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**Chapter 4(b): Fundamentals of JavaScript**

4.1 Capabilities 4.2 Essential Terminology 4.3 Structure of JavaScript Code 4.4 Data and Objects 4.5 Tokens, Operators, Expressions, and Statements 4.6 The JavaScript math Object 4.7 Comparison Operators and Decision-Making Structures 4.8 Loop Structures 4.9 Using JavaScript to Change Values in Form Fields

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**Chapter 4.8: Loop (Repetition) Structures**

Loops provide a structured way to perform repetitive calculations. Count-controlled loops perform calculations a pre-defined number of times. Conditional loops perform calculations until, or as long as, some predefined condition is true.

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**Count-Controlled Loops**

for (counter=start; {expression based on high (or low) value of counter}; expression controlling incrementing (or decrementing) of counter} ) Document 4.7 (counter2.htm) <html> <head> <title>Counter</title> <script> var k; document.write("Here's a simple counter: "+"<br />"); for (k=0; k<=10; k++) document.write(k+"<br />"); </script> </head> <body> </body> </html>

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**More Count-Controlled Loops**

Document 4.8 (countdown2.htm) <html> <head> <title>Countdown</title> <script> var k; document.write("Start launch sequence!" +"<br />"); for (k=10; k>=0; k--) document.write(k+"<br />"); document.write("FIRE!!"); </script> </head> <body> </body> </html>

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Conditional Loops Pre-test loops – may not be executed at all, depending on initial settings. while ( {logical expression} ) { {statements that result in changing the value of the pre-test logical expression} } Post-test loops – always executed at least once. do { {statements that result in changing the value of the post-test logical expression} } while ( {logical expression} );

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**The Elevator Problem Document 4.9 (gorilla1.htm, partial)**

<script language="javascript" type="text/javascript"> var totalWeight=0.,limitWeight=500.,maxWeight=500.; var newWeight; do { newWeight=Math.floor(Math.random()*(maxWeight+1)); if ((totalWeight + newWeight) <= limitWeight) { totalWeight += newWeight; document.write("New weight = " + newWeight + " total weight = " + totalWeight + "<br />"); newWeight=0.; } else document.write("You weigh " + newWeight " lb. I'm sorry, but you can't get on."); } while ((totalWeight + newWeight) <= limitWeight); </script>

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**Newton's Square Root Algorithm**

Given a number n: 1. Make a guess for the square root of n. n/2 is a reasonable guess. 2. Replace g with (g + n/g)/2. 3. Repeat step 2 until the absolute difference between g*g and n is smaller than some specified value. Document 4.10 (newtonSqrt2.htm) <html> <head> <title>Newton's square root algorithm</title> <script language="javascript" type="text/javascript"> var n=parseFloat(prompt("Enter a positive number:")); var g=n/2; do { g = (g + n/g)/2.; } while (Math.abs(g*g-n) > 1e-5); alert(g+" is the square root of "+n+"."); </script> </head> <body> </body> </html>

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**Using JavaScript to Change Form Field Values**

The basic use for JavaScript is to access and change the values in <input /> elements used in forms "Event handlers" allow us to do this from within forms.

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Pressure Calculation <form> Fill in elevation and sea-level pressure: <input type="text" name="elevation" value="0" size="8" maxlength="7" /> (m) <input type="text" name="sea_level_pressure" value=" " size="8" maxlength="7" /> (mbar) <br /> <input type="button" name="Calculate" value="Click here to get station pressure:" onclick="result.value= parseFloat(sea_level_pressure.value)- parseFloat(elevation.value)/9.2;" /> <input type="text" name="result" value=" " size="8" maxlength="7" /> (mbar)<br /> <input type="reset" value="Reset all fields." /> </form>

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**Rectangular Rule Integration**

How do we do it?

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**Rectangular Rule Integration**

Use pseudocode to develop the algorithm: INITIALIZE integral = 0 (Initialize the value to 0.) LOOP for i = 0 to n – 1, x = x0 + i•dx + dx/2 y=x•x (This could be any function of x.) integral = integral + y END LOOP ASSIGN integral = integral•dx

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The code: Document 4.13 <html><head><title></title></head> <body> <h2>Rectangular Rule integration</h2> for f(x)=x<sup>2</sup> <form> x<sub>0</sub>: <input type="text" name="x0" value="1" /> <br /> x<sub>1</sub>: <input type="text" name="x1" value="3" /> <br /> <input type="button" value="Click here to integrate." onclick="var x,X0,X1,i,n=20,integral=0,y; //y=x*x X1=parseFloat(x1.value); X0=parseFloat(x0.value); dx=(X1-X0)/n; for(i=0; i<n; i++) { x=X0 + i*dx + dx/2; y=x*x; integral+=y; } result.value=integral*dx; "/> <input type="text" name="result" value="result" /></br /> </form></body></html>

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