CMP 131Introduction to Computer

Programming

Violetta Cavalli-Sforza

Week 3, Lecture 1

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

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

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

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

Brief Review (Week 2)• Computer Languages

– History– Different Types

• Software Development Lifecycle• Software Engineering Process

1. Problem statement 2. Analysis3. Design4. Implementation5. Testing & verification6. Maintenance

• Top-down design, step-wise refinement• Introduction to the TurboPascal IDE

– Editing, compiling, running programs

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.

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?

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

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

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

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

Let’s 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

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

Mean of 3 Numbers

• INPUT: 3 numbers

• OUTPUT: Mean of 3 numbers

• PROCESSING: Get 3 numbers Sum them Compute Mean as Sum / 3

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 <Enter>.'); readln (Score1, Score2, Score3); Average := (Score1 + Score2 + Score3) / 3; writeln (Average:20:3)END.

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

Mean of 20 Numbers: Refinement

PROCESSING:

Do the following 20 times:Get a NumberAdd it to the Sum

Compute Mean as Sum / 20

QUESTION: Is this good enough?

Mean of 20 Numbers: Refinement

PROCESSING:

Initialize Sum to 0

Do the following 20 times:Get a NumberAdd 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.

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

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

Mean of N Take 1: RefinementPROCESSING:

Initialize Sum to 0Prompt for value of CountRead value of CountDo 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.

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.

Mean of N Numbers (Take 2): Refinement

PROCESSING: Initialize Sum to 0Initialize Count to 0repeat

Prompt for NumberRead NumberAdd Number to SumIncrement Count by 1

until no more numbersCompute Mean as Sum / Count

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)

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

PROCESSING:Initialize Sum to 0Initialize Count to 0repeat

Prompt for NumberRead NumberAdd Number to SumIncrement Count

until Number = -99999Compute Mean as Sum / Count

Mean of N (Take 2): RefinementPROCESSING:

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

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

Mean of N (Take 3)PROCESSING:

Initialize Sum to 0Initialize Count to 0Prompt for NumberRead Numberwhile Number is not equal to -99999 do:

Add Number to Sum Increment Count

Prompt for NumberRead 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