## Bug #7458

### Depending on value of BigDecimal::limit, addition can give inaccurate results

Status: | Closed | ||
---|---|---|---|

Priority: | Normal | ||

Assignee: | Kenta Murata | ||

ruby -v: | 2.0.0dev | Backport: |

**Description**

=begin

This bug is present in ruby-trunk and also in 1.8.7 and 1.9.3p125.

I apologize that I cannot figure out what exactly is going wrong and how to patch, but at least I have a small reproducible test case.

When adding very small numbers (like 7E-19) to 1, the result should be very close to 1. However, depending on the limit to BigDecimal's precision, this is not always what is resulted, as this script demonstrates:

require 'bigdecimal'

one = BigDecimal('1')

smallnum = BigDecimal('7E-19')

0.upto(20) do |lim|

BigDecimal::limit(lim)

result = one + smallnum

puts "result is #{result}, as float is #{result.to_f} with limit #{lim}"

end

When run in trunk (or 1.8.7), the results it gives are:

result is 0.10000000000000000007E1, as float is 1.0 with limit 0

result is 0.1E1, as float is 1.0 with limit 1

result is 0.1E1, as float is 1.0 with limit 2

result is 0.1E1, as float is 1.0 with limit 3

result is 0.1E1, as float is 1.0 with limit 4

result is 0.1E1, as float is 1.0 with limit 5

result is 0.1E1, as float is 1.0 with limit 6

result is 0.1E1, as float is 1.0 with limit 7

result is 0.1E1, as float is 1.0 with limit 8

result is 0.1E1, as float is 1.0 with limit 9

result is 0.2E1, as float is 2.0 with limit 10

result is 0.2E1, as float is 2.0 with limit 11

result is 0.2E1, as float is 2.0 with limit 12

result is 0.2E1, as float is 2.0 with limit 13

result is 0.2E1, as float is 2.0 with limit 14

result is 0.2E1, as float is 2.0 with limit 15

result is 0.2E1, as float is 2.0 with limit 16

result is 0.2E1, as float is 2.0 with limit 17

result is 0.2E1, as float is 2.0 with limit 18

result is 0.1000000000000000001E1, as float is 1.0 with limit 19

result is 0.10000000000000000007E1, as float is 1.0 with limit 20

So the results with limit <=9 or >=19 are as expected, but in between the result returned is equal to 2.0

A test that demonstrates this (suitable for inclusion in (({test/bigdecimal/test_bigdecimal.rb})) is:

def test_limited_one_plus_small

one = BigDecimal('1')

smallnum = BigDecimal('0.7E-18')

BigDecimal::save_limit do

1.upto(20) do |lim|

BigDecimal::limit(lim)

result = (one + smallnum).to_f.to_s

assert_equal("1.0", result, "Error adding a small number to one with BigDecimal::limit(#{lim})")

end

end

end

I think this is different from #7404 (from what I can tell by google translate), but I apologize for the duplicate if not.

=end

### Associated revisions

Append [Bug #7458] in the previous Changelog entry

Append [Bug #7458] in the previous Changelog entry

### History

#### #1 Updated by Usaku NAKAMURA about 2 years ago

**Category**set to*ext***Status**changed from*Open*to*Assigned***Assignee**set to*Kenta Murata***Target version**set to*2.0.0*

#### #2 Updated by Yusuke Endoh about 2 years ago

**Target version**changed from*2.0.0*to*next minor*

#### #3 Updated by Bradford Powell over 1 year ago

=begin

Still present in

(({ruby 2.1.0dev (2013-09-22 trunk 43011) [x86_64-linux]}))

=end

#### #4 Updated by Kenta Murata about 1 year ago

**Status**changed from*Assigned*to*Closed*

I've fix this issue in r44153.