CMP 131 Introduction to Computer Programming

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CMP 131Introduction to Computer Programming

• Violetta Cavalli-Sforza
• Week 6, Lecture 1 (Monday)

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TODAY

• Midterm exam will be Monday April 23
• Still waiting for programs from homework 2
• Go over remainder homework 2, as part of review
  for midterm exam
• Formatting with write, writeln
• New material for today/this week/next week
• Selection. Making decisions.
• Different types of choices Yes/No choice.
  Either/or choice. Multiple choices.
• Boolean expressions
• Conditional statements in some detail
• Nesting conditional statements

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Homework 2 Review
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Exercises 1.4, 2 12 points

• Valid
• b) Payroll
• c) Room222
• e) A
• f) A1
• k) ListOfEmployees
• Invalid
• a) 7up
• d) Name List
• g) 1A
• h) TimePlace
• i) CONST
• j) XY
• l) Lima, Ohio

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Exercises 1.4, 4 3 points

• The three main sections of a Pascal program are
• Program heading
• Declaration section
• Executable section / program body
• You tell me what is in each part.

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Exercises 1.4, 6 6 points

1. CONST Company General MotorsVAR Salary
   real
2. VAR Age 25
3. VAR Days integer Ch charCONST
   Name John Smith

Swap order
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1. CONST Car Cadillac
2. CONST Score integer
3. VAR X, Y, Z real Score, Num
   integer

This is okay!
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Exercises 1.4, 7 2 points

• Semicolon is a statement separator
• Semicolon is not needed before an END because END
  is not a statement
• A semicolon is also not needed before an ELSE
  (well see this later)
• Stmt stmt stmt ? separator (correct)
• Stmt stmt stmt ? terminator (accepted)

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Exercises 1.5, 16 6 points

• What type of data is appropriate for each of the
  following
• Your age integer
• Your grade point average (GPA) real
• Your name string (not char)
• A test score integer or real
• The average test score real
• Your grade char, integer, real, string

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Part c)

• Following the example of calculating the mean
  that we used in class during Week 3 of the
  course, develop the problem statement, analysis,
  and design for a program that calculates the
  maximum of a set of numbers. You don't know how
  many numbers you will be given. You can assume
  that all numbers will be greater than 0.
• Use example from Week 3.1, Slide 29.

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Maximum of N

• PROCESSING
• Initialize Old to 0Prompt for New numberRead
  Newwhile New is not equal to -99999 do
• IF New gt Old THEN Old New Prompt for
  New Read New
• Write the value of New New stores the
  maximum value

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.
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Minimum of N (very similar to Maximum)

• PROCESSING
• Initialize Old to MaxIntPrompt for New
  numberRead Newwhile New is not equal to -99999
  do
• IF New lt Old THEN Old New Prompt for
  New Read New
• Write the value of New New stores the
  minimum value

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.
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New Material
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Types of Statements

• Input/Output for user-program communication
• Assignment to store (intermediate) results
• Control Structures to determine which other
  statements are executed and when
• conditional or selection statements content is
  executed once, if at all
• looping, repetition, or iteration statements
  content is executed several times

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What about computations?

• Computations are performed through expressions.
  Examples
• 3 X
• a lt b
• If you dont use them in other statements you
  might as well not perform them
• Assign them to variables with an assignment
  statement
• Ouput them via output statements
• Use them in control structures to determine what
  statements are executed.

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Looping/Iteration/Repetition Statements

• Three terms for the same idea Performing an
  action or a set of actions a number of times
• Fixed or definite repetition
• You know how many repetitions before you enter
  the loop
• Also counter-controlled repetition
• Pascal uses FOR loop
• Example printing out a rectangle
• Variable or indefinite repetition
• You dont know how many repetitions you will do
  before you enter the loop
• Sentinel-controlled loops
• Pascal uses two types of loops
• WHILE loop
• Pretest loop condition is tested at the top of
  the loop and therefore before ever entering the
  loop
• Body of loop executed 0 or more times
• REPEAT loop
• Postest loop condition is tested at the top of
  the loop
• Body of loop executed at least one time in order
  to reach the condition
• Examples of all of these in slides from Week
  3.1.
• We will look at these in more detail later

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Conditional / Selection Statements

• The basic idea, at the level of the algorithm, is
  that there is a choice to be made
• Three types of choices
• Yes/no choice. If yes, do something, otherwise
  do nothing
• Either/or choice. A 2-way choice. Do something
  different in each case.
• Multiple choice. An N-way chice. Do something
  different in each case. It can be expressed as a
  series of 2-way choices.
• Like most programming languages, Pascal provides
  a way of expressing these choices.

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Combining Statements

• The various types of statements can be combined
  in many ways.
• In particular, control statements can be nested
  inside each other. Nesting is a visual/physical
  relationship but also a control relationship.
• One loop can be performed inside another
• A conditional statement can control whether a
  loop is executed
• A conditional statement can be executed several
  times inside a loop
• When several statements are controlled by another
  statement we often use BEGIN and END to create
  compound statements

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Example Drawing Rectangles

• FOR I 1 to HeightBEGIN FOR J 1 TO
  Width write() writelnEND

This FOR loop controls only the input/output
statement write().
This FOR loop controls a compound statement
enclosed by the BEGIN and END.
The inner FOR statement is nested inside the
outer FOR statement.