Averages of Angles

Question

Story, or why we are doing this.

None. This exercise is completely pointless ... unless you are Stephen Hawking.

The Challenge

Given a list of angles, find the average of those angles. For example the average of 91 degrees and -91 degrees is 180 degrees. You can use a program or function to do this.

Input

A list of degree values representing angle measures. You may assume that they will be integers. They can be inputted in any convenient format or provided as function arguments.

Output

The average of the inputted values. If there are more than one value found for the average, only one should be outputted. The average is defined as the value for which

is minimized. The output must be within the range of (-180, 180] and be accurate to at least two places behind the decimal point.

Examples:

> 1 3
2
> 90 -90
0 or 180
> 0 -120 120
0 or -120 or 120
> 0 810
45
> 1 3 3
2.33
> 180 60 -60
180 or 60 or -60
> 0 15 45 460
40
> 91 -91
180
> -89 89
0

As usual with codegolf, the submission with the least bytes wins.

Leaderboard

Here is a Stack Snippet to generate both a regular leaderboard and an overview of winners by language.

var QUESTION_ID=60538,OVERRIDE_USER=32700;function answersUrl(e){return"http://api.stackexchange.com/2.2/questions/"+QUESTION_ID+"/answers?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+ANSWER_FILTER}function commentUrl(e,s){return"http://api.stackexchange.com/2.2/answers/"+s.join(";")+"/comments?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+COMMENT_FILTER}function getAnswers(){jQuery.ajax({url:answersUrl(answer_page++),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){answers.push.apply(answers,e.items),answers_hash=[],answer_ids=[],e.items.forEach(function(e){e.comments=[];var s=+e.share_link.match(/\d+/);answer_ids.push(s),answers_hash[s]=e}),e.has_more||(more_answers=!1),comment_page=1,getComments()}})}function getComments(){jQuery.ajax({url:commentUrl(comment_page++,answer_ids),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){e.items.forEach(function(e){e.owner.user_id===OVERRIDE_USER&&answers_hash[e.post_id].comments.push(e)}),e.has_more?getComments():more_answers?getAnswers():process()}})}function getAuthorName(e){return e.owner.display_name}function process(){var e=[];answers.forEach(function(s){var r=s.body;s.comments.forEach(function(e){OVERRIDE_REG.test(e.body)&&(r="<h1>"+e.body.replace(OVERRIDE_REG,"")+"</h1>")});var a=r.match(SCORE_REG);a&&e.push({user:getAuthorName(s),size:+a[2],language:a[1],link:s.share_link})}),e.sort(function(e,s){var r=e.size,a=s.size;return r-a});var s={},r=1,a=null,n=1;e.forEach(function(e){e.size!=a&&(n=r),a=e.size,++r;var t=jQuery("#answer-template").html();t=t.replace("{{PLACE}}",n+".").replace("{{NAME}}",e.user).replace("{{LANGUAGE}}",e.language).replace("{{SIZE}}",e.size).replace("{{LINK}}",e.link),t=jQuery(t),jQuery("#answers").append(t);var o=e.language;/<a/.test(o)&&(o=jQuery(o).text()),s[o]=s[o]||{lang:e.language,user:e.user,size:e.size,link:e.link}});var t=[];for(var o in s)s.hasOwnProperty(o)&&t.push(s[o]);t.sort(function(e,s){return e.lang>s.lang?1:e.lang<s.lang?-1:0});for(var c=0;c<t.length;++c){var i=jQuery("#language-template").html(),o=t[c];i=i.replace("{{LANGUAGE}}",o.lang).replace("{{NAME}}",o.user).replace("{{SIZE}}",o.size).replace("{{LINK}}",o.link),i=jQuery(i),jQuery("#languages").append(i)}}var ANSWER_FILTER="!t)IWYnsLAZle2tQ3KqrVveCRJfxcRLe",COMMENT_FILTER="!)Q2B_A2kjfAiU78X(md6BoYk",answers=[],answers_hash,answer_ids,answer_page=1,more_answers=!0,comment_page;getAnswers();var SCORE_REG=/<h\d>\s*([^\n,]*[^\s,]),.*?(\d+)(?=[^\n\d<>]*(?:<(?:s>[^\n<>]*<\/s>|[^\n<>]+>)[^\n\d<>]*)*<\/h\d>)/,OVERRIDE_REG=/^Override\s*header:\s*/i;

body{text-align:left!important}#answer-list,#language-list{padding:10px;width:290px;float:left}table thead{font-weight:700}table td{padding:5px}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/codegolf/all.css?v=83c949450c8b"> <div id="answer-list"> <h2>Leaderboard</h2> <table class="answer-list"> <thead> <tr><td></td><td>Author</td><td>Language</td><td>Size</td></tr></thead> <tbody id="answers"> </tbody> </table> </div><div id="language-list"> <h2>Winners by Language</h2> <table class="language-list"> <thead> <tr><td>Language</td><td>User</td><td>Score</td></tr></thead> <tbody id="languages"> </tbody> </table> </div><table style="display: none"> <tbody id="answer-template"> <tr><td>{{PLACE}}</td><td>{{NAME}}</td><td>{{LANGUAGE}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table> <table style="display: none"> <tbody id="language-template"> <tr><td>{{LANGUAGE}}</td><td>{{NAME}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table>

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## Language Name, N bytes

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Here is a chatroom for any questions about the problem: http://chat.stackexchange.com/rooms/30175/room-for-average-of-angles

Answer 1

Python 3, 129 bytes

lambda l:min([sum(((b-a)%360)**2for b in l)*len(l)-s*s,180-(180-a-s/len(l))%360]for a in l for s in[sum((b-a)%360for b in l)])[1]

This problem seems to have generated quite a lot of confusion. Intuitively, the idea is to cut the circle of angles at some point, unwrap the circle to a line, compute the arithmetic mean on that line, and then wrap the result back to the circle. But there are many different points where you could choose to cut the circle. It is not enough to arbitrarily pick one, such as 0° or 180°. You need to try them all and see which one results in the smallest sum of squared distances. If your solution is significantly less complicated than this, it is probably wrong.

Answer 2

Python 3, 85 bytes

lambda l:180-min(range(72000),key=lambda x:sum((180-(x/200+i)%360)**2for i in l))/200

Takes advantage of the the answer only needing to be accurate to two decimal points by trying all possible angles with increments of 1/200 of a degree. This takes less than a second on my machine.

Because Python doesn't let us conveniently list arithmetic progressions of floats, we represent the possible angles as whole number [0,72000), which convert to an angle as (-180,180] as x -> 180 - x/200. We find the one of these that gives the minimum sum of squared angular differences.

For two angles with an angular displacement of d, the squared angular distance is found by transforming to an equivalent angle in (-180,180] as 180-(d+180)%360, then squaring. Conveniently, the angle given by x/200 is already offset by 180 degrees.

Answer 3

Octave, 97 95 bytes

p=pi;x=p:-1e-5:-p;z=@(L)sum((p-abs(abs(x-mod(L,360)*p/180)-p)).^2);@(L)x(z(L)==min(z(L)))*180/p

This produces an anonymous function that just searches the minimum of the given function on a grid that is just fine enough. As input the function accepts column vectors, e.g. [180; 60; -60]. For testing you need to give the function a name. So you could e.g. run the code above and then use ans([180, 60; -60]).

Answer 4

Javascript ES6, 87 bytes

with(Math)f=(...n)=>(t=>180/PI*atan(t(sin)/t(cos)))(f=>n.reduce((p,c)=>p+=f(c*PI/180)))

Example runs (Tested in Firefox):

f(-91,91)     // -0
f(-90,90)     // 0
f(0,-120,120) // 0
f(0,810)      // 44.999999999999936

Work in progress

This version takes a slightly different approach than the average-everything-then-do-modular-math. Rather, the angles are converted to vectors, the vectors are added and the angle of the resulting vector is then computed. Unfortunately, this version is very unstable with the trig and I'll be working on a modular-math version.

Answer 5

CJam,  44  40 bytes

Ie3_2*,f-:e-2{ea:~f{-P*180/mcmC2#}:+}$0=

Try it online in the CJam interpreter.

Test cases

$ for i in 1\ 3 90\ -90 0\ -120\ 120 0\ 810 1\ 3\ 3 180\ 60\ -60 0\ 15\ 45\ 460 91\ -91 -89\ 89
> do cjam <(echo 'Ie3_2*,f-:e-2{ea:~f{-P*180/mcmC2#}:+}$0=') $i
> echo
> done
2.0
180.0
0.0
45.0
2.33
60.0
40.0
180.0
0.0

Idea

We compute the deviation for all potential averages from -179.99 to 180.00 with steps of size 0.01, and select the one with the lowest deviation.

For this purpose, it doesn't matter if we take the angular distances degrees or radians. Rather than mapping the differences δ of angles from input and potential averages in [0,360°) and conditionally subtracting the result from 180°, we can simply calculate arccos(cos(πδ÷180°)), since cos is both periodic and even, and arccos always yields a value in [0,π).

Code

Ie3        e# Push 18e3 = 18,000.
_2*        e# Copy and multiply by 2. Pushes 36,000.
,          e# Push the range [0 ... 35,999].
f-         e# Subtract each element from 18,000. Pushes [18,000 ... -17,999].
:e-2       e# Divide each element by 100. Pushes [180.00 ... -179.99].
{          e# Sort; for each element A of that array:
  ea:~     e#   Push and evaluate the array of command-line arguments.
  f{       e#   For each input angle, push A and the angle; then:
    -      e#     Subtract the angle from A.
    P*180/ e#     Convert from degrees to radians.
    mcmC   e#     Apply cos, then arccos to the result.
    2#     e#     Square.
  }        e#
  :+       e#   Add the squares. This calculates the deviation.
}$         e# A's with lower deviations come first.
0=         e# Select the first element of the sorted array.

Answer 6

Scala, 96 bytes

Modified from @Roman Czyborra's answer.

---

Golfed version. Try it online!

x=>(-179 to 180).minBy(a=>x.map(i=>pow((a-i%360)match{case x if x>180=>x-360;case y=>y},2)).sum)

Ungolfed version. Try it online!

import scala.math._

object Main {
  def main(args: Array[String]): Unit = {
    println(a(List(0, 15, 45, 460)))
  }

  def a(x: List[Int]): Int = {
    (-179 to 180).minBy(a => x.map(i => pow((a - i % 360) match {
      case x if x > 180 => x - 360
      case y => y
    }, 2)).sum)
  }
}

Answer 7

MATLAB, 151

p=360;n=mod(input(''),p);a=0:0.01:p;m=[];for b=a e=b-n;f=mod([e;-e],p);c=min(f);d=c.^2;m=[m sum(d)];end;[~,i]=min(m);a=a(i);if a>180 a=a-p;end;disp(a);

Ok, so until I can actually understand what the methodology is, this is what I have come up with. It is a little bit of a hack, but as the question states that the answer must be correct to 2.d.p it should work.

I basically check every angle between 0 and 360 (in 0.01 increments) and then solve the formula in the question for each of those angles. Then the angle with the smallest sum is picked and converted into -180 to 180 range.

---

The code should with Octave. You can try it with the online interpreter

Answer 8

JavaScript (ES6) 138

Not having the faintest idea of an algorithm, this tries all possibile values with 2 digits precision (-179.99 to 180.00). Quite fast with the test cases anyway.

Test running the snippet below in an EcmaScript 6 compliant browser (implementing arrow functions and default parameters - AFAIK Firefox)

A=l=>(D=(a,b,z=a>b?a-b:b-a)=>z>180?360-z:z,m=>{for(i=-18000;i++<18000;)l.some(v=>(t+=(d=D(v%360,i/100))*d)>m,t=0)||(m=t,r=i)})(1/0)||r/100

// Test
console.log=x=>O.innerHTML+=x+'\n'

;[[1,3],[89,-89],[90,-90],[91,-91],[0,120,-120],[0,810],[1,3,3],[180,60,-60],[0,15,45,460],[1,183]]
.forEach(t=>console.log(t+' -> '+A(t)))

// Less golfed

A=l=>{
  D=(a,b,z=a>b?a-b:b-a) => z>180?360-z:z; // angular distance
  m=1/0;
  for(i=-18000;i++<18000;) // try all from -179.99 to 180
  {
    t = 0;
    if (!l.some(v => (t+=(d=D(v%360,i/100))*d) > m))
    {
      m = t;
      r = i;
    }  
  }  
  return r/100;
}

<pre id=O></pre>

Answer 9

julia, 101 bytes

This version works to machine precision.

The idea is that If each entry of the input vector \$x\in \mathbf{R}^n\$ satisfies \$x_i \in (0, 360]\$, and if \$x\$ is sorted in ascending order, then the angle average is the arithmetic mean of $$ x^{(k)} :=x + (\overbrace{360, \dotsc, 360}^{k\text{ entries}}, 0, \dotsc, 0) $$ for some \$k \in \{1, \dotsc, n\}\$

Hence, the problem reduces to finding $$ k^* = \underset{k=1, \dotsc, n}{\mathrm{argmin}} \sum_{i=1}^n (x_i^{(k)} - \bar{x}^{(k)})^2. $$

The corresponding arithmetic mean is then $$ \bar{x}^{(k^*)} = \bar{x} + k^* \cdot \frac{360}{n} $$

Finally, we put everything on the \$(-180, 180]\$ scale by adding \$180\$ to the input, computing everything mod \$360\$ and subtracting \$180\$ at the end.

---

Expects a Vector{Float64} and returns a Float64.

y->(| =mod1;~=sum;s=360;x=sort(y.+.5s.|s);n=~y.^0;~x+argmin(k->(x[k]+=s;~(x.-~x/n).^2),1:n)s)/n|s-.5s

Attempt This Online!

Answer 10

Haskell, 134 bytes, with fractional precision

a x=(/100)$fromIntegral$snd$minimum$map(\a->(sum$map((^2).(\x->last$x:[x-36000|x>18000]).(`mod`36000).(a-).(100*))x,a))[-17999..18000]

Try it online!

Haskell, 97 bytes, with integer precision

a x=snd$minimum$map(\a->(sum$map((^2).(\x->last$x:[x-360|x>180]).(`mod`360).(a-))x,a))[-179..180]

Try it online!

Answer 11

Julia 1.7, 61 bytes

p=180
!y=argmin(i->sum(j->((i-j%2p+p)%2p-p)^2,y),.01-p:.01:p)

Attempt This Online!

argmin(f, itr) needs julia 1.7 or later