Understanding Syntax and Semantics in Programming

1. CHAPTER 2 Variables, Expressions, & Output

2. Syntax vs. Semantics • Syntax • language rules • how valid instructions are written in a programming language • GRAMMAR English

3. Semantics • rules determining MEANING • of instructions in programming language

4. Identifiers

5. What is an identifier? • A name associated with a process or object used to refer to that process or object • John, Sue, Larry • mass, volume, pay

6. What are rules for valid identifiers? • Identifiers are made of • letters • digits • underscore • The first character must be a • letter • underscore

7. VALID IDENTIFIERS • sum_of_squares J9 GetData • box_22A count Bin3D4 • INVALID • 40Hours Get Data Box-22 • stock_# if return • RESERVED WORDS • word that has special meaning in cannot be used as identifier (if)

8. Why is it important to use meaningful identifier names? • Easier to understand • Self documenting • Compare printtopportion PRINTTOPPORTION PrintTopPortion • Last is best

9. Case Sensitive • Identifiers with different capitalization are entirely distinct. • Total total • Two different identifiers.

10. Variable Declarations

11. What is a declaration? • Associating identifier (name) with • data object • function • So programmer • can refer to that item • by name (naming object)

12. What is a variable? • A location in memory • referenced by an identifier • in which a data value is stored • value can change

13. Example Declarations • var Identifier [, Identifier, …]; • var empNum; • var studentCount, maxScore, sumOfScores; • or better • var studentCount; // Description • var maxScore; // Description 2 • var sumOfScores; // etc. • best, declare when needed

14. What Does a Variable Declaration Do? var ageOfDog; var taxRateY2K; var middleInitial; • Tells the compiler to • allocate enough memoryto hold value of this type • associate the identifierwith this location. 4 bytes for taxRateY2K 1 byte for middleInitial

15. When to Declare? • Declare vars when needed • NOT at top of the main block

16. Assignment Statements

17. What is an assignment statement? • A statement • gives the value of an expression • to a variable.

18. Valid Assignment Statements • Variable = Expression; • var num; var alpha; • var rate; var ch; • alpha = 2856; • rate = 0.36; • ch = 'B'; • num = alpha;

19. Valid Expressions • constant variable • constant and/or variables combined with operators • Examples • alpha + 2 rate - 6.0 • 4 - alpha rate • alpha * num 12

20. Unary and Binary Operators • Unary Operator • an operator that has just one operand • + - • Binary Operator • an operator that has two operands • + - * / %

21. Modulo Arithmetic (%) • % operator • yields remainder of integer division • Use with integers only • 5 % 3 = 2 • 9 % 4 = 1 • 8 % 2 = 0 • 3 % 5 = 3

22. 3 + 6 = ? 9 3.4 - 6.1 = ? -2.7 2 * 3 = ? 6 8 / 2 = ? 4 8.0 / 2.0 = ? 4.0 8 / 8 = ? 1

23. 8 / 9 = ? 0 8 / 7 = ? 1 8 % 8 = ? 0 8 % 9 = ? 8 8 % 7 = ? 1 0 % 7 = ? 0

24. Valid Assignment Statements • Variable = Expression; • var num = i; var alpha = j; • alpha = num + 6; • alpha = num / 2; • num = alpha * 2; • num = 6 % alpha;

25. What is a literal value? • Any constant value written in a program. • Integers, real numbers, characters, strings of characters: • 34 921.2196 • 'D' "Hi There"

26. Output

27. How do you output data? • println("Hello"); • println (Processing, not JS) • Writes to the text area of the Processing environment's console. • Each call to this function creates a new line of output. • Individual elements can be separated with quotes ("") and joined with the string concatenation operator (+).

28. Items are separated by + • literals are in quotes, • variable names or expressions allowed. • println(ExprOrString + ExprOrString);

29. Examples: • var x; var y; var z; • x = 1; y = 2.3; z = 'A'; • println("x =" + x); • println("y =" + y + "z =" + z); • println("***"); • Output: x =1 y =2.3z =A *** • Insert white spaces in the output where needed!

30. var x = -12345; • var y = 1.23456789; • var z = 'B'; • println(x + " " + y + " " + z); Output: -12345 1.23457 B

31. The assignment statement is not an equality. • Var = Expr; • The assignment statement • takes the value of the expression • and assigns it to the variable. • It does NOT determine if the values are equal.

32. Assignment Statement not equality • alpha = alpha + 1; • --Legal assignment, not equality • num = num + alpha;

33. Increment and Decrement • ++ -- • num++; • ++num; ===> num = num + 1; • num--; • --num; ===> num = num - 1; • Use as standalone statements.

34. String Literals • a string is a sequence of characters enclosed in double quotes • string literal sample values "Hello" "Year 2000" "1234" • the empty string (null string) contains no characters and is written as ""

35. String Concatenation (+) • concatenation is a binary operation that uses the + operator • at least one of the operands must be a string variable or named constant--the other operand can be string type or var type

36. Concatenation Example var WHEN = "Tomorrow" ; var EXCLAMATION = '!' ; var message1 ; var message2 ; message1 = "Yesterday " ; message2 = "and " ; message1 = message1 + message2 + WHEN + EXCLAMATION ; println(message1);

37. How does one add comments to a program? • Comments giving explanatory notes • They may appear anywhere in a program. • /* */ around text. • // Comment the rest of a line • It is good programming style to add descriptive comments to your program. • Paste document.cpp to top of program