1. CMP 131 Introduction to Computer Programming Violetta Cavalli-Sforza Week 3, Lecture 1

2. Quiz: This Wednesday, March 14 Covers material from Weeks 1 & 2 Slides Reading (Ch 1.1-1.3) Combination of true/false, and short- answer questions, long-answer question

3. Quiz – Be Familiar With Basic hardware concepts (computer components) Basic hardware/software concepts –Binary vs. decimal representations and conversion –Binary addition and outcomes, logical operations Different types of software & applications Basic types of PLs and their differences (assembly, machine, high-level) –NO: details about the different programming language paradigms (first part of Week 2, Lecture 1) –YES: structured, imperative, modular, and object- oriented PLs

4. Quiz – Be Familiar With (2) What happens to your program: –Compiling, linking, executing, interpreting Different types of program errors: syntactic, logical, runtime Software development lifecycle Software engineering process Top-down design, stepwise refinement

5. Brief Review (Week 1) History of Hardware and Software Hardware components Hardware/Software interface: –Layers of the Machine –Kinds of Software What happens to your program? –Compilation, Linking, Execution –Compilation vs. Interpretation –Program errors Syntax, Semantics, Grammars

6. Brief Review (Week 2) Computer Languages –History –Different Types Software Development Lifecycle Software Engineering Process 1. Problem statement 2. Analysis 3. Design 4. Implementation 5. Testing & verification 6. Maintenance Top-down design, step-wise refinement Introduction to the TurboPascal IDE –Editing, compiling, running programs

7. This Week More top-down design and stepwise refinement examples & practice Introduction to basic algorithm concepts: –sequential, selection and repetition concepts –stacking vs. nesting Some basics of the Pascal language Reading: Chs 1.4-1.5, Ch 2. –Read through once or twice well. –You are not expected to know everything in there –We’ll go over the most important concepts in class.

8. Case Study 3: Program for Grades Revised Problem Statement A program that will let me enter and store the score that I receive for each assessment in the course, the maximum score that I could have obtained in that assessment, and the name of the assessment. When I run the program, it will: –Retrieve and display the scores it already knows in a table with suitable headers –Give me the option to enter data for one or more additional assessments or to exit (AssessmentName, MyScore, MaxScore) –If I choose to enter data for an assessment it will prompt me for all three values on one line –After I enter the values it will again give me the option to enter new data or to exit. When I exit the program –all the data (old and new) will be stored to a new file so that it can be retrieved next time the program is run. –all the data it has for me will be shown. Are you starting to think that you should use a spreadsheet instead?

9. Case Study 3: I/O Analysis Design (Algorithm): –Input: Existing data from file –Need to specify file name or not? New data from keyboard –Output: Existing data from file to the screen Existing data plus new data to file

10. Case Study 3: Processing Analysis Processing: 1.Display existing data Print table header For each item in the file containing old data, print the values 2.Prompt user to exit or enter new data If user chooses to exit, then continue to Step 3, otherwise –Prompt user to enter data: Name of assessment, My score, Max score –Do something with that data… Repeat step 2 3.Display all the information the program has for me 4.Finalize files 5.Exit

11. Refinement Step 2 Prompt user to exit or enter new data –Alternative 1: A single prompt that can accept either data or an indication to exit. –Alternative 2: Two different prompts: Exit/Continue Prompt for data (if continue) –Selection: If user chooses to exit, then continue to Step 3 (display all data, finalize, exit) else Prompt user to enter data: AssessmentName, MyScore, MaxScore Do something with that data… (e.g. add it to the end a file) Repeat step

12. Case Study 4: A Simple Statistics Calculator A program like those in graphic calculators that allows entering the data and will automatically display mean / average, the frequency, the median, the mode … –We can do a conceptual version of it

13. Let’s review definitions Mean: (n 1 + n 2 + … + n m ) / m Median: Given a set of m numbers, the value v such that 50% of the numbers are smaller than it and 50% of the numbers are larger than it. –If m is odd, v will be one of the numbers –If m is even, v will be an average of two of the numbers Mode: The most frequent value in a set of numbers –May need to cluster into intervals / ranges Frequency: –May need to cluster into intervals / ranges

14. Consider the different requirements Mean: –Don’t need to keep track of the numbers, just How many of them Their sum Median: –Need to order (sort) them Mode, Frequency: –Possibly if they are integers, certainly if they are real, need to find: minimum and maximum create intervals count frequency in each interval

15. Mean of 3 Numbers INPUT: 3 numbers OUTPUT: Mean of 3 numbers PROCESSING: Get 3 numbers Sum them Compute Mean as Sum / 3

16. Mean of 3 Numbers: Program PROGRAM ComputeAverage (input,output); {A simple program that computes an average.} VAR Score1, Score2, Score3 : integer; Average : real; BEGIN writeln ('Enter three scores and press.'); readln (Score1, Score2, Score3); Average := (Score1 + Score2 + Score3) / 3; writeln (Average:20:3) END.

17. Mean of 20 Numbers INPUT: 20 numbers OUTPUT: Mean of 20 numbers PROCESSING: Get 20 numbers one at a time, keeping a running sum Compute Mean as Sum / 20

18. Mean of 20 Numbers: Refinement PROCESSING: Do the following 20 times: Get a Number Add it to the Sum Compute Mean as Sum / 20 QUESTION: Is this good enough?

19. Mean of 20 Numbers: Refinement PROCESSING: Initialize Sum to 0 Do the following 20 times: Get a Number Add it to Sum Compute Mean as Sum / 20 This is an example of definite looping: You know how many times you go around the loop when you start.

20. Mean of N Numbers INPUT: Several numbers OUTPUT: Mean of numbers PROCESSING: Get numbers and how many of them (Count) Add to Sum Compute Mean as Sum / Count

21. Mean of N Numbers: Take 1 PROCESSING: Ask user how many numbers (store in Count) Ask user for that many numbers and compute Sum Compute Mean as Sum / Count

22. Mean of N Take 1: Refinement PROCESSING: Initialize Sum to 0 Prompt for value of Count Read value of Count Do the following Count times: Prompt for Number Read Number Add Number to Sum Compute Mean as Sum / Count This is ALSO an example of definite looping : You know how many times you go around the loop when you start.

23. Mean of N Numbers: Take 2 PROCESSING: repeat Prompt for Number Read Number Add Number to Sum Increment Count by 1 (add 1 to Count) until no more numbers Compute Mean as Sum / Count This is an example of indefinite looping : You DO NOT know how many times you go around the loop until you finish.

24. Mean of N Numbers (Take 2): Refinement PROCESSING: Initialize Sum to 0 Initialize Count to 0 repeat Prompt for Number Read Number Add Number to Sum Increment Count by 1 until no more numbers Compute Mean as Sum / Count

25. Questions How do we know there are no more numbers? –If you were reading from a file, you would get a signal that you were at the end of the file –From the terminal, there must be either: 2 different prompts: –1) enter more data? yes/no –2) (if yes) then ask for data a special value that is a signal to end –Should be value that won’t be confused with the other data, i.e. is not a legal data value –e.g. -99999 (if you are computing an average of scores, this is not a legal score)

26. Is the logic okay with a sentinel value? Or is there a problem? PROCESSING: Initialize Sum to 0 Initialize Count to 0 repeat Prompt for Number Read Number Add Number to Sum Increment Count until Number = -99999 Compute Mean as Sum / Count

27. Mean of N (Take 2): Refinement PROCESSING: Initialize Sum to 0 Initialize Count to 0 repeat Prompt for Number Read Number if Number is not equal to -99999 then Add number to Sum Increment Count until Number = -99999 Compute Mean as Sum / Count

28. Can we make it better? Or different? In previous, we test if Number is the special value of -99999 twice: 1.To decide whether it is a legal number, to add it to Sum 2.To decide whether to exit the loop. This is not very elegant, and it is also inefficient (though equality tests are cheap) Rewrite the loop to test only once

29. Mean of N (Take 3) PROCESSING: Initialize Sum to 0 Initialize Count to 0 P rompt for Number Read Number while Number is not equal to -99999 do: Add Number to Sum Increment Count Prompt for Number Read Number Compute Mean as Sum / Count This is also an example of indefinite looping : You DO NOT know how many times you go around the loop until you finish. called a sentinel value because it guards the loop