Bug #17804: Math.cbrt(0.0) returns NaN

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-linux]
• Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[chasonr (Ray Chason)]

Math.cbrt(0.0) returns NaN. It should return 0.0.

From reading the implementation, I expect this bug to happen only if glibc is in use. The failing section reads:

#if defined GLIBC
if (isfinite(r)) {
r = (2.0 * r + (f / r / r)) / 3.0;
}
#endif

and this produces a NaN when f and r are both 0.

[xtkoba (Tee KOBAYASHI)]

FWIW, that code was introduced by commit c995315153f7403636ec0d6119b7885668b32039.

[jeremyevans0 (Jeremy Evans)]

I've submitted a pull request to fix this: https://github.com/ruby/ruby/pull/4425

• Status changed from Open to Closed

Applied in changeset git|406ae7fb03640e11e200382ef61cd450b952b7aa.

---

Fix Math.cbrt(0.0) on glibc

This should return 0, but on glibc it returned NaN.

Fixes [Bug #17804]