0001503: Potential new M_ constants for math.h

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0001503

[Issue 8 drafts] Base Definitions and Headers

Editorial

Enhancement Request

2021-08-07 20:59

2021-08-07 21:41

Reporter

andras_farkas

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public

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normal

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Open

Status

New

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Name

Andras Farkas

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User Reference

Section

math.h

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Final Accepted Text

Summary

0001503: Potential new M_ constants for math.h

Description

There may be five useful new M_ constants to be included in the math.h header.

The latest standard for C++, known as C++20, introduces a header called <numbers>, which contains many math constants, and is similar in intent to XSI's M_ constants in math.h. There's lots of overlap, but the list of M_ constants and list of constants in <numbers> do differ.

Constants in both XSI and <numbers>:
M_E, e
M_LOG2E, log2e
M_LOG10E, log10e
M_LN2, ln2
M_LN10, ln10
M_PI, pi
M_1_PI, inv_pi
M_SQRT2, sqrt2

Constants only in XSI:
M_PI_2
M_PI_4
M_2_PI
M_2_SQRTPI
M_SQRT1_2

Constants only in <numbers>:
inv_sqrtpi (1 divided by the square root of pi, approximately 0.5641)
sqrt3 (Square root of 3, approximately 1.7320)
inv_sqrt3 (1 divided by the square root of 3, approximately 0.5773)
egamma (Euler-Mascheroni constant, approximately 0.5772)
phi (golden ratio, approximately 1.6180)

References:
XSI:
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/math.h.html [^]
C++20 draft standard (specifically section 26.9, page 1224):
https://isocpp.org/files/papers/N4860.pdf [^]
P0631R8 Math Constants (The original paper on <numbers>):
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0631r8.pdf [^]
Notably, this paper references XSI.

I'm not a mathematician, so I'm not sure whether adding those five new constants from <numbers> would be considered useful, or considered clutter. Please chime in.
Personally, I've only ever used M_PI out of all the M_ constants, but I'm glad it's there when I do use it, and I'm glad all the constants are there.

Desired Action

I'd like for people to examine the usefulness of the five new constants. If they're judged useful:
Add to the section on math.h, immediately after the other M_ constants (or interspersed with them), and also XSI-marked:
M_1_SQRTPI
Value of 1/sqrt(pi)
M_SQRT3
Value of sqrt(3)
M_1_SQRT3
Value of 1/sqrt(3)
M_EGAMMA
Value of [lower-case gamma], Euler-Mascheroni constant
M_PHI
Value of [lower-case phi], ( (1 + sqrt(5))/2 ), golden ratio constant

Of course, the "Value of" text would use more fitting symbols than I was able to use here, for the values.

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Notes
(0005430) andras_farkas (reporter) 2021-08-07 21:32	Actually, there could be some changes to my desired action proposal. M_1_SQRT3 should certainly be M_SQRT1_3 in order to match M_SQRT1_2 M_1_SQRTPI could also be M_SQRT1_PI to match.

(0005431) andras_farkas (reporter) 2021-08-07 21:41	Thus, desiring to be consistent with the standard as it already exists, this is my new desired action: Add to the section on math.h, immediately after the other M_ constants (or interspersed with them), and also XSI-marked: M_SQRT1_PI Value of 1/sqrt(pi) M_SQRT3 Value of sqrt(3) M_SQRT1_3 Value of 1/sqrt(3) M_EGAMMA Value of [lower-case gamma], Euler-Mascheroni constant M_PHI Value of [lower-case phi], ( (1 + sqrt(5))/2 ), golden ratio constant

Issue History
Date Modified	Username	Field	Change
2021-08-07 20:59	andras_farkas	New Issue
2021-08-07 20:59	andras_farkas	Name	=> Andras Farkas
2021-08-07 20:59	andras_farkas	Section	=> math.h
2021-08-07 21:32	andras_farkas	Note Added: 0005430
2021-08-07 21:41	andras_farkas	Note Added: 0005431

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