School of Engineering and Applied Science Department of Computer Science CSci 53 -- Introduction to Software Development  http://www.seas.gwu.edu/~csci53/fall02  Prof. Michael B. Feldman mfeldman@seas.gwu.edu

Normally, lecture notes are not formally distributed in this course. On the other hand, viewgraphs that present material not covered in the course textbook will appear, from time to time, in this file. New notes will simply be added to the top of the file. Watch the date at the top to see if anything has been changed.

Thursday, Nov. 7, 2002

More on Classes and Methods

• An object has state (a set of data values, sometimes called fields or attributes) and behavior (a set of methods). The methods act on the data values, which changes the overall state.
• The data values are usually declared to be private, so that a calling program must call methods to modify them and can't modify them directly.
• The methods are usually declared to be public, so a calling program can use them. Sometimes there are also private methods; why would we want these?

Common kinds of methods

• Constructor: creates and returns object from a class. Usually has a list of parameters, and uses these to set the object's data values. The constructor's name is always that of the class; it has no return statement.
• "Setter": usually has parameters, which it uses to modify one or more of an existing object's data values.
• "Getter": usually has no parameters; it just returns one of the object's data values.
• input: reads an objects data values from the keyboard or from a file.
• toString: collects (some of) an object's data values together into a string, suitable for displaying or writing to a file.

Aggregation

Thursday, Oct. 24, 2002

Robust Input

See programs53/ShowInputLoop.java.

Writing Your Own Classes and Methods

• Sets of reusable classes which provide generally usable methods are the basis for all modern software development, regardless of language or operating system.

 
• There are a number of kinds of methods:

 
• A public method is available to any program that imports the class; a private method is available only to other methods within the class. At this time we'll consider only public ones.

 
• A static method is associated with the class; a non-static method is associated with each object. At this time we'll consider only static ones.

 
• A value-returning method is one that returns a value to the calling program; a void method is one that does not return a value. At this time we'll consider both value-returning and void methods.

See these files in programs53:Screen.java, Smiley.java, MinMax.java, MinMaxThree.Java.
 

Tuesday, Oct. 15, 2002

Objectives

Let's revisit the pizza problem

Find the best value among an arbitrary number of pizzas (not just 3 anymore).

Initial Algorithm for Improved Solution to Pizza Problem
 

1. For each size of pizza, read in the pizza size and price and compute unit cost. Compare the unit cost just computed with the previous unit costs and save the size and price of the pizza whose unit cost is the smallest so far.
2. Display the size and price of the pizza with the smallest unit cost.

Step 1 Refinement

1 Repeat the following steps for each size of pizza:
 1.1. Read in the next pizza size and price.
 1.2. Compute the unit price.
 1.3. If the new unit price is the smallest one so far, then save this pizza’s size, price, and unit price.

Step 1 specifies the repetition of a group of steps: step 1.1 (read), step 1.2 (compute), and step 1.3 (compare). We will repeat these steps as many times as necessary until all unit prices are computed.

Each time we compute a new unit price, step 1.3 compares it to the others, and the current pizza’s size and price are saved if its unit price is smaller than any others computed so far. If the unit price is not the smallest so far, the current pizza’s size and price are not saved.

Step 1.3 is a selection step because it selects between the two possible outcomes: (a) save the pizza’s data and (b) do not save the pizza’s data.

Algorithms: Repetitive Control Structures (loops)

• count-controlled
• sentinel-controlled
• flag-controlled

• count-controlled: before starting the repetition, calculate (or query the user about) how many iterations there will be
• sentintel-controlled: start, and continue, the repetition until a problem-specific value appears in the input data
• flag-controlled: after each iteration, ask the user whether to quit or continue

How could we use each of these in solving the pizza problem?

• set it to 0 before beginning the repetition
• make the first pizza a special case: read its data before beginning the repetition; this is called a priming read

How do we initialize the min and max?
priming read?
set the min to the maximum possible value; set the max to the minimum possible value.
Java provides Integer.MIN_VALUE, Integer.MAX_VALUE, Float.MIN_VALUE,Float.MAX_VALUE, etc.

Thursday, Oct. 10, 2002

Decision ("if") Constructs

• Tuesday: Office hours 2-5:30 PM, CSci 53 7:30-8:45 PM
• Wednesday: Office hours 3-5:30 PM
• Thursday: Office hours 2-5:30 PM, CSci 53 7:30-8:45 PM

Basic Algorithm
 

1. Prompt user to enter a day of the week.
2. Display the schedule items for that day

Step 2 refinement
    if day is Monday, display "work at home"
    if day is Tuesday, display Tuesday's items
    if day is Wednesday, display Wednesday's items
    if day is Thursday, display Thursday's items
    if day is Friday, display "work at home"
    if day is Saturday, display "have a nice weekend"
    if day is Sunday, display "have a nice weekend"

if (day == 1)          // Monday
{
  System.out.println ("Work at home today");
}

if (day == 2)          // Tuesday
{
  System.out.println ("Office hours, 2-5:30 PM");
  System.out.println ("CSci 53, 7:30-8:45 PM");
}

if (day == 3)          // Wednesday
{
  System.out.println ("Office hours, 3-5:30 PM");
}
 

if (day == 4)          // Thursday
{
  System.out.println ("Office hours, 2-5:30 PM");
  System.out.println ("CSci 53, 7:30-8:45 PM");
}

if (day == 5)          // Friday
{
  System.out.println ("Work at home today");
}

if (day == 6)          // Saturday
{
  System.out.println ("Have a nice weekend");
}

if (day == 7)          // Sunday
{
  System.out.println("Have a nice weekend");
}

if ((day == 2) || (day == 4))       // Tuesday, Thursday
{
  System.out.println ("Office hours, 2-5:30 PM");
  System.out.println ("CSci 53, 7:30-8:45 PM");
}

if (day == 3)                       // Wednesday
{
  System.out.println ("Office hours, 3-5:30 PM");
}

if ((day == 6) || (day == 7))       // Saturday, Sunday
{
  System.out.println ("Have a nice weekend");
}

Nested if Statement

Simplifying Nested if Statements

Switch Statements

Thursday, Sept. 26, 2002

Algorithms: the Foundation of Systematic Problem-Solving

QUESTION: Suppose a program is designed not to halt in a finite amount of time? Is it implementing an algorithm?

Algorithms:the Foundation of Systematic Problem-Solving

"The British mathematician Alan Turing proved [1936] that an algorithmic approach can be used to solve any mathematical or logical problem for which a solution is known to exist. This means that one can deduce that any solvable problem can be handled by a computer, provided the correct algorithm is used.

"In computing, an algorithm is the plan, routine, or process of defining a set of instructions so that a computer program can be written to find the answer / solution to a given problem or task."

source: Prentice-Hall's Illustrated Dictionary of Computing (3rd ed.) (Prentice-Hall, 1998), p. 19

The Power of Algorithms

Yet they are made up of careful combinations of very simple operations

Simple operations are combined in the following ways

• sequence - one operation (or group of operations) directly follows another
• condition (selection) - an operation (or group of operations) is executed, or not executed, based on the results of a yes/no (true/false) decision
• repetition (loop) - an operation (or group of operations) is executed repeatedly, a given number of times, or until a given condition is true

Thursday, Sept. 19, 2002

How are programming languages different from natural (human) languages?

• Human languages evolve naturally; programming languages are carefully designed
• Computer languages have very fussy rules, but these are easier to learn because they have fewer exceptions than human languages do
• A compiler's function is to examine a program in a higher-level programming language, then - if the program follows all the rules - translate it to machine code. Here are the phases:
• lexical analysis: classify every symbol according to its "part of speech"
• syntactic analysis: check for proper "sentence structure" - do all the parts of speech occur in the proper order?
• semantic analysis: determine the meaning of the program - does every symbol have a well-defined meaning?
• translation into machine code

Syntactic and semantic analysis

• Syntactic analysis compares statements in a language to the grammar, or set of rules, for writing a well-formed statement in that language.
• Semantic analysis analyzes well-formed statements of a language to determine whether they are meaningful.
• Monkey ate banana.
• Monkey ate the banana.
• The monkey ate the banana.
• George ate the banana.
• The banana ate the monkey.
• The banana ate George.
• Time flies like an arrow; fruit flies like a banana.
• “We'll see,” said the blind man, as he picked up his hammer and saw.
• Outside of a dog, a person's best friend is a book. Inside of a dog, it's too dark to read.