ucd computer science comp-2001 1 comp-2001 data structures & algorithms i (then in january:...

1UCD Computer Science COMP-2001

COMP-2001

• Data structures & algorithms I (then in January: COMP-2006: DS&A II)

• Eleni Mangina / [email protected]

• Course web: http://www.cs.ucd.ie/staff/emangina/COMP2001

Lectures 1 & 2


PracticalsSessions as assigned; attendance mandatory10 practicals (nearly 1 per week)

Each will probably require:2-4 hours prior to your session1.5 hours in lab2 hours after session

Several problems each, lifted directly from text-bookProgramming requires practice!30% of total course mark!Examination will require you understand the practicals!

1 week late? Maximum of 50% > 1 week late? No marks earned


Practicals

Don’t cheat

Cooperation and group studying are encouraged.But it’s easy to detect, andCheating will be dealt with according to theDepartment’s harsh plagiarism policy.


Course outline• Java refresher (1 week)• Object-oriented and modular design (1 week)• Mathematical tools for analyzing programs (1 week)• Remaining 9 weeks on a variety of useful topics

– Stacks– Queues– Lists– Vectors– Trees– Priority queues– Heaps

• Topics relevant to all programming languages 2001 is not just a course on “more Java”

A useful “bag of tools”that forms the foundationof all large software projects


Textbook• Goodrich & Tomassia -- Buy it!

Many useful resources at book’s web site.

• You Must read assigned sections before corresponding lecture!– Otherwise you’ll be lost


Java refresher• Forget Java over the summer? Try the COMP-2001

notes, or any of the excellent books or Web tutorials (see course Web for pointers)

• For Thursday: Read G&T Chapter 1• Start Practical #1 (due Friday 29/9 @ 5pm)

• Some of the following details may be bewildering or overly complicated at first -- come back to these notes or resources later when you understand what’s going on


Basic Concepts• Classes: a conceptual “box” that holds data (“instance

variables”) and methods that can be invoked in the data.• Objects: instances of a given class• Types: classes + primitive types (int, float, ...) +

built-in data structures (array)• Methods: a.k.a. “functions”, “subroutines”• Access modifiers: public, private, static, …• Expressions: create new values from combinations of

existing values• Control flow: if, for, return, ...• Packages: how classes are organized


Classesclass Fish {

int nfins; // instanceString name; // variables

Fish(int _nfins, String _name) { // constructornfins = _nfins; // methodname = _name;

}

void morefins(int delta) { // “regular” method

nfins += delta;}

}


A complete program• A Java program doesn’t “do anything” unless is

contains a special main method:

class Fish {int nfins;String name;Fish(int _nfins, String _name) {

nfins = _nfins;name = _name;

}void morefins(int delta) {

nfins += delta;}public static void main(String args[]) {

Fish f = new Fish(3, "Fred"); // object creationf.morefins(4); // method invocationSystem.out.println("Fred has " + f.nfins + "fins");

}}


Files, packages, classes• A single Java application typically comprises many

classes, grouped into several packages, much of which might be shared with other applications:

Payroll Program Human Resources Program

package A

package B

package C package DC1

C2

C4

C3

C6

C7

C5

C9

C10

C8


Files, packages, classes - continued• There must be a 1-to-1 correspondence between:

Classes - Java source files Packages - directories comtaining the class files

/app/acounting/packageA/C1.java /C2.java /packageB/C3.java /C4.java /sharedstuff/packageC/C5.java /C6.java /C7.java /human-res/packageD/C8.java /C9.java /C10.java


Creating objects and valuestype variablename = expression

int nfingers = 4;double weight; // very occasionally, initial value inappropriate/unnessaryboolean defective = weight<0; // initial value can be any expression

Integer ntoes = new Integer(4); // primitive data types aren’t objects

Boolean problem = new Boolean(defective || (nfingers<0);

String name = "Bob";

Fish f = new Fish(3, "Bob");


Simple Input/Output• Java has an incredibly complicated hierarchy of classes for

performing input/output. • Output

System.out.println(f.name + " has " + f.nfins + " fins");

• Inputimport java.io.*; // enable access to all Java I/O classes

…

BufferedReader stdin =

new BufferedReader(new InputStreamReader(System.in));

String line = stdin.readLine(); // read 1 line of text

Next step depends on what data type the user is expected to enter. For example…int xnfins = Integer.valueOf(line).intValue(); // expecting an integer

… or … float weight = Float.valueOf(line).floatValue(); // expecting a float


Exceptions• Any number of exceptional circumstances may

arise during program execution that cause troubleimport java.io.*;class IOExample { public static void main(String[] args) { try { BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in)); String line; System.out.print("Enter the number of fins > "); // wait til user enters something other than just RETURN while ((line=stdin.readLine()) == null) ; int nfins = Integer.valueOf(line).intValue(); } catch (IOException x) { // oops -- something strange went wrong System.out.println("Is your keyboard OK?! -- " + x); } catch (NumberFormatException x) { // entered something like “frog” System.out.println("Integers only! -- " + x); } }}


Control flowif (nfins > 10) { // execute block only if condition holds

System.out.println(“Are you sure??!??!”);}

int a = 10, b = 20;while (a>0) { // repeatedly execute body until a=0

b = b+a;if (b > 60) break; // immediately terminate while loopa = a-1;

}// now a=5 and b=65for (int i = 0; i < b; i++) { // set i=0, 1, 2, …, 64

a++; // shorthand for “a = a+1”}// now a=70 (and b is still 65)


Expressions• float appendageCost = (nfins + ntoes) * costPerAppendage;

• int remainder = numerator % divisor; // eg 5%2 = 1

• boolean probablySick = (whiteCellCount > 145);

• x = ++j; // increment j, and set x to the new value• y = j++; // increment j, and set y to the current value

• String fullname = firstname + " " + surname;

• double w = 45; // set w to the real number 45.0• int x = 45; // set x to the integer 45• double y = w/10; // set y = 4.5• double z = x/10; // set z = 4.0 not 4.5 !!??!!• double z = ((double)x)/10; // set z = 4.5

• int q = 3 + 4 * 5; // sets q to 23, not 60 !!?!?


Arrays• Java has one built-in “primitive” data structure

• When array is created, its capacity must be specified (with any integer expression!) and then can’t change

System.out.println("Enter the number of cities >");

int ncities = Integer.valueOf(stdin.readLine()).intValue();

int rainfall[ncities];

String name[ncities];

for (int i = 0; i < cities.length; i++) {

System.out.print("City " + i + " name > ");

name[i] = stdin.readLine();

System.out.print("City " + i + " rainfall > ");

rainfall[i] = Integer.valueOf(stdin.readLine()).intValue();

}

(danger -- no exception handling for simplicity)


Defining methodsclass Rectangle {

int height, width;

int area() {

return height*width;

}

void wider(int delta) {

width += delta;

}

void taller(int delta) {

height += delta;

}

RETURN-TYPE NAME( ARG1, ARG2, … ) {

BODY

}

}


The Dereference (“Dot”) Operator• If X is an an object of some class C that contains

an instance variable Y, then X.Y accesses that value of Y in object X

• Methods work exactly the same way

• stdin.readLine().toLowerCase().indexOf(“dog”)

the next string entered at the keyboard

the keyboard standard-input object

the position in that string of the first occurrence of “dog”

the string formed by replacing all UPPER case letters with lower


Sample practical problem• R1.13 - write a Java function that takes an integer n and

returns the sum of the odd integers smaller than n.

class R113 {static int oddsum(int x) {

…return …;

}}

Assumption: presumably the sum of all positive ints x !

We need to loop over all odd integers less than x

We need a variable to store the sum


R113 - continuedint sum = 0;

for (int i = 1; i<=x; i+=2) {… // i=1,3,5,…

}

Before proceeding… does this loop stop at the correct value of i??


R113 - continuedint sum = 0;

for (int i = 1; i<x; i+=2) {sum += i;

}// now sum = sum of all odd integers less than x

Does this handle negative values of x properly?Does this handle zero properly?Does this handle one properly?

Hint: what is the minimum number of timesany “for” loop will execute!


R113 - putting it all togetherclass R113 { static int oddsum(int x) { if (x<2) return 0; // special case! int sum = 0; for (int i=1; i<x; i+=2) { sum += i; } return sum; } public static void main(String[] args) { int i = 6; // “regular” input int osi = oddsum(i); System.out.println("got " + osi + " should be 9"); int j = -10; // problem with negative inputs? int osj = oddsum(j); System.out.println("got " + osj + " should be 0"); int k = 5; // problem with odd inputs? int osk = oddsum(k); System.out.println("got " + osk + " should be 4"); }}

ucd computer science comp-2001 1 comp-2001 data structures & algorithms i (then in january:...

Documents

ucd computer science

nfins method

int nfins string

nfins fins

constructor nfins

organized slide

regular method nfins

int nfins instance string