Centre and radius of a circle passing through 3 points in a plane

Centre and radius of a circle passing through 3 points in a plane is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Write a function which returns the centre and radius of a circle passing through three point in a plane. Demonstrate the function using the points (22.83,2.07) (14.39,30.24) and (33.65,17.31)

C++

This follows the lines of the C++ code here.

#include <iostream>
#include <math.h>
using namespace std;
 
void findCircle(float x1, float y1, float x2, float y2, float x3, float y3) {
    float x12 = x1 - x2;
    float x13 = x1 - x3; 
    float y12 = y1 - y2;
    float y13 = y1 - y3;
    float y31 = y3 - y1;
    float y21 = y2 - y1;
    float x31 = x3 - x1;
    float x21 = x2 - x1;
    
    float sx13 = pow(x1, 2) - pow(x3, 2);
    float sy13 = pow(y1, 2) - pow(y3, 2);
    float sx21 = pow(x2, 2) - pow(x1, 2);
    float sy21 = pow(y2, 2) - pow(y1, 2);
 
    float f = ((sx13) * (x12) + (sy13) * (x12) + (sx21) * (x13) + (sy21) * (x13))
            / (2 * ((y31) * (x12) - (y21) * (x13)));
    float g = ((sx13) * (y12) + (sy13) * (y12) + (sx21) * (y13) + (sy21) * (y13))
            / (2 * ((x31) * (y12) - (x21) * (y13)));
 
    float c = -pow(x1, 2) - pow(y1, 2) - 2 * g * x1 - 2 * f * y1;
    float h = -g;
    float k = -f;
    float r = sqrt(h * h + k * k - c);
 
    cout << "Centre is at (" << h << ", " << k << ")" << endl;
    cout << "Radius is " << r;
}
 
int main() {
    findCircle(22.83, 2.07, 14.39, 30.24, 33.65, 17.31);
 
    return 0;
}

Dart

Translation of: C++

import 'dart:math';

void findCircle(
    double x1, double y1, double x2, double y2, double x3, double y3) {
  double x12 = x1 - x2;
  double x13 = x1 - x3;
  double y12 = y1 - y2;
  double y13 = y1 - y3;
  double y31 = y3 - y1;
  double y21 = y2 - y1;
  double x31 = x3 - x1;
  double x21 = x2 - x1;

  double sx13 = x1 * x1 - x3 * x3;
  double sy13 = y1 * y1 - y3 * y3;
  double sx21 = x2 * x2 - x1 * x1;
  double sy21 = y2 * y2 - y1 * y1;

  double f = (sx13 * x12 + sy13 * x12 + sx21 * x13 + sy21 * x13) /
      (2 * (y31 * x12 - y21 * x13));
  double g = (sx13 * y12 + sy13 * y12 + sx21 * y13 + sy21 * y13) /
      (2 * (x31 * y12 - x21 * y13));

  double c = -pow(x1, 2) - pow(y1, 2) - 2 * g * x1 - 2 * f * y1;
  double h = -g;
  double k = -f;
  double r = sqrt(h * h + k * k - c);

  print("Centre is at ($h, $k)");
  print("\nCheck radius as the distance between the centre and the first point: $r");
}

void main() {
  findCircle(22.83, 2.07, 14.39, 30.24, 33.65, 17.31);
}

Output:

Centre is at (18.978515660148815, 16.265410797715866)

=={{header|F_Sharp|F#}}==
<syntaxhighlight lang="fsharp">
// Centre and radius of a circle passing through 3 points in a plane. Nigel Galloway: February 20th., 2024
let c (a,b) (c,d) (e,f)=(0.5*((a*a+b*b)*(f-d)+(c*c+d*d)*(b-f)+(e*e+f*f)*(d-b))/(a*(f-d)+c*(b-f)+e*(d-b)),
                         0.5*((a*a+b*b)*(e-c)+(c*c+d*d)*(a-e)+(e*e+f*f)*(c-a))/(b*(e-c)+d*(a-e)+f*(c-a)))
let d n g = let n,g=fst n-fst g,snd n-snd g in sqrt(n*n+g*g)/2.0
let circ P1 P2 P3 = let c=c P1 P2 P3 in (c,d c P1)

let centre,radius=circ (22.83, 2.07) (14.39, 30.24) (33.65, 17.31)
printfn $"Centre = %A{centre}, radius = %f{radius}"
</syntaxhighlight>
{{out}}
<pre>
Centre = (18.97851566, 16.2654108), radius = 7.354312

FreeBASIC

Translation of: Wren

Sub findCircle(x1 As Double, y1 As Double, x2 As Double, y2 As Double, x3 As Double, y3 As Double)
    Dim x12 As Double = x1 - x2
    Dim x13 As Double = x1 - x3
    Dim y12 As Double = y1 - y2
    Dim y13 As Double = y1 - y3
    Dim y31 As Double = y3 - y1
    Dim y21 As Double = y2 - y1
    Dim x31 As Double = x3 - x1
    Dim x21 As Double = x2 - x1
    
    Dim sx13 As Double = x1 * x1 - x3 * x3
    Dim sy13 As Double = y1 * y1 - y3 * y3
    Dim sx21 As Double = x2 * x2 - x1 * x1
    Dim sy21 As Double = y2 * y2 - y1 * y1
    
    Dim f As Double = (sx13 * x12 + sy13 * x12 + sx21 * x13 + sy21 * x13) / (y31 * x12 - y21 * x13) / 2
    Dim g As Double = (sx13 * y12 + sy13 * y12 + sx21 * y13 + sy21 * y13) / (x31 * y12 - x21 * y13) / 2
    
    Dim c As Double = -x1 * x1 - y1 * y1 - 2 * g * x1 - 2 * f * y1
    Dim h As Double = -g
    Dim k As Double = -f
    Dim r As Double = Sqr(h * h + k * k - c)
    
    Print "Centre is at (" & h & ", " & k & ")"
    Print "Radius is"; r
    Print
    Print "Check radius as the distance between the centre and the first point:"
    Print Sqr((22.83 - h)^2 + (2.07 - k)^2)
End Sub

findCircle(22.83, 2.07, 14.39, 30.24, 33.65, 17.31)

Sleep

Output:

Centre is at (18.97851566014882, 16.26541079771587)
Radius is 14.70862397833418

Check radius as the distance between the centre and the first point:
 14.70862397833418

Phix

Translation of: F#

with javascript_semantics
function circle(sequence p1, p2, p3)
    atom {a,b} = p1, {c,d} = p2, {e,f} = p3,
         a2b2 = a*a+b*b,  ae = a-e,  db = d-b,
         c2d2 = c*c+d*d,  bf = b-f,  ec = e-c,
         e2f2 = e*e+f*f,  ca = c-a,  fd = f-d,
         cx = 0.5*(a2b2*fd+c2d2*bf+e2f2*db)/(a*fd+c*bf+e*db),
         cy = 0.5*(a2b2*ec+c2d2*ae+e2f2*ca)/(b*ec+d*ae+f*ca),
         r = sqrt(power(cx-e,2)+power(cy-f,2))
    return {{cx,cy},r}
end function

atom {{cx,cy},r} = circle({22.83,2.07},{14.39,30.24},{33.65,17.31})
printf(1,"Centre = %.8f, %.7f, radius = %.6f\n",{cx,cy,r})

Output:

Centre = 18.97851566, 16.2654108, radius = 14.708624

Wren

This follows the lines of the C++ code here.

var findCircle = Fn.new { |x1, y1, x2, y2, x3, y3|
    var x12 = x1 - x2
    var x13 = x1 - x3
    var y12 = y1 - y2
    var y13 = y1 - y3
    var y31 = y3 - y1
    var y21 = y2 - y1
    var x31 = x3 - x1
    var x21 = x2 - x1

    var sx13 = x1 * x1 - x3 * x3
    var sy13 = y1 * y1 - y3 * y3
    var sx21 = x2 * x2 - x1 * x1
    var sy21 = y2 * y2 - y1 * y1

    var f = sx13 * x12 + sy13 * x12 + sx21 * x13 + sy21 * x13
    f = f / (y31 * x12 - y21 * x13) / 2

    var g = sx13 * y12 + sy13 * y12 + sx21 * y13 + sy21 * y13
    g = g / (x31 * y12 - x21 * y13) / 2

    var c = -x1 * x1 - y1 * y1 - 2 * g * x1 - 2 * f * y1
    var h = -g
    var k = -f
    var r = (h * h + k * k - c).sqrt
    return [h, k, r]
}

var hkr = findCircle.call(22.83, 2.07, 14.39, 30.24, 33.65, 17.31)
System.print("Centre is at %([hkr[0], hkr[1]])")
System.print("Radius is %(hkr[2])")

System.print("\nCheck radius as the distance between the centre and the first point:")
System.print(((22.83 - hkr[0]).pow(2) + (2.07 - hkr[1]).pow(2)).sqrt)

Output:

Centre is at [18.978515660149, 16.265410797716]
Radius is 14.708623978334

Check radius as the distance between the centre and the first point:
14.708623978334