CSC 1051 – Algorithms and Data Structures I

CSC 1051 M.A. Papalaskari, Villanova University

Algorithms

Dr. Mary-Angela Papalaskari

Department of Computing Sciences

Villanova University

Course website:

www.csc.villanova.edu/~map/1051/f13/

Some slides in this presentation are adapted from the slides accompanying Java Software Solutions by Lewis & Loftus or from Dr. Daniel Joyce’s slides for this course.

CSC 1051 – Algorithms and Data Structures I

http://www.csc.villanova.edu/~map/1051/f13/

http://www.csc.villanova.edu/~map/1051/f13/

Source: http://xkcd.com/627/


Algorithms in everyday life

http://xkcd.com/627/



Algorithm Example

Statement of GPA problem:

Write a program that inputs the credits and quality points earned and outputs the gpa.


variables: qp, credits, gpa

Algorithm:

1. Input qp2. Input credits3. gpa = qp / credits4. Print gpa

Algorithms

An algorithm is a specific set of instructions for carrying out a procedure or solving a problem, usually with the requirement that the procedure terminate at some point. Specific algorithms sometimes also go by the name method, procedure, or technique. The word "algorithm" is a distortion of al-Khwārizmī, a Persian mathematician who wrote an influential treatise about algebraic methods.

Sources: http://mathworld.wolfram.com/Algorithm.html and Wikipedia (http://en.wikipedia.org/wiki/Mu%E1%B8%A5ammad_ibn_M%C5%ABs%C4%81_al-Khw%C4%81rizm%C4%AB )


http://mathworld.wolfram.com/Method.html

http://mathworld.wolfram.com/Procedure.html

http://mathworld.wolfram.com/Technique.html

http://mathworld.wolfram.com/Algorithm.html



http://en.wikipedia.org/wiki/Mu%E1%B8%A5ammad_ibn_M%C5%ABs%C4%81_al-Khw%C4%81rizm%C4%AB

http://en.wikipedia.org/wiki/Mu%E1%B8%A5ammad_ibn_M%C5%ABs%C4%81_al-Khw%C4%81rizm%C4%AB

Java Program

//*************************************************************

// GPA.java Author: Joyce/Papalaskari// Demonstrates the use of Scanner input and simple computation.//*************************************************************import java.util.Scanner;

public class GPA{ public static void main (String[] args) //------------------------------------------------------------ // Inputs the quality points and credits and calculates GPA. //------------------------------------------------------------ { double qp, credits, gpa;

Scanner scan = new Scanner(System.in);

// get input System.out.print ("Enter Quality Points > "); qp = scan.nextInt(); System.out.print ("Enter Credits > "); credits = scan.nextInt();

// output information entered System.out.println ("\nQuality Points: " + qp); System.out.println ("Credits: " + credits); // calculate and output GPA gpa = qp / credits; System.out.println ("\n\tGPA: " + gpa); }}


Algorithm:



http://www.csc.villanova.edu/~map/1051/f13/examples/GPA.java

http://www.csc.villanova.edu/~map/1051/f13/examples/GPA.java

Pseudocode: a way to describe what an algorithm does without writing a program.



Algorithm:


Writing an algorithm in pseudocode• List the variables used. • List the steps for

solving the problem, in order.

• Try to be brief and unambiguous; use Java expressions only when it is simpler to specify a step in java than in English.



Algorithm:1. Input qp

2. Input credits

3. gpa = qp / credits

4. Print gpa

Writing an algorithm in pseudocode• List the variables used. • List the steps for

solving the problem, in order.

• Try to be brief and unambiguous; use Java expressions only when it is simpler to specify a step in java than in English.


variables: qp, credits, gpa (use floating point)

Algorithm:1. Input qp

2. Input credits

3. gpa = qp / credits (Note: use floating point division)

4. Print gpa

When the type is not obvious you can add a note.

Another example:


Write an application that reads values representing a time duration in hours, minutes, and seconds and then prints the equivalent total number of seconds. (For example, 1 hour, 28 minutes, and 42 seconds is equivalent to 5322 seconds.)

PP 2.8 (textbook, Chapter 2 Programming projects)

Can we reverse this calculation?


Create a version of the previous project that reverses the computation. That is, read a value representing a number of seconds, then print the equivalent amount of time as a combination of hours, minutes, and seconds. (For example, 9999 seconds is equivalent to 2 hours, 46 minutes, and 39 seconds.)

The next 3 slides will help us visualize this problem.

PP 2.9 (textbook, Chapter 2 Programming projects)

Algorithm for PP 2.9


How many of each can you pack in the black box?



Topic Thread• 2.1 Character Strings• 2.2 Variables, Assignment• 2.3 Data Types, in particular int, double• 2.4 Expressions (simple)• 2.6 Interactive Programs• 5.1 Boolean Expressions• 5.2 The if Statement• 5.5 The while Statement


Java Program

//*************************************************************

// GPA.java Author: Joyce/Papalaskari// Demonstrates the use of Scanner input and simple computation.//*************************************************************import java.util.Scanner;

public class GPA{ public static void main (String[] args) //------------------------------------------------------------ // Inputs the quality points and credits and calculates GPA. //------------------------------------------------------------ { double qp, credits, gpa;

Scanner scan = new Scanner(System.in);


// output information entered System.out.println ("\nQuality Points: " + qp); System.out.println ("Credits: " + credits); // calculate and output GPA gpa = qp / credits; System.out.println ("\n\tGPA: " + gpa); }}


Algorithm:



What if credits = 0 ????

Previous Example

http://www.csc.villanova.edu/~map/1051/examples/GPA.java

Updated Algorithm



Algorithm:

1. Input qp2. Input credits3. if credits = 0

• Print “No gpa yet”else

• gpa = qp / credits• Print gpa

4. Print gpa goodbye message


Algorithm:




4. Print gpa goodbye message

Java code

if (credits == 0)

System.out.println (“\n\tGPA: None");

else

{

gpa = qp / credits;

System.out.println (“\n\tGPA: " + gpa);

}


//*************************************************************// GPA_updated.java Author: Joyce/Papalaskari//// Demonstrates the use of conditional statements.//*************************************************************

import java.util.Scanner;

public class GPA_Updated{ public static void main (String[] args) //---------------------------------------------------------- // Reads the quality points and credits and calculates GPA. //---------------------------------------------------------- { double qp, credits, gpa; Scanner scan = new Scanner(System.in);


// output information entered System.out.println ("\nQuality Points: " + qp); System.out.println ("Credits: " + credits); // calculate and output GPA, if possible if (credits == 0) System.out.println ("\n\tGPA: None"); else { gpa = qp / credits; System.out.println ("\n\tGPA: " + gpa); } // Print goodbye message System.out.println ("Goodbye and thank you for using my GPA calculator."); }}


Updated program


Algorithm:




4. Print goodbye message

http://www.csc.villanova.edu/~map/1051/f13/examples/GPA_updated.java

• Conditional statements alter the linear flow of control. They use boolean expressions to determine what to do next.Example:

if (credits == 0)

System.out.println ("GPA: None");

else { gpa = qp / credits; System.out.println ("\n\tGPA: " + gpa); }


A boolean expression

Control flow• Sequence of statements that are actually executed in a program

• Conditional and Repetition statements: enable us to alter control flow


statement 1

statement 2

statement 3

statement 4

boolean 1

boolean 2

statement 1

statement 2true

true

false

false

statement 3

This slide dapted from Doug Clark’s course http://www.cs.princeton.edu/courses/archive/spring13/cos126/lectures.php

Java relational operators

• relational operators can be used with numeric types and produce boolean results:

== equal to

!= not equal to

< less than

> greater than

<= less than or equal to

>= greater than or equal to

• Note the difference between the equality operator (==) and the assignment operator (=)


Boolean Expressions• The reserved words true and false are the only valid

values for a boolean type

• Example boolean declarations:

A boolean expression using a relational operator


boolean aboveAgeLimit = false;

boolean usePlural = hours > 1;

Example• An if statement with its boolean condition:

if (hours != 1) System.out.print("s");

• Another way, using a boolean variable:

boolean usePlural = hours != 1;

if (usePlural)

System.out.print("s");

• See also Age.java


http://www.csc.villanova.edu/~map/1051/Chap05/Age.java

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CSC 1051 – Algorithms and Data Structures I

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