Guest Speaker - Cryptography

Wednesday (Apr. 22) – Prof. abhi shelat

Final Exam Review

CS101: Introduction to Computer Science • Slides adapted from Sedgewick and Wayne • Copyright © 2007 • http://www.cs.Princeton.EDU/IntroCS

Built-In Datatypes

Variables

A variable has three parts:1. Name (e.g., “x”)2. Type (e.g., int, double, String)3. Value

How does a variable get a value?– Can be initialized in the program

E.g., int x = 10– Can be computed when the program is running

E.g., x = y + z;

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Built-in Data Types

Data type. A set of values and operations defined on those values.

add, subtract, multiply, divide

3.14156.022e23

floating point numbers

double

add, subtract, multiply, divide

1712345integersint

and, or, nottruefalsetruth valuesboolean

sequences of characters

characters

set of values operationsliteral valuestype

compare'A''@'

char

String concatenate"Hello World""CS is fun"

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Math Library

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Type Conversion

Type conversion. Convert from one type of data to another. Automatic: no loss of precision; or with strings. Explicit: cast; or method.

Conditionals and Loops

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Control Flow Summary

Control flow. Sequence of statements that are actually executed in a

program. Conditionals and loops: enables us to choreograph the control

flow.

Straight-lineprograms

All statements areexecuted in the order given.

ConditionalsCertain statements are

executed depending on thevalues of certain variables.

ifif-else

LoopsCertain statements are

executed repeatedly untilcertain conditions are met.

whilefor

do-while

Control Flow Description Examples

Nesting Conditionals and Loops

Nest conditionals within conditionals.

Nest loops within loops. Nest conditionals within loops

within loops.

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Monte Carlo Simulation

General Code Structure:

class MonteCarloSimulation {

for(i=0; i<num_trials; i++) {

Perform one independent trial {

Generate random input;Deterministic computation on that input;

}

}Compute results aggregated over all trials;

}

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Single-Dimensional Arrays

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Arrays in Java

Java has special language support for arrays. To make an array: declare, create, and initialize it. To access element i of array named a, use a[i]. Array indices start at 0.

Compact alternative. Declare, create, and initialize in one statement. Default initialization: all numbers automatically set to zero.

int N = 10; double[] a; // declare the arraya = new double[N]; // create the arrayfor (int i = 0; i < N; i++) // initialize the array a[i] = 0.0; // all to 0.0

int N = 10;double[] a = new double[N]; // declare, create, init

Array Layout in Memory

Memory is split into equal sized units known as “addresses”. (Address

Space)

Arrays occupy consecutive addresses in this linear space:

int[] a = {5, 6, 10};

Key Point: Name of the array indirectly references its starting address.

int[] b = new int[3]; b=a;

//b is an alias to a. REFERENCE TYPE

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0 1 2 . . . N-2 N-1

0 1 2 3 4 5 N-2 N-1 a[0] a[1] a[2] 5 6 10

Addresses

Array Length

The array length can be obtained using the .length operator

for(i=0; i< myArray.length; i++)

The array length cannot be changed after memory allocation

Can use arraylists to get around this limitation

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Auto-increment

Auto-increment and auto-decrement operators used with arrays. ++i means “add one to i’s value and use the updated value” i++ means “use i’s value now and then add one to the old

value” Can use on line by itself as a single statement

– In which case they do exactly the same thing! Differences between the auto-increment operators are

important when used in expressions: y = 0; x = y++; y = 0; z = ++y;

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Y=0; x = y; y = y+1;Y=0; y = y+1; z = y;

Multidimensional Arrays

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Two Dimensional Arrays in Java

Array access. Use a[i][j] to access element in row i and column j.

Zero-based indexing. Row and column indices start at 0.

double[][] a = new double[10][3];for (int i = 0; i < 10; i++) { for (int j = 0; j < 3; j++) { a[i][j] = 0.0; }}

Declaring and Initializing a 10-by-3 Array

Memory Layout 2-D Arrays

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Row

array[0]

array[1]

array[2]

array[3]

array[0][0]

array[1][2]

Ragged Arrays

Row lengths can be non-uniform Can use .length

int[][] a = ...;

for (int rows=0; rows < a.length; rows++) {for (int cols=0; cols < a[rows].length; cols++)

System.out.print(" " + a[rows][cols]);System.out.println("");

}

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Number of rows

Number of columns for a given row

Functions/Static Methods

Components of a Function

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Flow Control – Call by Value

Call by reference – For reference types (arrays, objects)

Things to Remember About Functions

Functions can be overloaded Different argument types Different number of arguments Different return value is NOT overloading

Scoping Rules for functions and conditional and loop code-blocks

Recursion

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Recursion

What is recursion? When one function calls itself directly or indirectly.

Gcd. Find largest integer d that evenly divides into p and q.

base casereduction step

public static int gcd(int p, int q) { if (q == 0) return p; else return gcd(q, p % q);}

base case

reduction step,converges to base case

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How-To’s on Writing Recursive Functions

Base Case: You must check if we’ve reached the

base case before doing another level of recursion!

Make Progress Towards Base Case: Your recursive calls must be on a smaller

or simpler input. Eventually this must reach the base

case (and not miss it).

Multiple recursive calls: Sometimes more than one recursive call.

– H-Tree, Towers of Hanoi Are their return values chosen,

combined?

Objects

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Objects

Object. An entity that can take on a data type value.

An object’s “value” can be returned to a client or be changed by one of the data type’s operations.

length, substring, comparesequence of charactersString

OperationsSet of ValuesData Type

get red component, brighten24 bitsColor

Picture get/set color of pixel (i, j)2D array of colors

Things to Remember About Objects

Class API Constructor Instance Methods

Difference between constructors and methods

Assigning one object/array to another object/array results in an alias

Cannot use == operator to check the equality of objects

Use instance method equals(). Can define this method for any class.

Mutable vs. Immutable Objects Immutable – Can’t change the value of an

object after its created Aliases to immutable objects are less

problematic

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Objects are Reference Types

Principles of Object Oriented Programming

Encapsulation: Combine data and the functions that operate on that data into a single unit (object)

Data Hiding: Clients should not be able to manipulate data in objects directly

Declare instance variables to be “private”– Use getter methods to read data within objects

Any object should be able to invoke the instance methods– Declare instance methods to be “public”

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Summary of Classes

(“Charge” Class Discussed in the Textbook)

Data Structures

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Data Structures

Lists A collection of a variable number of items

Typical Operations on Lists Add an item to the list Remove an item from the list Read an item from the list Check whether the list is empty Get the current size of the list

All of these are provided via the Java ArrayList class

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Generics. Parameterize the datatype used in the data structure.

You need to import the library: import java.util.ArrayList;

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Generics

ArrayList<Apple> list= new ArrayList<Apple>();Apple a = new Apple();Orange b = new Orange();list.add(a);list.add(b); // compile-time errora = list.get(0); // returns an Apple object

sample client

3737

Autoboxing

Generic ArrayList implementation. Only permits reference types.

Wrapper type. Each primitive type has a wrapper reference type. Ex: Integer is wrapper type for int.

Autoboxing. Automatic cast from primitive type to wrapper type.Autounboxing. Automatic cast from wrapper type to primitive type.

ArrayList<Integer> list= new ArrayList<Integer>();list.add(17); // autobox (int -> Integer)int a = list.get(i); // autounbox (Integer -> int)

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Lists Can Be Organized in Different Ways

Linked List Linear sequence of elements

Queue Remove the item least recently added. First-In, First-Out (FIFO)

Stack Remove the item most recently added. Last-In, First-Out (LIFO)

Array Implementation vs. ArrayList Implementation

Guest Speaker - Cryptography

Wednesday (Apr. 22) – Prof. abhi shelat