CMSC104

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Lecture 18: Algorithms and Design

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Reading Assigned:   5.10 - 5.11

Homework Due:  Homework 08

Homework Assigned:  Homework 09

Classwork Assigned:  None

Topics Covered:

• Quiz 3 on Monday - Topics
• Homework 8 Solution and Solution with extra credit
• Classwork 10 Solution and Solution with extra credit
• Algorithms
  • Algorithms are a series of steps used to accomplish a goal
  • Each of our programs is an implementation of an algorithm
  • Algorithms take a set of inputs, and will generate a set of outputs
  • Part of computer science is understanding a description of a problem, and developing an algorithm to solve the problem
• Design of algorithms
  • How should you approach designing an algorithm?
  • Requirements
    • Most important part is understanding your requirements
    • Requirements - what does your algorithm need to do?
    • If there are questions on the requirements, ask them early
    • Read through all of a problem, and make sure you understand what the program should do, before you start
    • Unlike a computer language, English is imprecise
  • Top-down design (section 3.3 of book)
    • Start with high level view of problem
    • Break problem up into smaller sub-problems
    • Break sub-problems up into smaller sub-sub-problems
    • Continue until you have a problem you know how to easily solve
    • Implement the solution to the easy problem
    • Now go back up a step, and use the solution to the easy problem to solve the larger problem
    • Example from our homework 08:

      Problem: Draw a diamond consisting of stars
      Sub-problem: Print a line of spaces and stars
      Sub-Sub-Problem: Print a number of spaces
      Sub-Sub-Sub-Problem: Print one space
      

    • The flow of how the resulting algorithm will work:

      /* Draw diamond */
      loop over number of lines to print for our diamond
      {
        /* Draw a line */
        loop over number of spaces to print
        {
          print a space
        }
        loop over number of stars to print
        {
          print a star
        }
        print newline
      }
      

    • Note: Look for similar code and try to make then functions
    • Example above: printing a character a set number of times
    • You probably want to test the low level solutions before you use them to solve higher level problems
  • Limiting problem space
    • Also look at limiting problem space to make the problem easier
    • I got the top of diamond working, then tried to get bottom correct
    • In classwork to print sin wave, I got positive amplitude working, then worked on negative amplitude
    • Ignore the hard parts until the easy part is done
    • Leave comments in your code to remind you what you still need to do
  • An example to design in class
    • Problem: Write a program to calculate the minimum score that you must get in future assignments to get a given grade in the class
    • ...