Computer Programs

• Computers are programmed to perform many different tasks
• Computers execute very basic instructions in rapid succession
• A computer program is a sequence of instructions and decisions
• Programming is the act of designing and implementing computer programs.
• The physical computer and peripheral devices are collectively called the hardware.
• The programs the computer executes are called the software.

Self Check 1.1

What is required to play a music CD on a computer?

• Answer: A program that reads the data on the CD and sends output to the speakers and the screen.

Self Check 1.2

Why is a CD player less flexible than a computer?

• Answer: A CD player can do one thing — play music CDs. It cannot execute programs.

Self Check 1.3

What does a computer user need to know about programming in order to play a video game?

• Answer: Nothing

Class Question Jan 23 - 1

Where is a program like Microsoft Word usually stored when it is not currently running?

1. On the printer
2. On a memory stick
3. On a hard drive
4. In the cloud

The Java Programming Language

• Safe
• Portable
• Platform-independent
  • Distributed as instructions for a virtual machine
• Vast set of library packages
• Designed for the Internet

Becoming Familiar with Your Programming Environment

• An editor is a program for entering and modifying text, such as a Java program.
• Java is case sensitive.
• Java compiler translates source code into class files
• Class files contain instructions for the Java virtual machine.

Becoming Familiar with Your Programming Environment

1. Start the Java development environment.
2. Write a simple program.
3. Run the program.
4. Organize your work

Becoming Familiar with Your Programming Environment

 

Figure 7 From Source Code to Running Program

Self Check 1.10

What do you do to protect yourself from data loss when you work on programming projects?

• Answer: You back up your files and folders.

In the real world

• Take a look at a simple program in Bluej

Analyzing Your First Program: Methods

• Each class contains declarations of methods.
• Each method contains a sequence of instructions.
• A method contains a collection of programming instructions that describe how to carry out a particular task
• A method is called by specifying the method and its arguments.

Analyzing Your First Program: main Method

• Every Java application contains a class with a main method
  • When the application starts, the instructions in the main method are executed
• Declaring a main method

  public static void main(String[] args) 
  { 
     . . . 
  }
  

Analyzing Your First Program: Statements

• The body of the main method contains statements
• Our method has a single statement:

  System.out.println("Hello, World!");

• It prints a line of text:

  Hello, World!

Analyzing Your First Program: Method Call

• A method call

  System.out.println("Hello, World!");

• A method call requires:
  1. The method you want to use (in this case, System.out.println)
  2. Any values the method needs to carry out its task enclosed in parentheses (in this case, "Hello, World!")
• The technical term for such values is an arguments

Syntax 1.1 Java Program

Analyzing Your First Program: Strings

• String: a sequence of characters enclosed in double quotation marks:

  "Hello, World!"

Analyzing Your First Program: Printing

• You can print numerical values

  System.out.println(3 + 4);

  • evaluates the expression 3 + 4
  • displays the number 7.
• System.out.println method prints a string or a number and then starts a new line.
  • The sequence of statements

    System.out.println("Hello");
    System.out.println("World!");

  • Prints two lines

    Hello
    World!

• There is a second method, System.out.print, that you can use to print an item without starting a new line

Class Question Jan 28 - 1

How do you modify the HelloPrinter program to greet you if your name is Carlos

1. System.out.println(Hello, Carlos);
2. System.out.println("Hello, Carlos");
3. system.out.println("Hello, Carlos");
4. System.print("Hello, Carlos");

Class Question Jan 28 - 2

What does the following set of statements print?

System.out.print("My lucky number is");
System.out.println(3 + 4 + 5);

1. My lucky number is 12
2. My lucky number is12
3. My lucky number is
   12
4. It does not print anything. There is a syntax error

Errors

• A compile-time error (syntax error)
  • is a violation of the programming language rules
  • detected by the compiler.

    System.ou.println("Hello, World!");

• A run-time error (logic errors)
  • causes a program to perform an action that the programmer did not intend.

    System.out.println("Hello, Word!");

Errors

• Syntax errors - compile time error
• Exception - a type of run-time error
  • Generates an error message from the Java virtual machine
  • This statement

    System.out.println(1 / 0)

  • Generates this run-time error message

    "Division by zero"

• logic error - another type of rune type error

Class Question Jan 28 - 3

Suppose you omit the "" characters around Hello, World! from the HelloPrinter.java program. Is this a compile-time error or a run-time error?

1. compile-time
2. run-time
3. It is not an error

Class Question Jan 28 - 4

Suppose you change println to printline in the HelloPrinter.java program. Is this a compile-time error or a run-time error?

1. compile-time
2. run-time
3. It is not an error

Class Question Jan 28 - 5

Suppose you change main to hello in the HelloPrinter.java program. Is this a compile-time error or a run-time error?

1. compile-time
2. run-time
3. It is not an error

Class Question Jan 28 - 6

When you used your computer, you may have experienced a program that "crashed" (quit spontaneously) or "hung" (failed to respond to your input). Is that behavior a compile-time error or a run-time error?

1. compile-time
2. run-time
3. It is not an error

Notes

• Need a notetaker
• If you were not here last class, come tell me after class
• Bring paper and pencil next class.

Problem Solving: Algorithm Design

• Algorithm: A sequence of steps that is:
  • unambiguous
  • executable
  • terminating

An Algorithm for Solving an Investment Problem

• The problem:
  • You put $10,000 into a bank account that earns 5 percent interest per year. How many years does it take for the account balance to be double the original?
• Calculating by hand
  

An Algorithm for Solving an Investment Problem - continued

• The steps in the algorithm
  • Start with a year value of 0, a column for the interest, and a balance of $10,000.
    
    
  • Repeat the following steps while the balance is less than $20,000
    Add 1 to the year value.
    Compute the interest as balance x 0.05 (i.e., 5 percent interest).
    Add the interest to the balance.
    
  • Report the final year value as the answer.

Pseudocode

• Pseudocode: An informal description of of a sequence of steps for solving a problem
  • Describe how a value is set or changed:

    total cost = purchase price + operating cost
    Multiply the balance value by 1.05.
    Remove the first and last character from the word.

  • Describe decisions and repetitions:

    If total cost 1 < total cost 2s
    While the balance is less than $20,000
    For each picture in the sequence

  • Use indentation to indicate which statements should be selected or repeated

    For each car
         operating cost = 10 x annual fuel cost
         total cost = purchase price + operating cost

  • Indicate results:

    Choose car1.
    Report the final year value as the answer.

From Algorithm to Programs

Class Question Jan 30 - 1

Suppose the interest rate was 20 percent. How long would it take for the investment to double?

1. 3
2. 4
3. 5
4. There is not enough information to tell

• Answer: 4 years:

  0 10,000 
  1 12,000 
  2 14,400 
  3 17,280 
  4 20,736

Self Check 1.22

Suppose your cell phone carrier charges you $29.95 for up to 300 minutes of calls, and $0.45 for each additional minute, plus 12.5 percent taxes and fees. Give an algorithm to compute the monthly charge for a given number of minutes.

Use paper and pencil

• Answer:

  Is the number of minutes at most 300? 
  a. If so, the answer is $29.95 × 1.125 = $33.70. 
  b. If not, 
     1. Compute the difference: (number of minutes) – 300. 
     2. Multiply that difference by 0.45. 
     3. Add $29.95. 
     4. Multiply the total by 1.125. That is the answer.

Self Check 1.23

Consider the following pseudocode for finding the most attractive photo from a sequence of photos:

Pick the first photo and call it "the best so far".
For each photo in the sequence
   If it is more attractive than the "best so far"
      Discard "the best so far".
      Call this photo "the best so far".
The photo called "the best so far" is the most attractive photo in the sequence. 

Use pencil and paper

Is this an algorithm that will find the most attractive photo?

• Answer: No. The step If it is more attractive than the "best so far" is not executable because there is no objective way of deciding which of two photos is more attractive.

Self Check 1.24

Suppose each photo in Self Check 23 had a price tag. Give an algorithm for finding the most expensive photo

Use pencil and paper

• Answer:
  

  Pick the first photo and call it "the most expensive so far".
  For each photo in the sequence
     If it is more expensive than "the most expensive so far"
        Discard "the most expensive so far".
        Call this photo "the most expensive so far".
  The photo called "the most expensive so far" is the most expensive photo in the sequence

Self Check 1.26

Suppose you have a random sequence of colored marbles. Consider this pseudocode:

Repeat until sorted
   Locate the first marble that is preceded by a marble of a different color, and switch them.

Why is this not an algorithm?

• Answer: The sequence doesn’t terminate. Consider the input . The first two marbles keep getting switched.