| 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. |
| 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? | ||
| 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 | |||
| 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 $8.41 or a time zone or an action | |||
| In computing names can change values | |||
| Example: Alpha and Bet in Alphabetize CDs changed | |||
| Names must change values in a program because programs specify a transformation of input into output … as the transformation proceeds the things named change values | |||
| variable is the term for program names that can change value | |||
| The term “variable” reminds us the value can change | |||
| The names used for variables are arbitrary, provided: | |||
| Variable names must begin with a letter | |||
| Variable names can contain any letter, numeral or _ | |||
| Variable names should be meaningful and accurate | |||
| total, averageOverClass,
average_over_class but not o0OO0o, bet. Also (for now) not i, n, x, etc. |
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| Most languages are case sensitive: a ¹ A | |||
| A variable can be thought of as a “named container” | ||
Variables name computer memory locations, so the value of a variable is the quantity stored in its memory |
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| Variables can take on different types of values | ||
| Whole numbers or integers: 2, -9, 1048576 | ||
| Character sequences or strings: “2”, “&^%$#@”, “ ” | ||
| 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 | ||
| 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 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. | ||
| 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 |
| 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 | ||
| 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 |
| 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 |
| 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 | |||
| 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 | ||
| 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 | |||
| 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 | ||
| An advantage of the general conditional is that the statement lists can contain other conditionals | |||
| 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 |
| 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 |