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Programming Basics To specify algorithms, we must be precise. To be precise, we need a language that is more exact than English.  A programming language offers this advantage.  All programming languages have a basic set of features.

Recapping Alphabetize CDs Alphabetize CDs illustrates an intuitively understandable process not involving a computer The Alphabetize CDs program demonstrated several features of algorithms and programs … The program illustrated the 5 properties of algorithms -- input and output specs, definiteness, effectiveness, finiteness In order to reference the different slots, we used two “pointers” called Alpha and Bet Alpha referenced all slots but the last, and for each slot Alpha referenced, Bet referenced each slot to its right Can you “visualize” Alphabetize CDs’ processing strategy?

An Approach To Programming Though Alphabetize CDs was precise enough for a person to execute successfully, computers demand greater precision from programs The plan … Adopt a better notation than English to express algorithms General ideas are given in lecture VB6 will be used in lecture and lab Discuss standard ways of using a programming language Practice the ideas by writing programs Add a few more language features and describe their use Practice with a few more programs

Variables In normal language, names are (usually) tightly fixed to their values -- “penny” means 1 cent … it doesn’t change its meaning, and sometimes refer to $8.41 or a time zone or an action In computing names can change values Example: Alpha and Bet in Alphabetize CDs changed Names must change values in a program because programs specify a transformation of input into output … as the transformation proceeds the things named change values variable is the term for program names that can change value

On Variable Names The term “variable” reminds us the value can change The names used for variables are arbitrary, provided: Variable names must begin with a letter Variable names can contain any letter, numeral or _ Variable names should be meaningful and accurate total, averageOverClass, average_over_class but not o0OO0o, bet.  Also (for now) not i, n, x, etc. Most languages are case sensitive:   a ¹ A

On Variable Values A variable can be thought of as a “named container” Variables name computer memory locations, so the value of a variable is the quantity stored in its memory Variables can take on different types of values Whole numbers or integers: 2, -9, 1048576 Character sequences or strings: “2”, “&^%$#@”, “ ” Floating point numbers or doubles: 2.0, 3.14159, -999.99 (numbers that can have some digits after the decimal point) A variable’s values have a specific type Variables are declared and their type is specified Dim averageOverClass As Double

Assignment Computers must be told what value to assign to variables, using an assignment statement such as averageOverClass = 21.14 mayor = “Paul Schell” The general form of an assignment statement is <variable name> <assignment symbol> <expression> Languages use different assignment symbols:    = :=   ¬ Read assignment as “is assigned”, or “becomes” or “gets” All three components must always be present Fundamental property of assignment The “flow” of information is always right-to-left destination = source changedVariable = value

Expressions Expressions are formulae made from variables and operators, e.g. calculator operations: +, -, *, /, ^ weeks = days / 7 divide value of days by 7 grossPay = hours * rate multiply the two values area = pi * radius ^ 2 p times radius squared In the last example, the ^ operator has precedence over the * operator. We could also write area = pi * (radius ^ 2) When in doubt, use extra parentheses in expressions!  It’s always safe. See the Snyder text for more about precedence, and page 77 of the VB book for a complete table of operator precedence in VB.

Mini-Exercise #1 Suppose you have a variable that represents the total amount of a loan.  What is a good name for this variable? Suppose the computer executes the following statements. What is the value of total at the end?      x = 1      total = x+3 What is the value of squid after executing these statements?      clam = 1      squid = 4 + 2*clam

Mini-Exercise #1 -- Answers Suppose you have a variable that represents the total amount of a loan.  What is a good name for this variable?      loanAmount or loan_amount Suppose the computer executes the following statements. What is the value of total at the end?      x = 1      total = x + 3 total is 4 What is the value of squid after executing these statements?      clam = 1      squid = 4 + 2*clam squid is 6

Fundamental Rule of Assignment Fundamental rule of assignment The expression is evaluated before the assignment is made score = score + 3 shotClock = shotClock - 1

Mini-Exercise #2 Suppose the computer executes the following statements. What is the value of total at the end?      total = 1      total = total + 5 Harder:      x = 0      x = x+4      x = x*2

Mini-Exercise #2 -- Answers Suppose the computer executes the following statements. What is the value of total at the end?      total = 1      total = total + 5 total is 6 Harder:      x = 0      x = x+4      x = x*2 x is 8

Operators Most programming languages have more operators than a pocket calculator Operators like + taking 2 operands are called binary:  a + b Operators like -  taking 1 operand are called unary:  - a A very useful operator is concatenate, & in VB6, which connects two strings together: plural = “dog” & “s” The relational operators are: a < b   less than          a > b   greater than a <= b less than or equal to a >= b greater than or equal a = b equal to          a <> b not equal

Conditionals Programs must frequently test if some condition holds, e.g. are two CDs in alphabetical order Conditional statements have been invented to make tests If temp < 32 Then waterState = “frozen” General form of basic conditional: If <T/F expression> Then <assignment statement> The meaning is that the <T/F expression> is evaluated If the outcome is true, then the assignment statement is performed If the outcome is false, then the assignment statement is skipped

More Complex Conditionals The basic conditional is too limited, so generalize it General form of an If-statement If <T/F expression> Then <statement list> End If Example: If temp >= 212 Then state = “gaseous” form = “steam” End If

General Conditional Statement When operations must be performed for the true outcome and different operations are need for a false outcome, use the If-Then-Else statement General form If <T/F expression> Then <statement list> Else <statement list> End If

Example of If-Then-Else An advantage of the general conditional is that the statement lists can contain other conditionals

Mini-Exercise #3 Suppose the computer executes the following statements. What is the value of total at the end?      total = 1      total = total + 5      if total > 8 then         total = 0      else         total = 10      end if

Mini-Exercise #3 -- Answer Suppose the computer executes the following statements. What is the value of total at the end?      total = 1      total = total + 5      if total > 8 then         total = 0      else         total = 10      end if total is 10