CMP 131 Introduction to Computer Programming Violetta CavalliSforza

CMP 131 Introduction to Computer Programming Violetta Cavalli-Sforza Week 3, Lecture 2

Today’s Plan • • Questions Quiz (1/2 hour) Break (brief) Lecture: Important Programming Concepts

Program Structure • Programs are made up of: – Program headers – Declaration statements: • Name things • Allocate memory • Examples: – Constant Definitions – Type Definitions – Variable Declarations – Executable statements • Statements that perform actions

Executable Statements • • Assignment statements Selection statements Looping statements ….

Important Structured Programming Concepts • Selection • Repetition/Looping/Iteration • Sequential execution (stacking) • Nesting • Blocks & Compound Statements (Most high-level PLs provide constructs for these important structured programming concepts. Pascal was one of the first PLs to provide them. )

Selection • Conditional statements – Condition controlled execution – Single branch : IF <condition> THEN – Two branches : IF <condition> THEN <statements> ELSE <statements> – Multiple branches: CASE statement

Selection Examples • Single branch: – IF a condition is true THEN perform an action – The condition is tested once – The action is performed once • Examples: – IF it’s cold THEN wear a coat – IF a homework is due THEN do it –…

Selection Examples • Two branches: – IF a condition is true THEN perform an action ELSE perform a different action – The condition is tested once – One of the two actions is performed once • Examples: – To take the absolute value of a number: IF N >= 0 THEN return N ELSE return -N – IF the time is before noon THEN say good morning ELSE say good afternoon –…

Selection Examples • Multiple branches: – – The condition is tested once Several values of the condition are possible Several actions are possible One of the actions is performed once • Examples: – CASE Score OF 10, 9 : Grade is A 8, 7 : Grade is B 6, 5 : Grade is C OTHERWISE Grade is F –…

Repetition / Looping / Iteration • Fixed / Definite – Know before entering how many times will repeat loop body – While the “how many times” may be a variable: • Its value is set before entry into the loop • Its value may not (should not) be changed inside the loop – FOR loop • Variable Condition / Indefinite – Don’t know before entering how many times will repeat loop body – Entry into / exit from loop is controlled by a variable that can be changed inside the loop – REPEAT…UNTIL : tests condition at end – WHILE … DO : tests condition at start

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.

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.

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

Mean of N (Take 3) PROCESSING: Initialize Sum to 0 called a sentinel value because it guards the loop Initialize Count to 0 Prompt 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.

Stacking • Two meanings: – sequential execution vs. nested execution • one statement/block of code after the other, vs. • one statement/block of code inside the other – a way of storing and retrieving data: • Last-In First-Out (LIFO) regime • Used in a certain style of processor architecture (stack machines) • Used for memory management at runtime with procedure and function calls. • Used for certain types of applications (e. g. search)

Nesting • A block of code is visually placed inside another block of code AND • The execution of the internal block of code is controlled by the execution of the external block of code. • Ex. Do the following 20 times: Get a Number Add it to Sum

PROGRAM Silly (output); CONST Name = 'George'; Age = 26; VAR J, Sum : integer; BEGIN Sum : = 0; FOR J : = 1 TO 10 DO Sum : = Sum + J; writeln ('My name is ': 28, Name); writeln ('My age is ': 27, Age); writeln ('The sum is ': 28, Sum) END.

PROGRAM Silly (output); CONST Name = 'George'; Age = 26; VAR J, Sum : integer; BEGIN Sum : = 0; FOR J : = 1 TO 10 DO Sum : = Sum + J; writeln ('My name is ': 28, Name); writeln ('My age is ': 27, Age); writeln ('The sum is ': 28, Sum) END.

Blocks of Code • Most statements like FOR, IF, etc. . control only one other statement: – What do I mean by that? • To make them control multiple statements, you need to enclose those statements in a bracketing mechanism: BEGIN … END. • BEGIN … END – essentially says: treat all my contents as if they were a single complex statement – creates a compound statement