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
Source: http://xkcd.com/627/
CSC 1051 M.A. Papalaskari, Villanova University
Algorithms in everyday life
Algorithm Example
Statement of GPA problem:
Write a program that inputs the credits and quality points earned and outputs the gpa.
CSC 1051 M.A. Papalaskari, Villanova University
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 )
CSC 1051 M.A. Papalaskari, Villanova University
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); }}
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. gpa = qp / credits4. Print gpa
CSC 1051 M.A. Papalaskari, Villanova University
Pseudocode: a way to describe what an algorithm does without writing a program.
CSC 1051 M.A. Papalaskari, Villanova University
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. gpa = qp / credits4. 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.
CSC 1051 M.A. Papalaskari, Villanova University
variables: qp, credits, gpa
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.
CSC 1051 M.A. Papalaskari, Villanova University
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:
CSC 1051 M.A. Papalaskari, Villanova University
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?
CSC 1051 M.A. Papalaskari, Villanova University
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
CSC 1051 M.A. Papalaskari, Villanova University
How many of each can you pack in the black box?
How many of each can you pack in the black box?
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
CSC 1051 M.A. Papalaskari, Villanova University
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); }}
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. gpa = qp / credits4. Print gpa
CSC 1051 M.A. Papalaskari, Villanova University
What if credits = 0 ????
Previous Example
Updated Algorithm
CSC 1051 M.A. Papalaskari, Villanova University
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. if credits = 0
• Print “No gpa yet”else
• gpa = qp / credits• Print gpa
4. Print gpa goodbye message
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. if credits = 0
• Print “No gpa yet”else
• gpa = qp / credits• Print gpa
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);
}
CSC 1051 M.A. Papalaskari, Villanova University
//*************************************************************// 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);
// 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, 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."); }}
CSC 1051 M.A. Papalaskari, Villanova University
Updated program
variables: qp, credits, gpa
Algorithm:
1. Input qp2. Input credits3. if credits = 0
• Print “No gpa yet”else
• gpa = qp / credits• Print gpa
4. Print goodbye message
• 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); }
CSC 1051 M.A. Papalaskari, Villanova University
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
CSC 1051 M.A. Papalaskari, Villanova University
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 (=)
CSC 1051 M.A. Papalaskari, Villanova University
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
CSC 1051 M.A. Papalaskari, Villanova University
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
CSC 1051 M.A. Papalaskari, Villanova University