Loops - Monday, Week 4  What are loops?  While loops  Classic loop example - the for loop  Parts of a loop  Examples

What are loops?  Loops are the way we can do something multiple times  Sometimes it’s the same thing repeated, other times we want to change something slightly each repetition.  A loop will repeat a set of statements over and over until a stop condition is met.

While Loops  A while loop repeats a set of statements while a logical expression is true.  For example, we could repeat asking for a big number as long the number given was less than 10.

While Loops  Syntax: while (logical expression) { some statements }  Example: while (myInt < 10) { ask for another integer; }

While Loop Structure Group of Statements A while ( logical expression ) {.. Group of Statements B. } Group of Statements C

Parts of a Loop  Initialize some variables  Loop while some condition is true.  Change some part of the loop conditions so the loop will stop.

Parts of a Loop  In general a for loop makes the parts of a loop explicit.  for (init; test; step) { statements; }  for (i=0; i<11; i++) { document.writeln(“i = “+i+” ”); }

Classic Loop Example  Frequently we want to do something to all items in an array. This requires being able to generate the numbers from 0 to n.  A for loop is an easy way to do this.  Print out the numbers from 0 to 10: for (i=0; i<11; i++) { document.writeln(“i = “+i+” ”); }

Classic Loop Example  Looping through the elements of an array can be done easily with a for loop.  for (var i=0; i < StudentArray.length; i++) { document.writeln (“Name: “ + StudentArray[i].name + “ phone: “ + StudentArray[i].phone + “ ”); }

Examples Placing images on your web page var i; for ( i = 0; i < colors.length; i++ ) { // write document.write('<IMG SRC="'); document.writeln(colors[i]+'ball.gif">'); }

Examples The following for loop and while loops are equivalent: for (i=0; i<11; i++) { document.writeln(“i = “+i+” ”); } i=0; while (i < 11) { document.writeln(“i = “+i+” ”); i++; } What happens in the while loop if you forget the i++?

MiniLab #5  Now we can show our entire array of records using a loop! Then we will put the data into a table format.