What is CS171 about

  • CS 171 is a continuation of CS 170:

      • Problem solving using computer programs

  • What exactly is a computer program ?

        (Text book by Niklaus Wirth)

      1. Algorithms --- steps used to solve problems

      2. Data structures --- efficient information storage of information used by the algorithms

What is an algorithm ?

  • Algorithm:

      • Algorithm = a method for solving a problem expressed as a sequence of steps that is suitable for execution by a computer (machine)

  • Different ways to express algorithms:

      • Natural languages (bad - ambiguous)

      • Flow charts (good for conceptualization of the algorithm)

      • Pseudo code (good for algorithm development !)

      • Programming languages (good for communicating the algorithm to a machine (computer) -- because it's unambiguous)

What is a data structure ?

  • Data structure:

      • Data structure = a way to organize (= insert, delete and retrieve) the data used by an algorithm

  • Think:

      • If you have a large collection of songs.....

        How do you organize them so you can find a song quickly ?

  • The data structure you learned in CS 170: array

Strength and weakness of the array data structures

  • Every data structure has its strengths and its weaknesses

  • Array:

      • The data is stored in consecutive array elements

      • The array elements are accessed efficiently using an index

  • Strength:   efficient (low) storage cost

  • Weakness:   not dynamic (cannot increase in size easily)

What's more important: algorithm or data structure ?

  • Quote from a well-known system programmer Linus Torvalds (creator of Linux):

      • "I will, in fact, claim that the difference between a bad programmer and a good one is whether he considers his code or his data structures more important.

        Bad programmers worry about the code.

        Good programmers worry about data structures and their (data structure's) relationships

  • In CS 171, you will learn some basic data structures

  • In CS 253, you will learn more advanced data structures

Measure the "goodness" of algorithms/data structures

  • in CS 171, you will also analyze algorithms

      • Is one algorithm better than another algorithm ??

  • Metrics used to measure the goodness of algorithms:

      • Running time -- how long does it take for the program to finish

      • Storage requirment -- how much memory does the program use to store its information)

  • In CS 171, we will:

      • Study how to categorize algorithms by their running time

      • Study running time of various algorithms