Notes

Outline

Announcements Put the hardcopy of Project 2a on the table

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 4 var j, A = new Array(5); for (j=0; j<5; i++) { A[i] = 4; } 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