# Array#float\_sum (like math.fsum of Python)

**URL:** <https://www.ruby-forum.com/t/array-float-sum-like-math-fsum-of-python/232538>\
**Category:** Development\
**Created:** [October 15, 2014, 1:21pm UTC](https://www.ruby-forum.com/t/array-float-sum-like-math-fsum-of-python/232538 "2014-10-15T13:21:14Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [October 15, 2014, 1:21pm UTC](https://www.ruby-forum.com/t/array-float-sum-like-math-fsum-of-python/232538/1 "2014-10-15T13:21:14Z")

</div>

Issue #10298 has been updated by Takeshi Nishimatsu.

Thank you for the quick patch for enum.c.

Timing and test.  
I see. performance is not improved.

```auto
$ uname -sprsv
Darwin 13.3.0 Darwin Kernel Version 13.3.0: Tue Jun 3 21:27:35 PDT 
2014; root:xnu-2422.110.17~1/RELEASE_X86_64 i386
$ sysctl -n machdep.cpu.brand_string
Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
$ ./miniruby --version
ruby 2.2.0dev (2014-10-15 trunk 47735) [x86_64-darwin13]
$ /usr/bin/time ./miniruby -e 'p Array.new(10000000,0.1).reduce(:+)'
999999.9998389754
        0.69 real 0.65 user 0.03 sys
$ mv optimized/enum.c .
$ make -j2 miniruby
compiling enum.c
linking miniruby
$ /usr/bin/time ./miniruby -e 'p Array.new(10000000,0.1).reduce(:+)'
1000000.0
        0.55 real 0.51 user 0.04 sys
$ ./miniruby -e 'p Array.new(10000000,0.1).unshift(0.0).reduce(:-)'
-1000000.0

```

I understand that Array is not only for Float.

```auto
$ ./miniruby -e '[1,2,3].float_sum'
-e:1: [BUG] Segmentation fault at 0x00000000000013

```

* * *

Feature #10298: Array#float\_sum (like math.fsum of Python)

> **[Feature #10298: Array#float\_sum (like math.fsum of Python) - Ruby master -...](https://bugs.ruby-lang.org/issues/10298#change-49459)**
>
> Redmine

- Author: Takeshi Nishimatsu
- Status: Feedback
- Priority: Low
- Assignee:
- Category: math
- Target version:

* * *

Here, I propose Array#float\_sum in array.c (or math.c).  
Array#float\_sum returns an accurate total summation of Float  
elements in an array using the Kahan summation algorithm  
[Kahan summation algorithm - Wikipedia](http://en.wikipedia.org/wiki/Kahan_summation_algorithm) .  
This algorithm can significantly reduce the numerical  
error in the total obtained by adding a sequence of  
finite precision floating point numbers, compared to the  
obvious approach. Python already have math.fsum  
[9.2. math — Mathematical functions — Python 2.7.18 documentation](https://docs.python.org/2/library/math.html#math.fsum) .

```auto
[0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1].float_sum #=> 1.0
[].float_sum #=> 0.0
Array.new( 10, 0.1).float_sum #=> 1.0
Array.new(100, 0.1).float_sum #=> 10.0
# cf.
Array.new( 10, 0.1).reduce(:+) #=> 0.9999999999999999
Array.new(100, 0.1).reduce(:+) #=> 9.99999999999998

```

The name of method can be fsum, sum\_float, etc., though  
I propose float\_sum.

This Array#float\_sum is inspired by Feature #9834  
Float#{next\_float,prev\_float}.

—Files--------------------------------  
array.float\_sum.patch (1.32 KB)