Chapter 1.

Programming and Programs

• A computer program is a sequence of instructions to be performed by a computer.
  • There are many different programming languages
  • There are many different ways to attempt this task.
• In this class, we will study C++
  • This is a widely used programming language
  • It is modern and supports a number of programming paradigms
  • It is the main language used in our program.
  • If you know this programming language well, you can learn others quickly.
• Programming is the process of planning, implementing, and debugging a computer program.
  • Programming is one of the major tools in our approach to computer science.
  • It is a skill, an art, and a science.
  • We will learn more than just C++ in this class.
  • This is the first in a series of classes to fully introduce computer programming.
• Programming consists of one or more of the following steps:
  1. Understand the problem.
  2. Develop an algorithm to solve the problem.
  3. Verify that the algorithm solves the problem.
  4. Implement the algorithm.
  5. Test the program.
  6. Use the program.
  7. Maintain the program.
• Steps 1-3 above are the Problem Solving Phase
• Steps 4-5 are the Implementation Phase
• Steps 6-7 are the Maintenance Phase
• You can try to skip steps 1-3 if you wish, but this will lead to troubles.
• Look at the picture on page 3 of the book.
• As the book notes, this is just one method of program development. Others are valid, but frequently require more skill and are difficult for the beginner.

Algorithms

• An algorithm is a set of instructions that solve a problem in a finite amount of time.
• There are several uses of an algorithm.
  1. State a concise way to solve a problem.
  2. Provide a map for creating a program.
• Consider the following problem: Place a set of items in order.
  • Each item has a location, sequentially numbered from 1 to the number of items (N).

    
     For position = 1  to N do
        Find the location of the  smallest item (position to N)
        Swap it with the item at location position
    
    	 

  • Trace this for 4, 7 , 2, 9, 8 , 0, 1
• This is an example of an algorithm given to state concisely how to solve a problem.
• This is called the "Selection Sort", and is a classical algorithm.
• A few notes about algorithm languages
  • Programming languages can be used to describe algorithms, but they probably contain too much detail.
  • Pseudo languages, or algorithm languages are frequently used, but you usually spend too much time learning the language.
  • I prefer English, with programming structures added in.
  • But a picture may work
  • Or perhaps a dance
• This algorithm demonstrates a problem we encounter when writing algorithms:
  • Step 2 is probably too vague.
    • It is not clear the steps needed to accomplish this step.
• We are probably more interested in the second use of algorithms.
  • Problem: A local store would like you to implement a system which will compute the change due at the end of a transaction. The input will be the total price and the amount paid. You should determine the change, as well as the actual coins and bills to be returned to the user.
  • Pass 1:

    • Give Change

    • This has a number of problem.
      • How much change?
      • How many of each coin?
      • How do you calculate these?
  • Pass 2:

    • 
       Get Price
       Get Amount Paid
       Change = Amount Paid - Price
       Give change
      
      	    

    • Notice that this corrects a few of the earlier problems
    • We know the amount of change to give, but we don't know how to give this change.
    • This does us a classic design pattern, break the problem into smaller pieces.
  • Pass 3:

    • 
       Get Price
       Get Amount Paid
       Change = Amount Paid - Price
       While the change is not correct
            Grab a random selection of coins, make this the change
       Give Change
      
      

    • This is not an algorithm!
      • Will it ever finish?
    • It is very inefficient as well.
  • Pass 4:

    
     Get Price
     Get Amount Paid
     Change = Amount Paid - Price
     For each coin type 
         While Change > coin value
             add one coin of this type
             subtract the coin value from Change
     Give Change
    
    

    • This seems to address the major issues
    • I am somewhat concerned about the order we look at the coin types.
    • Let's try it with a Price of $1.52 and an Amount Paid of 2.00
  • One last try:

    
     Get Price
     Get Amount Paid
     Change = Amount Paid - Price
     For each coin type  largest to smallest
         While Change > coin value
             add one coin of this type
             subtract the coin value from Change
     Give Change
    
       

  • Notice the use of mathematical expressions
  • The idea of looping (for, while, ..)
  • Try this with the problem of writing a check
  • Try this problem