COMP6015 An Introduction to Computer Programming Lecture 02

1. COMP6015 • An • Introduction • to • Computer Programming • Lecture 02 COMP6015 - An Introduction to Computer Programming : University of the West Indies

2. The Algorithmic Language • During development of an algorithm, the language gradually progresses from English towards a programming language notation. • An intermediate notation called pseudo-code is commonly used to express algorithms. COMP6015 - An Introduction to Computer Programming : University of the West Indies

3. The Algorithmic Language • Algorithmic Structure • Every algorithm should have the following sections, in the stated order: • Header : Algorithm’s name or title. • Declaration : A brief description of algorithm and variables. i.e. a statement of the purpose. • Body : Sequence of steps • Terminator : End statement COMP6015 - An Introduction to Computer Programming : University of the West Indies

4. The Algorithmic Language • How to write Pseudocode • An algorithm can be written in pseudocode using six (6) basic computer operations: • A computer can receive information. • A computer can output (print) information. • A computer can perform arithmetic operation • A computer can assign a value to a piece of data: • A computer can compare two (2) pieces of information and select one of two alternative actions. • A computer can repeat a group of actions. COMP6015 - An Introduction to Computer Programming : University of the West Indies

5. The Algorithmic Language • A computer can receive information. • Typical pseudocode instructions to receive information are: • Read name • Get name • Read number1, number2 • A computer can output (print) information. • Typical pseudocode instructions are: • Print name • Write "The average is", ave COMP6015 - An Introduction to Computer Programming : University of the West Indies

6. The Algorithmic Language • A computer can perform arithmetic operation • Typical pseudocode instructions: • Add number to total OR • Total = Total + Number • Ave = sum/total • A computer can assign a value to a piece of data: • e.g. to assign/give data an initial value: • Initialize total to zero • Set count to 0 • To assign a computed value: • Total = Price + Tax COMP6015 - An Introduction to Computer Programming : University of the West Indies

7. The Algorithmic Language • A computer can compare two (2) pieces of information and select one of two alternative actions. • Typical pseudocode e.g. • If number < 0 then • add 1 to neg_number • else • add one to positive number • end-if COMP6015 - An Introduction to Computer Programming : University of the West Indies

8. The Algorithmic Language • A computer can repeat a group of actions. • Typical pseudocode e.g. • Repeat until total = 50 • read number • write number • add 1 to total • end-repeat • OR • while total < = 50 do: • read number • write number • add 1 to total • end-while COMP6015 - An Introduction to Computer Programming : University of the West Indies

9. The Algorithmic Language • Now, let’s review the plan and write out the algorithm for the average problem in the specified format: • Header :Algorithm’s name or title. • Declaration : A brief description of algorithm and variables. i.e. a statement of the purpose. • Body : Sequence of stepsTerminator :End statement COMP6015 - An Introduction to Computer Programming : University of the West Indies

10. The Algorithmic Language • Algorithm Average • This algorithm reads a list of numbers and computes their average. • Let: SUM be the total of the numbers read • COUNTER be the number of items in the list • AVE be the average of all the numbers • Begin • Set SUM to 0, • Set COUNTER to 0. COMP6015 - An Introduction to Computer Programming : University of the West Indies

11. The Algorithmic Language • While there is data do: • Read number • COUNTER = COUNTER + 1 • SUM = SUM + number • end-while • if COUNTER = 0 then • AVE = 0 • else • AVE = SUM/ COUNTER • Write “Average = ” AVE • Stop. COMP6015 - An Introduction to Computer Programming : University of the West Indies

12. The Algorithmic Language • A Systematic Approach to Defining a Problem • Defining the problem is the first step towards a problem solution!!!! • A systematic approach to problem definition, leads to a good understanding of the problem. • A tested method for defining any given problem: • Divide problem into three (3) separate components: • (a) Input or source data provided • (b) Output or end result required • (c) Processing - a list of what actions are to be performed COMP6015 - An Introduction to Computer Programming : University of the West Indies

13. The Algorithmic Language • EXAMPLE 1: • A program is required to read three (3) numbers, add them and print their total. • Solution • The Defining Diagram: COMP6015 - An Introduction to Computer Programming : University of the West Indies

14. The Algorithmic Language • Pseudocode for EXAMPLE 1 • Algorithm Add _Numbers • <Description and variables’ definition> • Begin • Read Num1, Num2, Num3 • Total = Num1 + Num2 + Num3 • Print Total • Stop COMP6015 - An Introduction to Computer Programming : University of the West Indies

15. The Algorithmic Language • Top-Down Design Approach (Modularization) • Methodology involves: • breaking a problem into a set of sub-problems; • breaking each sub-problem into a set of tasks; • breaking each task into a set of actions. COMP6015 - An Introduction to Computer Programming : University of the West Indies

16. The Algorithmic Language • Example 1 • Problem: Add 23 and 35 • Refinement: No further refinement is required. • Example 2 • Problem: Turn on a light bulb • Refinement: Sub-problem 1: Locate switch • Sub-problem 2: Depress switch • Note each sub-problem in this case required only one task which also required only one action COMP6015 - An Introduction to Computer Programming : University of the West Indies

17. The Algorithmic Language • EXAMPLE 3 • Problem: Given a list of test scores, find the highest and lowest score and the average score. • Sub-problem 1: Read test scores • Sub-problem 2: Find highest score • Sub-problem 3: Find lowest score • Sub-problem 1 can be considered as one action and therefore needs no further refinement. • Sub-problems 2 and 3 can be further divided into a group of actions. This is left as an exercise for you. COMP6015 - An Introduction to Computer Programming : University of the West Indies

18. The Algorithmic Language • Advantages of the Top-Down Design Method • It is easier to comprehend the solution of a smaller and less complicated problem than to grasp the solution of a large and complex problem. • It is easier to test segments of solutions, rather than the entire solution at once. This method allows one to test the solution of each sub-problem separately until the entire solution has been tested. • It is often possible to simplify the logical steps of each sub-problem, so that when taken as a whole, the entire solution has less complex logic and hence easier to develop. COMP6015 - An Introduction to Computer Programming : University of the West Indies

19. The Algorithmic Language • Advantages of the Top-Down Design Method • A simplified solution takes less time to develop and will be more readable. • The program will be easier to maintain. COMP6015 - An Introduction to Computer Programming : University of the West Indies

20. Summary • Steps to Problem-Solving: • Imperative that the problem be defined PROPERLY! • Level 1 - Abstraction: • Clarify the nature of the problem • Decide how to find a solution to the problem • Judge the appropriateness of a potential solution to the problem • Evaluate the implementation of a solution to the problem COMP6015 - An Introduction to Computer Programming : University of the West Indies

21. Summary • Steps to Problem-Solving: • Level 2 - Implementation Level • Can you "see" what the problem needs in order to be solved? • Do you know of a technique which might be useful in solving the problem? • Do you know of a technique which might be useful in solving the problem if you could use it in a more abstract, or sophisticated way? COMP6015 - An Introduction to Computer Programming : University of the West Indies

22. Summary • CHARACTERISTICS of a Good Algorithm: • The Right Problem Has Been Solved • The Implementation is free of Errors • The Algorithm is Well Documented • The solution is Robust • The Solution Is Maintainable You can make your solution maintainable by: • dividing the solution into modules • making sure that the algorithm is well-documented • using symbolic constants COMP6015 - An Introduction to Computer Programming : University of the West Indies

23. Summary • Elements of Programming Style: • Choice of Identifiers • Comments • Indentation • Effective Use of White Spaces COMP6015 - An Introduction to Computer Programming : University of the West Indies

24. EXERCISE • The Electricity Company requires a program which computes charges for subscribers’ electricity consumption. The company maintains a file on all its subscribers – domestic and commercial. Each record in the file consists of the subscriber’s name, account number, account type, address and the number of kilowatt-hours consumed. • For commercial accounts, the rate charged is 25¢ per KWH. • For domestic accounts, the rates charged are as follows: • 10¢ per KWH on the first 100KWH • 25¢ per KWH on the next 200KWH • 55¢ per KWH on the remaining KWHs in excess of 300 KWH • e.g. The charge for 416 KWH would be: • (100 x 10¢) + (200 x 25¢) + (116 x 55¢) • Design an algorithm that will read all the records in the file and produce a report for subscribers as follows: • Subscribers Name Account No. KWH Used Amt Due . • xxxxxxx xxxxxxx xxx-xxx-xxxx xxx 999.99 • xxxxxxx xxxxxxx xxx-xxx-xxxx xxx 999.99 COMP6015 - An Introduction to Computer Programming : University of the West Indies