Positioning divs in a circle using JavaScript

Question

I am trying to position 15 div elements evenly in a circle with a radius of 150px. I'm using the following code, which seems to give an oddly eccentric ellipse that overlaps.

Fiddle

// Hold a global reference to the div#main element.  Initially assign it ... somewhere useful :)
var main = document.getElementById('main');
var circleArray = [];

// Move a circle based on the distance of the approaching mouse
var moveCircle = function(circle, dx, dy) {

};

// Look at all the circle elements, and figure out if any of them have to move.
var checkMove = function() {

};
var setup = function() {
  for (var i = 0; i < 15; i++) {
    //create element, add it to the array, and assign it's coordinates trigonometrically.
    //Then add it to the "main" div
    var circle = document.createElement('div');
    circle.className = 'circle number' + i;
    circleArray.push(circle);
    circleArray[i].posx = Math.round((150 * Math.cos(i * (2 * Math.PI / 15)))) + 'px';
    circleArray[i].posy = Math.round((150 * Math.sin(i * (2 * Math.PI / 15)))) + 'px';
    circleArray[i].style.position = "relative";
    circleArray[i].style.top = circleArray[i].posy;
    circleArray[i].style.left = circleArray[i].posx;
    main.appendChild(circleArray[i]);
  }
};
setup();
window.addEventListener('load', function() {

});

div {
  box-sizing: border-box;
}
div#main {
  position: absolute;
  left: 50%;
  top: 50%;
}
div.circle {
  position: absolute;
  width: 20px;
  height: 20px;
  border: 2px solid black;
  border-radius: 10px;
  -webkit-border-radius: 10px;
  -moz-border-radius: 10px;
}

<div id="main"></div>

Any suggestions as to what I might be doing wrong?

Answer 1

First of all, the equation for a co-ordinate on a circle is simply:

(x, y) = (r * cos(θ), r * sin(θ))

where, r is the radius of a circle and θ is the angle in radians.

---

The reason why your code is creating an eccentric ellipse is because when you assign the .top and .left CSS values, you are not considering that it will actually take the top-left corner as its reference. I've fixed your code and now it creates a perfect circle.

Changes made to your code:

1. Added an array theta that holds all the angles.

   var theta = [0, Math.PI / 6, Math.PI / 4, Math.PI / 3, Math.PI / 2, 2 * (Math.PI / 3), 3 * (Math.PI / 4), 5 * (Math.PI / 6), Math.PI, 7 * (Math.PI / 6), 5 * (Math.PI / 4), 4 * (Math.PI / 3), 3 * (Math.PI / 2), 5 * (Math.PI / 3), 7 * (Math.PI / 4), 11 * (Math.PI / 6)];
   

   The image below shows all the angles I've used.

   

2. Added an array colors that holds different colors.

   var colors = ['red', 'green', 'purple', 'black', 'orange', 'yellow', 'maroon', 'grey', 'lightblue', 'tomato', 'pink', 'maroon', 'cyan', 'magenta', 'blue', 'chocolate', 'DarkSlateBlue'];
   

3. Made changes to your trigonometric equations.

   circleArray[i].posx = Math.round(radius * (Math.cos(theta[i]))) + 'px';
   circleArray[i].posy = Math.round(radius * (Math.sin(theta[i]))) + 'px';
   

4. Changed the way .top and .left are assigned.

   circleArray[i].style.top = ((mainHeight / 2) - parseInt(circleArray[i].posy.slice(0, -2))) + 'px';
   circleArray[i].style.left = ((mainHeight / 2) + parseInt(circleArray[i].posx.slice(0, -2))) + 'px';
   

   where mainHeight is the height of the #main div.

---

[1] 16 divs

Demo on Fiddle

var setup = function() {
  var radius = 150;
  var main = document.getElementById('main');
  var mainHeight = parseInt(window.getComputedStyle(main).height.slice(0, -2));
  var theta = [0, Math.PI / 6, Math.PI / 4, Math.PI / 3, Math.PI / 2, 2 * (Math.PI / 3), 3 * (Math.PI / 4), 5 * (Math.PI / 6), Math.PI, 7 * (Math.PI / 6), 5 * (Math.PI / 4), 4 * (Math.PI / 3), 3 * (Math.PI / 2), 5 * (Math.PI / 3), 7 * (Math.PI / 4), 11 * (Math.PI / 6)];
  var circleArray = [];
  var colors = ['red', 'green', 'purple', 'black', 'orange', 'yellow', 'maroon', 'grey', 'lightblue', 'tomato', 'pink', 'maroon', 'cyan', 'magenta', 'blue', 'chocolate', 'DarkSlateBlue'];
  for (var i = 0; i < 16; i++) {
    var circle = document.createElement('div');
    circle.className = 'circle number' + i;
    circleArray.push(circle);
    circleArray[i].posx = Math.round(radius * (Math.cos(theta[i]))) + 'px';
    circleArray[i].posy = Math.round(radius * (Math.sin(theta[i]))) + 'px';
    circleArray[i].style.position = "absolute";
    circleArray[i].style.backgroundColor = colors[i];
    circleArray[i].style.top = ((mainHeight / 2) - parseInt(circleArray[i].posy.slice(0, -2))) + 'px';
    circleArray[i].style.left = ((mainHeight / 2) + parseInt(circleArray[i].posx.slice(0, -2))) + 'px';
    main.appendChild(circleArray[i]);
  }
};
setup();

div#main {
  height: 300px;
  width: 300px;
  position: absolute;
  margin: 0 auto;
  transform: translate(-50%, -50%);
  top: 50%;
  left: 50%;
}
div.circle {
  position: absolute;
  width: 20px;
  height: 20px;
  border: 2px solid black;
  border-radius: 50%;
}
body {
  margin: 0 auto;
  background: papayawhip;
}

<div id="main"></div>

---

[2] 15 divs Positioned Evenly

Demo on Fiddle

var setup = function() {
  var radius = 150;
  var main = document.getElementById('main');
  var mainHeight = parseInt(window.getComputedStyle(main).height.slice(0, -2));
  var theta = [0, (2 * (Math.PI / 15)), (4 * (Math.PI / 15)), (2 * (Math.PI / 5)), (8 * (Math.PI / 15)), (2 * (Math.PI / 3)), (4 * (Math.PI / 5)), (14 * (Math.PI / 15)), (16 * (Math.PI / 15)), (6 * (Math.PI / 5)), (4 * (Math.PI / 3)), (22 * (Math.PI / 15)), (8 * (Math.PI / 5)), (26 * (Math.PI / 15)), (28 * (Math.PI / 15))];
  var circleArray = [];
  var colors = ['red', 'green', 'purple', 'black', 'orange', 'yellow', 'maroon', 'grey', 'lightblue', 'tomato', 'pink', 'maroon', 'cyan', 'magenta', 'blue', 'chocolate', 'DarkSlateBlue'];
  for (var i = 0; i < 15; i++) {
    var circle = document.createElement('div');
    circle.className = 'circle number' + i;
    circleArray.push(circle);
    circleArray[i].posx = Math.round(radius * (Math.cos(theta[i]))) + 'px';
    circleArray[i].posy = Math.round(radius * (Math.sin(theta[i]))) + 'px';
    circleArray[i].style.position = "absolute";
    circleArray[i].style.backgroundColor = colors[i];
    circleArray[i].style.top = ((mainHeight / 2) - parseInt(circleArray[i].posy.slice(0, -2))) + 'px';
    circleArray[i].style.left = ((mainHeight / 2) + parseInt(circleArray[i].posx.slice(0, -2))) + 'px';
    main.appendChild(circleArray[i]);
  }
};
setup();

div#main {
  height: 300px;
  width: 300px;
  position: absolute;
  margin: 0 auto;
  transform: translate(-50%, -50%);
  top: 50%;
  left: 50%;
}
div.circle {
  position: absolute;
  width: 20px;
  height: 20px;
  border: 2px solid black;
  border-radius: 50%;
}
body {
  margin: 0 auto;
  background: papayawhip;
}

<div id="main"></div>

---

[3] Dynamically Position any number of divs on an Ellipse/Circle

The equation for a co-ordinate on an ellipse is:

(x, y) = (rx * cos(θ), ry * sin(θ))

where, rx is the radius along X-axis and ry is the radius along Y-axis.

---

In this case, the function generate(n, rx, ry, id) takes four arguments, where n is the number of divs, rx and ry are the radii along the X and Y-axis respectively and finally id is the id of the div that you want to append your elliptically arranged divs in.

Demo on Fiddle

var theta = [];

var setup = function(n, rx, ry, id) {
  var main = document.getElementById(id);
  var mainHeight = parseInt(window.getComputedStyle(main).height.slice(0, -2));
  var circleArray = [];
  var colors = ['red', 'green', 'purple', 'black', 'orange', 'yellow', 'maroon', 'grey', 'lightblue', 'tomato', 'pink', 'maroon', 'cyan', 'magenta', 'blue', 'chocolate', 'darkslateblue', 'coral', 'blueviolet', 'burlywood', 'cornflowerblue', 'crimson', 'darkgoldenrod', 'olive', 'sienna', 'red', 'green', 'purple', 'black', 'orange', 'yellow', 'maroon', 'grey', 'lightblue', 'tomato', 'pink', 'maroon', 'cyan', 'magenta', 'blue', 'chocolate', 'darkslateblue', 'coral', 'blueviolet', 'burlywood', 'cornflowerblue', 'crimson', 'darkgoldenrod', 'olive', 'sienna'];
  for (var i = 0; i < n; i++) {
    var circle = document.createElement('div');
    circle.className = 'circle number' + i;
    circleArray.push(circle);
    circleArray[i].posx = Math.round(rx * (Math.cos(theta[i]))) + 'px';
    circleArray[i].posy = Math.round(ry * (Math.sin(theta[i]))) + 'px';
    circleArray[i].style.position = "absolute";
    circleArray[i].style.backgroundColor = colors[i];
    circleArray[i].style.top = ((mainHeight / 2) - parseInt(circleArray[i].posy.slice(0, -2))) + 'px';
    circleArray[i].style.left = ((mainHeight / 2) + parseInt(circleArray[i].posx.slice(0, -2))) + 'px';
    main.appendChild(circleArray[i]);
  }
};

var generate = function(n, rx, ry, id) {
  var frags = 360 / n;
  for (var i = 0; i <= n; i++) {
    theta.push((frags / 180) * i * Math.PI);
  }
  setup(n, rx, ry, id)
}
generate(16, 150, 75, 'main');

div#main {
  height: 300px;
  width: 300px;
  position: absolute;
  margin: 0 auto;
  transform: translate(-50%, -50%);
  top: 50%;
  left: 50%;
}
div.circle {
  position: absolute;
  width: 20px;
  height: 20px;
  border: 2px solid black;
  border-radius: 50%;
}
body {
  margin: 0 auto;
  background: papayawhip;
}

<div id="main"></div>

---

Edit[9th December 2015]:

Here's a more flexible version with start offset, clock wise and anti-clock wise functionality.

/*
Usage: Position.ellipse(n, rx, ry, so, wh, idd, cls, cw);

where n = number of divs,
      rx = radius along X-axis,
      ry = radius along Y-axis,
      so = startOffset,
      wh = width/height of divs,
      idd = id of main div(ellipse),
      cls = className of divs;
      cw = clockwise(true/false)
*/

var Position = {
  ellipse: function(n, rx, ry, so, wh, idd, cls, cw) {
    var m = document.createElement('div'),
      ss = document.styleSheets;
    ss[0].insertRule('#' + idd + ' { position: absolute; left: 50%; top: 50%; transform: translate(-50%, -50%); border-radius: 50%; box-shadow: inset 0 0 ' + wh + 'px ' + wh / 4 + 'px black; background: rgba(0, 0, 0, 0.2); width: ' + String((rx * 2) + wh) + 'px; height: ' + String((ry * 2) + wh) + 'px; }', 1);
    ss[0].insertRule('.' + cls + '{ position: absolute; background: black; color: papayawhip; text-align: center; font-family: "Open Sans Condensed", sans-serif; border-radius: 50%; transition: transform 0.2s ease; width: ' + wh + 'px; height: ' + wh + 'px; line-height: ' + wh + 'px;}', 1);
    ss[0].insertRule('.' + cls + ':hover { transform: scale(1.2); cursor: pointer; background: rgba(0, 0, 0, 0.8); }', 1);
    m.id = idd;
    for (var i = 0; i < n; i++) {
      var c = document.createElement('div');
      c.className = cls;
      c.innerHTML = i + 1;
      c.style.top = String(ry + -ry * Math.cos((360 / n / 180) * (i + so) * Math.PI)) + 'px';
      c.style.left = String(rx + rx * (cw ? Math.sin((360 / n / 180) * (i + so) * Math.PI) : -Math.sin((360 / n / 180) * (i + so) * Math.PI))) + 'px';
      m.appendChild(c);
    }
    document.body.appendChild(m);
  }
}

Position.ellipse(20, 150, 150, 0, 42, 'main', 'circle', true);

@import url(http://fonts.googleapis.com/css?family=Open+Sans+Condensed:300);
 body {
  margin: 0 auto;
  background: rgb(198, 193, 173);
}

Answer 2

Other approach without JS

chipChocolate.py's anser is pretty complete but there is an other way to achieve your aim. It is simpler and doesn't require JS.

The point is to think "circle" and rotation rather than rely on [x,y] coordinates :

You need to nest all the elements and apply a rotation to them. As they are nested the n + 1 element will rotate according to it's direct parent's rotation. Here is a DEMO :

.circle, .circle div {
    width:24px; height:300px;
    position:absolute;
    left:50%; top:50px;
}
.circle:before, .circle div:before {
    content:'';
    display:block;
    width:20px; height:20px;
    border: 2px solid black;
    border-radius: 100%;
}
.circle div {
    top:0; left:0;
    -webkit-transform : rotate(24deg);
    -ms-transform : rotate(24deg);
    transform : rotate(24deg);
}

<div class="circle">
    <div><div><div><div><div><div><div><div><div><div><div><div><div><div><div>
    </div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
</div>

The diameter of the circle is controled by the height of the elements (in the demo height:300px) you can make that a percentage to make the circle responsive (see below).

The rotation must be set according to the number of elements you want around the circle. In the demo 15 elements so rotation = 360 / 15 = 24deg.

If you have a dynamic number of elements, you may use JS to add them and to calculate the rotation angle needed.

---

Responsive example

DEMO

.circle{
    position:relative;
    width:5%;padding-bottom:50%;
    margin-left:47.5%;
}
.circle div {
    position:absolute;
    top:0; left:0;
    width:100%; height:100%;
    -webkit-transform : rotate(24deg);
    -ms-transform : rotate(24deg);
    transform : rotate(24deg);
}
.circle:before, .circle div:before {
    content:'';
    position:absolute;
    top:0; left:0;
    width:100%; padding-bottom:100%;
    border-radius: 100%;
    border: 2px solid teal;
    background:gold;
}

<div class="circle">
    <div><div><div><div><div><div><div><div><div><div><div><div><div><div><div>
    </div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
</div>

Answer 3

Yet another solution, based on ideas from other solutions I've seen

http://jsfiddle.net/0hr1n7a2/6/

(function() {
  var radians, radius;

  radius = 150;
    
  var totalItems = 48
  var item = 0;
  function positionTarget() 
  {
    var x, y, angle = 0, step = (2*Math.PI) / totalItems;
    var width = $('#container').width()/2;
    var height = $('#container').height()/2;
    var itemW = 20, itemH = 2;
    var deg = 0;    
 while(item <= totalItems)
 {        
  x = Math.round(width + radius * Math.cos(angle) - itemW/2);
  y = Math.round(height + radius * Math.sin(angle) - itemH/2);        
        //console.log(x + "," + y);
     
  $('#container').append('<div id="'+ item +'"/>')
  $('div#'+item).css('position', 'absolute')
        .css('width', itemW+'px').css('height', itemH+'px')      
        .css('left', x+'px').css('top', y+'px')
        .css('background-color', 'blue')
        .css('transform-origin', x+'px' -y+'px')        
        .css('transform', 'rotate('+ deg +'deg)')
        .css('border', 'solid 1px #000');
     
        angle += step;
        ++item;
        deg += 360/totalItems;
     
        //console.log(deg)
    }
  }

  $('#theButton').on('click', function()
  {
        positionTarget();
  })
    
})();

#container { width: 600px; height: 600px; border: 1px solid #000; position: relative; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="button" id="theButton" value="Draw">    
<div id="container">
</div>