CSC 1051 Algorithms and Data Structures I. Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University

Transcription

1 Algorithms CSC 1051 Algorithms and Data Structures I 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.

2 Algorithms in everyday life Source:

3 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 qp 2. Input credits 3. gpa = qp / credits 4. Print gpa

4 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: and Wikipedia ( )

5 Java Program è // GPA.java Author: Joyce/Papalaskari // Demonstrates the use of Scanner input and s //********************************************* import java.util.scanner; public class GPA { public static void main (String[] args) // // Inputs the quality points and credits an // { double qp, credits, gpa; variables: qp, credits, gpa Algorithm: 1. Input qp 2. Input credits 3. gpa = qp / credits 4. Print 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: " System.out.println ("Credits: " + // calculate and output GPA gpa = qp / credits; System.out.println ("\n\tgpa: " + gpa);

6 Pseudocode: a way to describe what an algorithm does without writing a program. variables: qp, credits, gpa Algorithm: 1. Input qp 2. Input credits 3. gpa = qp / credits 4. Print gpa

7 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: 1. Input qp 2. Input credits 3. gpa = qp / credits 4. Print gpa

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

9 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.)

10 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.

11 Algorithm for PP 2.9

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

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

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

15 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

16 Previous Example Java Program è // GPA.java Author: Joyce/Papalaskari // Demonstrates the use of Scanner input and s //********************************************* import java.util.scanner; variables: qp, credits, gpa Algorithm: public class GPA { public static void main (String[] args) // // Inputs the quality points and credits an // { double qp, credits, gpa; Scanner scan = new Scanner(System.in); 1. Input qp 2. Input credits 3. gpa = qp / credits 4. Print gpa // 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: " System.out.println ("Credits: " + What if credits = 0???? } } // calculate and output GPA gpa = qp / credits; System.out.println ("\n\tgpa: " + gpa);

17 variables: qp, credits, gpa Algorithm: Updated 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

18 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 if (credits == 0) else { } Java code System.out.println ( \n\tgpa: gpa = qp / credits; None"); System.out.println ( \n\tgpa: " + gpa);

19 Updated program è 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 import java.util.scanner; public class GPA_Updated { public static void main (String[] args) // // Reads the quality points and credits and calcula // { 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 us

20 Conditional statements alter the linear flow of control. They use boolean expressions to determine what to do next. Example: A boolean expression if (credits == 0) System.out.println ("GPA: None"); else { } gpa = qp / credits; System.out.println ("\n\tgpa: " + gpa);

21 Control flow Sequence of statements that are actually executed in a program Conditional and Repetition statements: enable us to alter control flow statement 1 true boolean 1 false statement 2 statement 1 statement 3 boolean 2 true statement 2 statement 4 false statement 3 This slide dapted from Doug Clark s course

22 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 (=)

23 Boolean Expressions The reserved words true and false are the only valid values for a boolean type Example boolean declarations: boolean aboveagelimit = false; boolean useplural = hours > 1; A boolean expression using a relational operator

24 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) See also Age.java System.out.print("s");