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Add Tau constant to math module #6536

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merged 2 commits into from Dec 21, 2022
Merged

Add Tau constant to math module #6536

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708fc7f
stdlib: Improve documentation of math:pi/0
hauleth Dec 6, 2022
6aebff0
stdlib: Add math:tau/0 constant as 2*pi
hauleth Dec 19, 2022
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15 changes: 13 additions & 2 deletions lib/stdlib/doc/src/math.xml
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Expand Up @@ -102,9 +102,20 @@ erf(X) = 2/sqrt(pi)*integral from 0 to X of exp(-t*t) dt.</pre>

<func>
<name name="pi" arity="0" since=""/>
<fsummary>A useful number.</fsummary>
<fsummary>Ratio of the circumference of a circle to its diameter.</fsummary>
<desc>
<p>A useful number.</p>
<p>Ratio of the circumference of a circle to its diameter.</p>
<p>Floating point approximation of mathematical constant pi.</p>
</desc>
</func>

<func>
<name name="tau" arity="0" since="OTP @OTP-18361@"/>
<fsummary>Ratio of the circumference of a circle to its radius.</fsummary>
<desc>
<p>Ratio of the circumference of a circle to its radius.</p>
<p>This constant is equivalent to a full turn when described in radians.</p>
<p>The same as <c>2 * pi()</c>.</p>
</desc>
</func>

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6 changes: 4 additions & 2 deletions lib/stdlib/src/math.erl
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Expand Up @@ -19,7 +19,7 @@
%%
-module(math).

-export([pi/0]).
-export([pi/0,tau/0]).

%%% BIFs

Expand Down Expand Up @@ -154,5 +154,7 @@ tanh(_) ->
%%% End of BIFs

-spec pi() -> float().

pi() -> 3.1415926535897932.

-spec tau() -> float().
tau() -> 6.2831853071795864.
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Just a suggestion. Instead of writing out the full (yeah, I know) sequence of digits, I would put the value for pi in a constant and define both pi/0 and tau/0 in terms of it:

-define(PI, 3.1415926535897932).

pi() -> ?PI.

tau() -> 2 * ?PI.

You know, in case some day god decides to make math things simpler 😸

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I didn't wanted to do it to avoid loosing precision when compiling this module on different machines. We can make math simpler, but IEEE 754 will always stand in our way.

9 changes: 7 additions & 2 deletions lib/stdlib/test/math_SUITE.erl
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Expand Up @@ -28,15 +28,15 @@
init_per_testcase/2, end_per_testcase/2]).

%% Test cases
-export([floor_ceil/1, error_info/1]).
-export([floor_ceil/1, error_info/1, constants/1]).


suite() ->
[{ct_hooks,[ts_install_cth]},
{timetrap,{minutes,1}}].

all() ->
[floor_ceil, error_info].
[floor_ceil, error_info, constants].

groups() ->
[].
Expand All @@ -60,6 +60,11 @@ init_per_testcase(_Case, Config) ->
end_per_testcase(_Case, _Config) ->
ok.

constants(_Config) ->
3.1415926535897932 = math:pi(),
6.2831853071795864 = math:tau(),
ok.

floor_ceil(_Config) ->
MinusZero = 0.0/(-1.0),
-43.0 = do_floor_ceil(-42.1),
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