CMP 131 Introduction to Computer Programming

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CMP 131Introduction to Computer Programming

• Violetta Cavalli-Sforza
• Week 3, Lecture 1

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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

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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

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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

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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

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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

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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
• Well go over the most important concepts in
  class.

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Case Study 3 Program for GradesRevised 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?
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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

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Case Study 3 Processing Analysis

• Processing
• Display existing data
• Print table header
• For each item in the file containing old data,
  print the values
• 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
• Display all the information the program has for
  me
• Finalize files
• Exit

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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

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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

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Lets review definitions

• Mean (n1 n2 nm) / 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

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Consider the different requirements

• Mean
• Dont 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

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Mean of 3 Numbers

• INPUT 3 numbers
• OUTPUT Mean of 3 numbers
• PROCESSING Get 3 numbers Sum them
• Compute Mean as Sum / 3

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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
  ltEntergt.')
• readln (Score1, Score2, Score3)
• Average (Score1 Score2 Score3) / 3
• writeln (Average203)
• END.

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Mean of 20 Numbers

• INPUT 20 numbers
• OUTPUT Mean of 20 numbers
• PROCESSINGGet 20 numbers one at a time, keeping
  a running sum
• Compute Mean as Sum / 20

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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?
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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.
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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

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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

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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 NumberRead NumberAdd 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.
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Mean of N Numbers Take 2

• PROCESSING
• repeat
• Prompt for NumberRead NumberAdd Number to
  SumIncrement 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.
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Mean of N Numbers (Take 2) Refinement

• PROCESSING
• Initialize Sum to 0Initialize Count to 0
• repeat
• Prompt for NumberRead NumberAdd Number to
  SumIncrement Count by 1
• until no more numbers
• Compute Mean as Sum / Count

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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 wont 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)

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Is the logic okay with a sentinel value? Or is
there a problem?

• PROCESSING
• Initialize Sum to 0Initialize Count to 0
• repeat
• Prompt for NumberRead NumberAdd Number to
  SumIncrement Count
• until Number -99999
• Compute Mean as Sum / Count

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Mean of N (Take 2) Refinement

• PROCESSING
• Initialize Sum to 0Initialize Count to 0
• repeat
• Prompt for NumberRead Numberif Number is not
  equal to -99999
• then Add number to Sum Increment Count
• until Number -99999
• Compute Mean as Sum / Count

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Can we make it better? Or different?

• In previous, we test if Number is the special
  value of -99999 twice
• To decide whether it is a legal number, to add it
  to Sum
• 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

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Mean of N (Take 3)

• PROCESSING
• Initialize Sum to 0Initialize Count to 0
• Prompt for NumberRead Numberwhile Number is
  not equal to -99999 do
• Add Number to Sum Increment Count Prompt
  for Number Read Number
• Compute Mean as Sum / Count

called a sentinel value because it guards the loop
This is also an example of indefinite looping
You DO NOT know how many times you go around the
loop until you finish.