Bug #6944: BigDecimal#to_f breaks for numbers too small to be representable in Float

• Status: Closed
• Assignee: -
• Target version: 1.9.2
• ruby -v: ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-linux]

[adrianomitre (Adriano Mitre)]

BigDecimal numbers too small to be representable in Float are incorrectly converted to +/-Infinity instead of 0.0 (or -0.0). Things seem to broke down when exponent is about -308.

with 307, ok

BigDecimal('1e-307').to_f

=> 1.0e-307

BigDecimal('-1e-307').to_f

=> -1.0e-307

with 308, broken but signedness is correct

BigDecimal('1e-308').to_f

=> Infinity

BigDecimal('-1e-308').to_f

=> -Infinity

with 350 not even signedness is correct

BigDecimal('1e-350').to_f

=> -Infinity

BigDecimal('-1e-350').to_f

=> -Infinity

Workaround: resort to BigDecimal#to_s and Float(.)

x=BigDecimal('1e-350')
Float(x.to_s)

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r36854.
Adriano, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

bigdecimal.c: check underflow

• ext/bigdecimal/bigdecimal.c (BigDecimal_to_f): check underflow since
  strtod() sets errno to ERANGE at underflow too. [ruby-core:47342]
  [Bug #6944]