25 October

Conditionals and Loops

Presentations

Brendan: Cyberwarfare Casey: Online shopping (No current event today)

Announcements

Papers: Good batch of papers Feel free to talk to me about any

comments – content or style “Hygiene” – staple, title, page

numbers, proof

Documenting Your Code

Documenting your code is not necessary for the compiler or interpreter, but is typically needed by people – including yourself!

ALWAYS document what you write Information for people only are

comments

Comments Two Forms

/* … */ //

/* … */ Everything between the delimiters is a

comment and not processed Can cross multiple lines Can start anywhere

// Everything from there until the end of the

line is not processed

Comment Examples

var start = 0; // beginning of the string

var start = 0; /* identifies the place start processing */

var start = 0; /* beginning of the stringvar end = 0;

What to do with Variables

Simplest action is to assign a value: assignment statement Name = “Tom”; Price = 13.25;

Can use any type of expression on the right – both constants and variables

Expressions: Arithmetic There are rules of precedence

1+b*c Do I multiply or add first? If you use parentheses, you don’t have

to remember! Binary operations

+, - , /, *, % (modulus or remainder) Unary operations

Negation

Expressions: String

Primary one is concatenation Uses “+”

If with either entry is a string, means concatenate

Else means add

Expressions: Boolean

Comparisons Numeric or string >, <, == Not and combinations

Logical operations And, or, not

Logic Tables 0 = false, 1 = true And

0 && 0 = 0 0 && 1 = 0 1 && 0 = 0 1 && 1 = 1

Or 0 || 0 = 0 0 || 1 = 1 1 || 0 = 1 1 || 1 = 1

Not !0 = 1 !1 = 0

There are actually 8 different logical operations on two variables?

How would you convince yourself of that?

Practice

Write a statement that computes total price including tax

and shipping computes the area of a circle calculates the average speed of a car

trip

How to Change the Operations Conditional statements Iterations (doing it more than once) Functions (an algorithm that can be

executed more than once)

All need compound statements – multiple statements that can be processed as one { stmt; stmt; stmt; }

Conditional Statements Executes the statement only if a condition is

true Format: if (boolean) statement;

/* avoid divide by zero */if (n != 0)

m = m/n;

/* if purchase made, increase total purchase price and the number of items bought */if (purchased = true) {

total = total + price;items = items + quantity;};

Conditional with else Executes the first statement if a

condition is true or the second if it’s false Format: if (boolean) statement; else

statement;

/* take the absolute value */if (n < 0)

abs_val = !n;else

abs_val = n;

Conditional with else (cont.)

/* execute either add or subtract depending on button and keep track of how many of each operation is performed */

if (operation = add) {result = value1 + value2;add_ops = add_ops + 1;};

else { // must be subtractresult = value1 - value2;sub_ops = sub_ops + 1;};

Practice Write a statement that

computes total price including tax if the purchaser is in state and shipping in all cases

computes the area of a circle or a square depending on whether the radius or side was given

calculates the average speed of a car trip if the length of the trip was more than one hour

Arrays

Arrays are variables with dimensions Why are they interesting?

Dealing with a table of values Translations Series of different values

Processing a stretch of elements Strings Series of numbers

Arrays in JavaScript Declaration

var name = new Array (no-elems); Examples

var ZodiacSigns = new Array (12); Or can initialize

Var HalfZodiacSigns = new Array (“Aries”, “Taurus”, “Gemini”, “Cancer”, “Leo”, “Virgo”);

Referencing Arrays Array element can be referenced any

place that a constant or variable can be used

Form: array [ index ] Array is the name Index is which element of the array you want

Can be a constant, variable, or expression Must evaluate to an integer between 0 and one less

than the array size Example

MySign = HalfZodiacSigns[0]; What value does MySign have?

Iterations: Doing it more than once If I want to process all of the elements in

an array, I need to tell the computer to do something for each element:

Form that we will usefor ( itervar = 0; itervar < endval ; itervar++) {

Statement-list}

var++ is a shorthand that is equivalent to var = var + 1;

What it does

for ( i = 0 ; i < n ; i++) {Statement-list}

1. i = 0;2. execute statement-list3. i = i + 1;4. if i < n go to #2

Iteration Example

var numbers = new Array (10);var j;/* initialize an array to 1 to 10 */for ( j=0; j<10; j++) { numbers[j] = j+1; }

Practice

Write a loop that computes total price including tax

and shipping for 3 orders computes the area of a “n” circles calculate the average speed of 2 legs

of a car trip

Loops with conditionals

var values = new Array (10);var results = new Array (10);var j;for ( j=0; j<10; j++) { if (values[j] < 0)

results[j] = -j; else

results[j] = j; }