This proposal proposes to wrap the above abstraction into the following macros:
These will be defined in Zend/zend_float.h.
Fix of zend_strtod
Since the introduction of zend_strtod() (instead of strtod() which is locale dependent) the function suffers from the same problem as round(): It calculates in extended precision and then truncates the result to double on some platforms. Example:
printf ("%.20f\n", 0.002877);
Run that in C and run that in PHP on a Linux x86 32 bit box - you will get different results.
For this reason, my patch also fixes zend_strtod() by adding the proposed macros to the function. Then, C and PHP will yield the same results on all platforms (that support IEEE 754 arithmetics anyway).
New implementation of the round algorithm
This proposal proposes the following change to PHPs round() function to eliminate all the problems:
Usage of FP control word manipulation
The round function uses the new ZEND_FLOAT macros in order to ensure double precision arithmetics within the function body. This will make sure the final division works properly.
Create a function that does the actual integer rounding
A new static inline function php_round_helper(double value, int mode) was added that rounds a number to integer. It basically does the simple arithmetic rounding floor(value + 0.5) or ceil(value - 0.5). But it also supports the other rounding methods round-half-even, round-half-odd and round-half-down via the mode parameter.
Special handling for large places difference
This was taken from the 2004 proposal: If the numer of places are very large, then 10^places is very large, too, and cannot be represented in an exact manner anymore. This will cause inaccuracies with the final division, as explained earlier.
The solution for that is that the rounded double is converted to a string and e-places is added to that string. Take, for example, the number 5.3e-24 which you may want to round to 24 places precision (which it of course already is). After rounding, the float value is 1.0 and that has to be divided by 1e24. But 1e24 is too large to be exactly represented, so instead a string 1.0e-24 is generated and passed through strtod(). strtod() on the other hand will make sure that the nearest double representation for that number is chosen. This of course has a performance penalty, but anybody wanting to round such small numbers will probaby be willing to pay for it.
This change will make sure that very small (< 1e-22) or large (> 1e22) numbers will be rounded correctly to the given precision.
Pre-rounding to the value's precision if possible
The previous measures only concern the problems with the division but not the problem with the multiplication. Here, another measure is taken:
If the requested number of places to round the number is smaller than the precision of the number, then the number will be first rounded to its own precision and then rounded to the requested number of places.
Example: Round 1.255 to 2 places precision, expected value is 1.26. First step: Calculate 10^places = 10^2 = 100. Second step: Calculate 14 - floor(log10(value)) = 14 - 0 = 14 which indicates the number of places after the decimal point which are guaranteed to be exact by IEEE 754. Now, 2 < 14, so the condition applies. So, calculate 10^14 and multiply the number by that: 1.255 * 1e14 = 125499999999999.984375... Now, round that number to integer, i.e. 125500000000000. Now, divide that number by 10^(14 - 2) = 10^12 (the difference) and get 125.5 (exact). NOW round that number to decimal which yields 126 and divide it by 10^2 = 100 which gives 1.26 which is the expected result for that rounding operation.
Of course, one may argue that pre-rounding is not necessary and that this is simply the problem with FP arithmetics. This is true on the one hand, but the introduction of the places parameter made it clear that round() is to operate as if the numbers were stored as decimals. We can't revert that and this seems to me to be the best solutions for FP numbers one can get.
Additional parameter for the round() function
The round function now has an additional optional parameter for the selected rounding mode. The default mode is arithmetic rounding but other rounding modes may be selected.
The 2004 proposal introduced some optimizations that this proposal has chosen to use, just in a slightly shorter form. In order to calculate floor(log10(v)) a quick binary search lookup is used for small enough powers of 10. The same goes for calculating 10^places for small enough values: These are looked up in a table if they are small enough.
The patch was already applied to PHP_5_3 and HEAD.
I tested the patch under Linux 32 bit x86, Linux 64 bit x86_64, Windows 32 bit x86 and FreeBSD 32 bit x86. Additionally, the macros themselves were tested on various platforms (see the above link for details).
Comparison with the 2004 proposal
Quite a few ideas in this proposal came from the 2004 proposal:
- Conversion to string and back for places too large.
- The performance optimizations for log10 and pow(10)
- The FPU control word manipulation.
But there are four main differences between the 2004 proposal and this one:
The 2004 proposal also patches printf() in order to make rounding consistent. However, as I already explained, I don't think that changing printf()s behaviour is such a good idea since printf behaves a certain way in every other programming language that supports that function.
A note in the manual that the precision specifier in printf() is not suitable for rounding the values should be sufficient.
No pre-rounding to precision
The 2004 proposal does not pre-round after the multiplication and thus rounding 1.255 to 2 places with the 2004 code will not work correctly either.
Additional ini settings
The 2004 proposal also adds additional ini settings for rounding mode etc. In my eyes this is superfluous since the rounding mode can always be set as an additional parameter to the round() function.
Altering floating point arithmetics in PHP core
The 2004 proposal goes way beyond traditional rounding by altering FP arithmetics in PHP in such a way that after any operation (add, subtract, multiply, divide) the result is rounded to the 15 digits precision guaranteed by IEEE 754. This destroys traditional floating point semantics but allows simple decimal calculations to work as expected by users not familiar with floating point values. It is essentially the same thing some spreadsheet applications (e.g. Microsoft Excel) do.
I'm opposed to this kind of change of general semantics since PHP never said it implemented a decimal type and if people are using it wrong, it's their problem, we shouldn't break other legitimate applications for this.
The round() function is a slightly different case though, since the round() function itself claims to be able to round to decimal precision. For this reason, changing round()s behaviour in order to accomodate decimal semantics is OK, since users will expect it to work that way.
Nevertheless, what could be discussed separately is the introduction of a new type that automatically uses an arbitrary precision library internally, since writing $a * $b is much more natural than e.g. bcmul($a, $b). This, however, goes far beyond the scope of this proposal.
- 2008-12-02 Christian Seiler: Updated to current situation
- 2008-08-23 Christian Seiler: Created RFC