CMP 131 Introduction to Computer Programming

1. CMP 131Introduction to Computer Programming Violetta Cavalli-Sforza Week 6, Lecture 1 (Monday)

2. 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

3. Homework #2 Review

4. 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) Time&Place • i) CONST • j) X*Y • l) Lima, Ohio

5. 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.

6. Exercises 1.4, # 6 – 6 points • CONST Company : ‘General Motors’;VAR Salary : real; • VAR Age = 25; • VAR Days : integer; Ch : char;CONST Name = ‘John Smith’; Swap order

7. CONST Car : ‘Cadillac’; • CONST Score : integer; • VAR X, Y, Z : real; Score, Num : integer; This is okay!

8. 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 (we’ll see this later) Stmt ; stmt ; stmt  separator (correct) Stmt ; stmt ; stmt ;  terminator (accepted)

9. 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

10. 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.

11. Maximum of N called a sentinel value because it guards the loop PROCESSING: Initialize Old to 0Prompt for New numberRead Newwhile New is not equal to -99999 do: IF New > Old THEN Old := New Prompt for New Read New Write the value of New {New stores the maximum value} This is also an example of indefinitelooping : You DO NOT know how many times you go around the loop until you finish.

12. Minimum of N (very similar to Maximum) called a sentinel value because it guards the loop PROCESSING: Initialize Old to MaxIntPrompt for New numberRead Newwhile New is not equal to -99999 do: IF New < Old THEN Old := New Prompt for New Read New Write the value of New {New stores the minimum value} This is also an example of indefinitelooping : You DO NOT know how many times you go around the loop until you finish.

13. New Material

14. 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 selectionstatements : content is executed once, if at all • looping, repetition, or iteration statements : content is executed several times

15. What about computations? • Computations are performed through expressions. Examples: • 3 + X • a < b • If you don’t 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.

16. 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 don’t 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

17. 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.

18. 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

19. Example: Drawing Rectangles FOR I := 1 to HeightBEGIN FOR J := 1 TO Width write(‘*’); writeln;END 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.