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CSC 1051 – Algorithms and Data Structures I. Algorithms. Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: www.csc.villanova.edu/~map/1051 /f13/
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CSC 1051 – Algorithms and Data Structures I 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 M.A. Papalaskari, Villanova University
Algorithms in everyday life Source: http://xkcd.com/627/ CSC 1051 M.A. Papalaskari, Villanova University
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: • Input qp • Input credits • gpa = qp / credits • Print gpa CSC 1051 M.A. Papalaskari, Villanova University
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.htmland 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
//*************************************************************//************************************************************* // GPA.java Author: Joyce/Papalaskari// Demonstrates the use of Scanner input and simple computation.//*************************************************************importjava.util.Scanner;publicclass GPA{publicstaticvoid main (String[] args)//------------------------------------------------------------// Inputs the quality points and credits and calculates GPA.//------------------------------------------------------------{doubleqp, credits, gpa; Scanner scan = new Scanner(System.in);// get inputSystem.out.print ("Enter Quality Points > ");qp = scan.nextInt();System.out.print ("Enter Credits > "); credits = scan.nextInt();// output information enteredSystem.out.println ("\nQuality Points: " + qp);System.out.println ("Credits: " + credits);// calculate and output GPAgpa = qp / credits;System.out.println ("\n\tGPA: " + gpa); }} Java Program • variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • gpa = qp / credits • Print gpa CSC 1051 M.A. Papalaskari, Villanova University
Pseudocode: a way to describe what an algorithm does without writing a program. • variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • gpa = qp / credits • Print gpa CSC 1051 M.A. Papalaskari, Villanova University
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 • Algorithm: • Input qp • Input credits • gpa = qp / credits • Print gpa CSC 1051 M.A. Papalaskari, Villanova University
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: • Input qp • Input credits • gpa = qp / credits (Note: use floating point division) • Print gpa When the type is not obvious you can add a note. CSC 1051 M.A. Papalaskari, Villanova University
Another example: PP 2.8 (textbook, Chapter 2 Programming projects) 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.) CSC 1051 M.A. Papalaskari, Villanova University
Can we reverse this calculation? PP 2.9 (textbook, Chapter 2 Programming projects) 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. CSC 1051 M.A. Papalaskari, Villanova University
Algorithm for PP 2.9 CSC 1051 M.A. Papalaskari, Villanova University
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
//*************************************************************//************************************************************* // GPA.java Author: Joyce/Papalaskari// Demonstrates the use of Scanner input and simple computation.//*************************************************************importjava.util.Scanner;publicclass GPA{publicstaticvoid main (String[] args)//------------------------------------------------------------// Inputs the quality points and credits and calculates GPA.//------------------------------------------------------------{doubleqp, credits, gpa; Scanner scan = new Scanner(System.in);// get inputSystem.out.print ("Enter Quality Points > ");qp = scan.nextInt();System.out.print ("Enter Credits > "); credits = scan.nextInt();// output information enteredSystem.out.println ("\nQuality Points: " + qp);System.out.println ("Credits: " + credits);// calculate and output GPAgpa = qp / credits;System.out.println ("\n\tGPA: " + gpa); }} Previous Example Java Program • variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • gpa = qp / credits • Print gpa What if credits = 0 ???? CSC 1051 M.A. Papalaskari, Villanova University
Updated Algorithm • variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • if credits = 0 • Print “No gpa yet” • else • gpa = qp / credits • Print gpa • Printgpagoodbye message CSC 1051 M.A. Papalaskari, Villanova University
variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • if credits = 0 • Print “No gpa yet” • else • gpa = qp / credits • Print gpa • Printgpagoodbye 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.//*************************************************************importjava.util.Scanner;publicclassGPA_Updated{publicstaticvoid main (String[] args)//----------------------------------------------------------// Reads the quality points and credits and calculates GPA.//---------------------------------------------------------- {doubleqp, credits, gpa; Scanner scan = new Scanner(System.in);// get inputSystem.out.print ("Enter Quality Points > ");qp = scan.nextInt();System.out.print ("Enter Credits > "); credits = scan.nextInt();// output information enteredSystem.out.println ("\nQuality Points: " + qp);System.out.println ("Credits: " + credits);// calculate and output GPA, if possibleif (credits == 0)System.out.println ("\n\tGPA: None");else {gpa = qp / credits;System.out.println ("\n\tGPA: " + gpa); }// Print goodbye messageSystem.out.println ("Goodbye and thank you for using my GPA calculator."); }} Updated program • variables: qp, credits, gpa • Algorithm: • Input qp • Input credits • if credits = 0 • Print “No gpa yet” • else • gpa = qp / credits • Print gpa • Printgoodbye message CSC 1051 M.A. Papalaskari, Villanova University
Conditional statementsalter 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 CSC 1051 M.A. Papalaskari, Villanova University
Control flow • Sequence of statements that are actually executed in a program • Conditional and Repetition statements: enable us to alter control flow true statement 1 false statement 1 statement 2 boolean 1 boolean 2 true statement 3 statement 2 false statement 4 statement 3 This slide dapted from Doug Clark’s course http://www.cs.princeton.edu/courses/archive/spring13/cos126/lectures.php CSC 1051 M.A. Papalaskari, Villanova University
Java relational operators • relational operators can be used with numeric types and producebooleanresults: ==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 wordstrueandfalseare the only valid values for a boolean type • Example booleandeclarations: booleanaboveAgeLimit = false; booleanusePlural = hours > 1; A boolean expression using a relational operator CSC 1051 M.A. Papalaskari, Villanova University
Example • An ifstatement with its boolean condition: if(hours !=1) System.out.print("s"); • Another way, using a boolean variable: booleanusePlural = hours !=1; if (usePlural) System.out.print("s"); • See alsoAge.java CSC 1051 M.A. Papalaskari, Villanova University