CSC 1051 M.A. Papalaskari, Villanova University Algorithms Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: 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: 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 qp 2.Input credits 3.gpa = qp / credits 4.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.methodproceduretechnique Sources: and Wikipedia ( ) 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 qp 2.Input credits 3.gpa = qp / credits 4.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 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 Algorithm: 1.Input qp 1.Input credits 1.gpa = qp / credits 1.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 1.Input credits 1.gpa = qp / credits (Note: use floating point division) 2.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?

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 qp 2.Input credits 3.gpa = qp / credits 4.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 qp 2.Input credits 3.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 qp 2.Input credits 3.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 qp 2.Input credits 3.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 1boolean 2 statement 1 statement 2 true false statement 3 This slide dapted from Doug Clark’s course

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.javaAge.java CSC 1051 M.A. Papalaskari, Villanova University