View source for Change Equation of a Graph

This example shows how you can change the equation of a graph without creating the whole construction again. Dependend elements are updated automatically.
<html>
<link rel="stylesheet" type="text/css" href="http://jsxgraph.uni-bayreuth.de/distrib/jsxgraph.css" />
<script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/prototype.js"></script>
<script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/jsxgraphcore.js"></script>
<div style="width:960px">
<div id="jxgbox" class="jxgbox" style="width:600px; height:400px;"></div>
<p>
<input type="text" id="eingabe" value="Math.sin(x)*Math.cos(x)">
<input type="button" value="set" onClick="doIt()" style='margin:1em'> 
</p>
</div>
<div id="debug" style="display:none;"></div>
<script type="text/javascript">
    /* <![CDATA[ */

        board = JXG.JSXGraph.initBoard('jxgbox', {originX: 250, originY: 250, unitX: 40, unitY: 20});
        b1axisx = board.createElement('axis', [[1,0], [0,0]], {});
        b1axisy = board.createElement('axis', [[0,1], [0,0]], {});
		
	eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
	graph = board.createElement('curve', [function(x){ return x; }, function(x){ return f(x); }, "x", -10, 10]);						
	//graph = eval("board.createElement('curve', ['x', function(x){ return "+document.getElementById("eingabe").value+";}, 'x', -10, 10])",{curveType:'plot'});
        p1 = board.createElement('glider', [graph], {style:6, name:'P'});
        p2 = board.createElement('point', [function() { return p1.X()+1;}, function() {return p1.Y()+board.algebra.D(graph.Y)(p1.X());}], {style:1, name:''});
        l1 = board.createElement('line', [p1,p2],{});
        p3 = board.createElement('point', [function() { return p2.X();}, function() {return p1.Y();}],{style:1, name:''});
        pol = board.createElement('polygon', [p1,p2,p3], {});
        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));}]); 
            
        function doIt(){
        	eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
        	graph.Y = function(x){ return f(x); };
    		graph.updateCurve();
    		board.update();
        } 
        
  /* ]]> */
  </script>
</html>


== How to Create this Construction ==

=== HTML Part ===
'''Adding a text input field somewhere on the page together with a button'''

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

=== JavaScript Part ===
'''Setting up the board'''
<source lang="javascript">
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]], {});
</source>

'''Create a JavaScript Function f(x) From the Text Field'''
<source lang="javascript">
eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
</source>

'''Use f(x) for defining the Graph'''
<source lang="javascript">
graph = board.createElement('curve', [function(x){ return x; }, function(x){ return f(x); }, "x", -10, 10]);
</source>

'''Define Further Elements'''
<source lang="javascript">
//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}); 
</source>

'''Define JavaScript Function doIt() for Reacting on User Input'''
<source lang="javascript">
function doIt(){
  //redefine function f according to the current text field value
  eval("function f(x){ return "+document.getElementById("eingabe").value+";}");
  //change the Y attribute of the graph to the new function 
  graph.Y = function(x){ return f(x); };
  //update the graph
  graph.updateCurve();
  //update the whole board
  board.update();
} 
</source>

== 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.
[[Category:Examples]]