Change Equation of a Graph

This example shows how you can change the equation of a graph without creating the whole construction again.

HowTo Create this Construction

HTML Part

Adding a text input field somewhere on the page together with a button

<input type="text" id="eingabe" value="Math.sin(x)*Math.cos(x)">
<input type="button" value="set" onClick="doIt()">

JavaScript Part

Setting up the board

  board = JXG.JSXGraph.initBoard('jxgbox', {originX: 250, originY: 250, unitX: 40, unitY: 20});
  // Axes
  b1axisx = board.createElement('axis', [[1,0], [0,0]], {});
  b1axisy = board.createElement('axis', [[0,1], [0,0]], {});

Create a JavaScript Function f(x) From the Text Field

  eval("function f(x){ return "+document.getElementById("eingabe").value+";}");

Use f(x) for defining the Graph

  graph = board.createElement('curve', [function(x){ return x; }, function(x){ return f(x); }, "x", -10, 10]);

Define Further Elements

  //glider on the curve
  p1 = board.createElement('glider', [graph], {style:6, name:'P'});
  //define the derivative of f
  g = board.algebra.D(f);
  //a point on the tangent
  //                                 variable x coordinate           variable y coordinate depending on the derivative of f at point p1.X()
  p2 = board.createElement('point', [function() { return p1.X()+1;}, function() {return p1.Y()+board.algebra.D(graph.Y)(p1.X());}], {style:1, name:''});
  //the tangent 
  l1 = board.createElement('line', [p1,p2],{}); 
  //a third point fpr the slope triangle
  p3 = board.createElement('point', [function() { return p2.X();}, function() {return p1.Y();}],{style:1, name:''});
  //the slope triangle
  pol = board.createElement('polygon', [p1,p2,p3], {});
  //a text for displaying slope's value
  //                               variable x coordinate          variable y coordinate                        variable value
  t = board.createElement('text', [function(){return p1.X()+1.1;},function(){return p1.Y()+(p2.Y()-p3.Y())/2;},function(){ return "m="+(board.round(p2.Y()-p3.Y(),2));}],{color:ff0000});

Define JavaScript Function doIt() for Reacting on User Input

  function doIt(){
    //redefine function f according to the current text field value
    eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
    //change the yterm attribute of the graph to the new function 
    graph.yterm = function(x){ return f(x); };  // usually: e.g. "x^2"
    graph.Y = graph.yterm;
    //update the graph
    graph.updateCurve();
    //update the whole board
    board.update();
  }

Remarks

The doIt() function is only responsible for updating the graph. All other dependend objects are self-updating, especially the object p2 which depends on the derivative of function f. This is all done by aononymous functions of JavaScript.