Announcements

	Project 2A due Wednesday

Once Is Not Enough

	Repeating instructions is the source of great power in computing

Iteration

	“Iteration” is another term for “repeat”
			Iteration doesn’t suffer from the question of whether the first item is counted … in iteration it always is.  (Use “repeat” and “iterate” interchangeably unless it matters.)
			Iterating is usually called “looping” in programming
			Programming languages have many kinds of statements to help program loops
			In JS we will use the for-statement

Anatomy of for

	The for-statement syntax
	for ( <initialize>; <continue test>; <next iteration> ) {
			<statement list>
		}
			<initialize> -- gives iteration variable its first value
			<continue test> -- this test is performed before starting each cycle of loop; if false, quit
			<next iteration> -- the change to the iteration variable after each cycle

An Iteration

	Iterations can count ...

Iterations Control  Actions

	Iterations can replicate other things...

Key Points of Loops

	The most important features of loops:
			The starting value of the iteration variable
			The ending value of the iteration variable
			The amount the iteration variable changes
		As explained in the book, it is possible to completely control these features by properly setting the “control trio,” but
		programmers have gotten in the habit of writing a single kind of iteration: WFI

World Famous Iteration

	To loop n times the WFI has this form
			for ( i=0; i<n;  i++) {
			<statement list>
			}
			Advantages:
			Fast to type
			The number of iterations is the number after <
			0-origin makes it handy for most computations

“Off By 1” Error

	The most common error when working with iterations is to miscount by 1
			Everyone makes this mistake
			A common place where the “off by 1” error matters is in how many times a loop loops
			The importance of the WFI is it tells exactly
			for ( i=0; i<n;  i++) {
			<statement list>
			}

Using Iteration In JS

	Print out a row of things

Doubly Nested Loop

	A loop within a loop repeats repetitions

Arrays and Indexes

	We know about names with multiple instances: Rocky 3, QE 2, John Paul 2
			The number is called the name’s index
			The least index is called the index origin
			In programming, variables that can be indexed are called arrays
			Declare arrays in JavaScript:
			var <identifier> = new Array (<num elements>);
			JavaScript arrays are 0-origin
			Reference array elements w/ brackets: A[0]

Arrays and Loops

	Loops and arrays work together
			Declare an array and initialize elements to 8
			var j, A = new Array(5);
			for (j=0; j<5; j++) {
			A[j] = 8;
			}
			WFI and array’s indices both start at 0
			Notice what would change to have 1000 elements -- arrays and loops give power

Summary

	Iteration is very powerful because a small amount of code specifies a lot of computation
			for gives full range of looping limits, steps
			Use any form of for that works, but using the WFI is a good habit to adopt
			In a doubly nested loop one iteration has another iteration as its <statement list>
			Arrays are variables with many elements that are referred to by their index