314maro/rungekutta.html

## rungekutta.html
<!DOCTYPE html>
<html lang="ja">
<head>
<script>
function rungekutta(n,xn,f,x0,y0) {
  var h = (xn - x0) / n;
  var xi = x0, yi = y0;
  function step(x,y) {
    var k1 = f(x, y),
        k2 = f(x+h/2, y+h*k1/2),
        k3 = f(x+h/2, y+h*k2/2),
        k4 = f(x+h, y+h*k3);
    return y + h*(k1+2*k2+2*k3+k4)/6;
  }
  for (var i = 0; i < n; xi = x0 + h*++i) {
    yi = step(xi,yi);
  }
  return yi;
}
function solve() {
  var elem = document.getElementById("result"),
      n = document.getElementById("precision").value,
      x = document.getElementById("x").value,
      f = document.getElementById("function").value,
      x0 = document.getElementById("x0").value,
      y0 = document.getElementById("y0").value;
  var res = rungekutta(+n, +x, new Function("x","y",f), +x0, +y0);
  elem.value = res;
}
var dict = {
  // hoge: { xn: , f: , x0: , y0: },
  nothing: { xn: null, f: null, x0: null, y0: null },
  exp: { xn: 1, f: "return y", x0: 0, y0: 1 },
  arctan: { xn: 1, f: "return 4/(1+x*x)", x0: 0, y0: 0 }
}
window.onload = function () {
  var ex = document.getElementById("examples");
  for (var i in dict) {
    var opt = document.createElement("option");
    opt.value = i; opt.textContent = i;
    ex.appendChild(opt);
  }
}
function selectExample(ev) {
  var ex = dict[ev.target.value];
  document.getElementById("x").value = ex.xn;
  document.getElementById("function").value = ex.f;
  document.getElementById("x0").value = ex.x0;
  document.getElementById("y0").value = ex.y0;
}
</script>
</head>
<body>
  <p>precision is <input id="precision" value="100"></input></p>
  <p>dy/dx = <input id="function"></input></p>
  <p>y(<input id="x0"></input>)
    = <input id="y0"></input></p>
  <button onclick="solve()">Solve</button>
  <p>y(<input id="x"></input>)
    = <input type="text" id="result" size="25" readonly></input></p>
  <p>
    you can choose example:
    <select id="examples" onchange="selectExample(event)">
    </select>
  </p>
</body>
</html>
	<!DOCTYPE html>
	<html lang="ja">
	<head>
	<script>
	function rungekutta(n,xn,f,x0,y0) {
	var h = (xn - x0) / n;
	var xi = x0, yi = y0;
	function step(x,y) {
	var k1 = f(x, y),
	k2 = f(x+h/2, y+h*k1/2),
	k3 = f(x+h/2, y+h*k2/2),
	k4 = f(x+h, y+h*k3);
	return y + h(k1+2k2+2*k3+k4)/6;
	}
	for (var i = 0; i < n; xi = x0 + h*++i) {
	yi = step(xi,yi);
	}
	return yi;
	}
	function solve() {
	var elem = document.getElementById("result"),
	n = document.getElementById("precision").value,
	x = document.getElementById("x").value,
	f = document.getElementById("function").value,
	x0 = document.getElementById("x0").value,
	y0 = document.getElementById("y0").value;
	var res = rungekutta(+n, +x, new Function("x","y",f), +x0, +y0);
	elem.value = res;
	}
	var dict = {
	// hoge: { xn: , f: , x0: , y0: },
	nothing: { xn: null, f: null, x0: null, y0: null },
	exp: { xn: 1, f: "return y", x0: 0, y0: 1 },
	arctan: { xn: 1, f: "return 4/(1+x*x)", x0: 0, y0: 0 }
	}
	window.onload = function () {
	var ex = document.getElementById("examples");
	for (var i in dict) {
	var opt = document.createElement("option");
	opt.value = i; opt.textContent = i;
	ex.appendChild(opt);
	}
	}
	function selectExample(ev) {
	var ex = dict[ev.target.value];
	document.getElementById("x").value = ex.xn;
	document.getElementById("function").value = ex.f;
	document.getElementById("x0").value = ex.x0;
	document.getElementById("y0").value = ex.y0;
	}
	</script>
	</head>
	<body>
	<p>precision is <input id="precision" value="100"></input></p>
	<p>dy/dx = <input id="function"></input></p>
	<p>y(<input id="x0"></input>)
	= <input id="y0"></input></p>
	<button onclick="solve()">Solve</button>
	<p>y(<input id="x"></input>)
	= <input type="text" id="result" size="25" readonly></input></p>
	<p>
	you can choose example:
	<select id="examples" onchange="selectExample(event)">
	</select>
	</p>
	</body>
	</html>