HANDLING NONLOCAL REFERENCES

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1 SYMBOL TABLE A symbol table is a data structure kept by a translator that allows it to keep track of each declared name and its binding. Assume for now that each name is unique within its local scope. The data structure can be any implementation of a dictionary, where the name is the key. 1

2 HANDLING NONLOCAL REFERENCES 1. Each time a scope is entered, push a new dictionary onto the stack. 2. Each time a scope is exited, pop a dictionary off the top of the stack. 3. For each name declared, generate an appropriate binding and enter the name-binding pair into the dictionary on the top of the stack. 4. Given a name reference, search the dictionary on top of the stack: a) If found, return the binding. b) Otherwise, repeat the process on the next dictionary down in the stack. c) If the name is not found in any dictionary, report an error. 2

3 EXAMPLE DICTIONARY STACK 1 void sort (float a[ ], int size) { 2 int i, j; 3 for (i = 0; i < size; i++) 4 for (j = i + 1; j < size; j++) 5 if (a[j] < a[i]) { 6 float t; 7 t = a[i]; 8 a[i] = a[j]; 9 a[j] = t; 10 } 11 } At line 4 and 11: <j, 2> <i, 2> <size,1> <a, 1> <sort, 1> At line 7: <t, 6> <j, 2> <i, 2> <size,1> <a, 1> <sort, 1> 3

4 STATIC SCOPING For static scoping, the referencing environment for a name is its defining scope and all nested subscopes. The referencing environment defines the set of statements which can validly reference a name. 4

5 DYNAMIC SCOPING In dynamic scoping, a name is bound to its most recent declaration based on the program s call history. Used be early Lisp, APL, Snobol, Perl. Symbol table for each scope built at compile time, but managed at run time. Scope pushed/popped on stack when entered/exited. 5

6 1 int h, i; 2 void B(int w) { 3 int j, k; 4 i = 2*w; 5 w = w+1; } 8 void A (int x, int y) { 9 float i, j; 10 B(h); 11 i = 3; } 14 void main() { 15 int a, b; 16 h = 5; a = 3; b = 2; 17 A(a, b); 18 B(h); } 6

7 RUNTIME SYMBOL TABLE Call history main (17) A (10) B Function Dictionary B <w, 2> <j, 3> <k, 3> A <x, 8> <y, 8> <i, 9> <j, 9> main <a, 15> <b, 15> <h, 1> <i, 1> <B, 2> <A, 8> <main, 14> Reference to i (4) resolves to <i, 9> in A. 7

8 1 int h, i; 2 void B(int w) { 3 int j, k; 4 i = 2*w; 5 w = w+1; } 8 void A (int x, int y) { 9 float i, j; 10 B(h); 11 i = 3; } 14 void main() { 15 int a, b; 16 h = 5; a = 3; b = 2; 17 A(a, b); 18 B(h); } 8

9 RUNTIME SYMBOL TABLE (AGAIN) Same example: call history main (17) B Function Dictionary B <w, 2> <j, 3> <k, 3> main <a, 15> <b, 15> <h, 1> <i, 1> <B, 2> <A, 8> <main, 14> Reference to i (4) resolves to <i, 1> in global scope. 9

10 DISADVANTAGES OF DYNAMIC SCOPING Compromises the ability to statically type check references to non-local variables Variables in a function visible to other functions which call this function program less reliable Access to non-local variable follows chains of dynamic links more time consuming Therefore most modern languages prefer static scoping! 10

11 VISIBILITY A name is visible if its referencing environment includes the reference and the name is not redeclared in an inner scope. A name redeclared in an inner scope effectively hides the outer declaration. Some languages provide a mechanism for referencing a hidden name; e.g.: this.x in C++/Java. 11

12 EXAMPLE JAVA PROGRAM 1 public class Student { 2 private String name; 3 public Student (String name,...) { 4 this.name = name; } 7 } 12

13 OVERLOADING Overloading uses the number or type of parameters to distinguish among identical function names or operators. Examples: +, -, *, / can be float or int + can be float or int addition or string concatenation in Java System.out.print(x) in Java 13

14 LIFETIME The lifetime of a variable is the time interval during which the variable has been allocated a block of memory. Earliest languages used static allocation. Memory assigned at compile time Function only allocated space for arguments and return value recursive function not supported! Algol introduced the notion that memory should be allocated/deallocated at scope entry/exit. Modern languages are based on this idea but may break scope equals lifetime rule. 14

15 TYPES 15

16 BASICS A type is a collection of values and operations on those values. Example: Integer type has values..., -2, -1, 0, 1, 2,... and operations +, -, *, /, <,... The Boolean type has values true and false and operations,,. 16

17 BASICS Computer types have a finite number of values due to fixed size allocation; problematic for numeric types. Exceptions: Smalltalk uses unbounded fractions. Haskell type Integer represents unbounded integers. More problematic is the fixed sized floating point numbers 0.2 is not exact in binary. So 0.2 * 5 is not exactly 1.0 Floating point is inconsistent with real numbers in mathematics. 17

18 BASICS In the early languages, Fortran, Algol, Cobol, all of the types were built in. If needed a type color, could use integers; but what does it mean to multiply two colors? Purpose of types in programming languages is to provide ways of effectively modeling a problem solution. 18

19 TYPE ERRORS Machine data carries no type information. Basically, just a sequence of bits. Example: The floating point number The 32-bit integer 1,079,508,992 Two 16-bit integers and 0 Four ASCII X NUL NUL 19

20 TYPE ERRORS A type error is any error that arises because an operation is attempted on a data type for which it is undefined. Type errors are common in assembly language programming. High level languages reduce the number of type errors. A type system provides a basis for detecting type errors. 20

21 STATIC AND DYNAMIC TYPING A type system imposes constraints such as the values used in an addition must be numeric. Cannot be expressed syntactically in EBNF. Some languages perform type checking at compile time (e.g., C). Other languages (e.g., Perl) perform type checking at run time. Still others (e.g., Java) do both. 21

22 STATIC AND DYNAMIC TYPING A language is statically typed if the types of all variables are fixed when they are declared at compile time. A language is dynamically typed if the type of a variable can vary at run time depending on the value assigned. Can you give examples of each? Static: C/C++, Java, ML, Haskell Dynamic: Perl, Python, JavaScript, Prolog, Scheme 22

23 BASIC TYPES Terminology in use with current 32-bit computers: Nibble: 4 bits Byte: 8 bits Half-word: 16 bits Word: 32 bits Double word: 64 bits Quad word: 128 bits 23

24 FINITE SIZE IN TYPES Unlike mathematics: a + (b + c) (a + b) + c (why??) In most languages, the numeric types are finite in size. So a + b may overflow the finite range. Also in C-like languages, the equality and relational operators produce an int, not a Boolean 24

25 OVERLOADING An operator or function is overloaded when its meaning varies depending on the types of its operands or arguments or result. Java: a + b (ignoring size) integer add floating point add string concatenation Mixed mode: one operand an int, the other floating point 25

26 TYPE CONVERSION Type conversion is implicit or explicit change of the type of a value to a different one. Implicit conversion type coercion Explicit conversion type casting A type conversion is a narrowing conversion if the result type permits fewer bits, thus potentially losing information. Otherwise it is termed a widening conversion. Should languages ban implicit narrowing conversions? In PL/I: Why? Lose information declare (a) char (3); a = 123 ; a = a + 1; Unexpected results: 26

27 NONBASIC TYPES Enumeration: enum day {Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday}; enum day myday = Wednesday; In C/C++ the above values of this type are 0,..., 6. More powerful in Java: for (day d : day.values()) Sytem.out.println(d); 27

28 POINTERS C, C++, Ada, Pascal Java??? Value is a memory address Indirect referencing Operator in C: * 28

29 EQUIVALENCE OF ARRAYS AND POINTERS IN C float sum(float a[ ], int n) { int i; float s = 0.0; for (i = 0; i<n; i++) return s; s += a[i]; float sum(float *a, int n) { int i; float s = 0.0; for (i = 0; i<n; i++) s += *a++; return s; 29

30 STRCPY EXAMPLE void strcpy(char *p, char *q) { } while (*p++ = *q++) ; 30

31 POINTER OPERATIONS If T is a type and ref T is a pointer: & : T ref T * : ref T T For an arbitrary variable x: *(&x) = x 31

32 PITFALLS OF POINTERS Bane of reliable software development Error-prone Buffer overflow, memory leaks Particularly troublesome in C Other languages such as Java, Haskell, ML and Prolog completely remove pointers from the vocabulary of the language However internally these languages still makes heavy use of pointers Still can do dynamic allocation/deallocation of memory 32

33 ARRAYS AND LISTS Array: Indexed sequences of values of the same type. Example: int a[10]; float x[3][5]; /* odd syntax vs. math */ char s[40]; /* indices: 0... n-1 */ Default one-dimensional, but possible to have array of arrays: x[3][5] is an array of 5 3-element arrays (odd syntax!) 33

34 INDEXING Only operation for many languages Type signature [ ] : T[ ] x int T Example float x[3] [5]; type of x: float[ ][ ] type of x[1]: float[ ] type of x[1][2]: float 34

35 STRUCTURES Analogous to a tuple in mathematics Collection of elements of different types Used first in Cobol, PL/I Absent from Fortran, Algol 60 Common to Pascal-like, C-like languages Omitted from Java as redundant 35

36 STRUCTURE (CONT D) struct employeetype { int id; char name[26]; int age; float salary; char dept; }; struct employeetype employee;... employee.age = 45; Number of bytes employeetype requires? (assuming 32-bit int) Answer: 39 bytes Memory allocation may have special requirement Reverse order of allocation 32-bit int or float must be allocated at address multiple of 4 How many bytes are needed? Answer: 44 bytes 36

37 UNIONS C: union Pascal: case-variant record Logically: multiple views of same storage Useful in some systems applications 37

38 FUNCTIONS AS TYPES First-class citizens : Can be assigned a value Can be passed as an argument to a function Pascal example: function newton(a, b: real; function f: real): real; Know that f returns a real value, but the arguments to f are unspecified. Java interface class: public interface RootSolvable { double valueat(double x); } public double Newton(double a, double b, RootSolvable f); 38

39 SUBTYPES A subtype is a type that has certain constraints placed on its values or operations. In Ada subtypes can be directly specified: subtype one_to_ten is Integer range ; type Day is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday); subtype Weekend is Day range Saturday.. Sunday; type Salary is delta 0.01 digits 9 range _999_999.99; subtype Author_Salary is Salary digits 5 range ; 39

40 SUBTYPES (CONT D) Java implements subtypes using class hierarchy: Integer i = new Integer(3);... Number v = i;... Integer x = (Integer) v; //Integer is a subclass of Number, // and therefore a subtype Type cast required In general, t: T, s: S, t = s is possible only if: T = S, or S is a subtype of T 40

41 POLYMORPHISM AND GENERICS Greek: poly means many, morph means form/shape A function or operation is polymorphic if it can be applied to any one of several related types and achieve the same result. An advantage of polymorphism is that it enables code reuse. 41

42 POLYMORPHISM Example: overloaded built-in operators and functions + - * / ==!=... Java: + also used for string concatenation Ada 83 Ada, C++: define for new types Java overloaded methods: number or type of parameters Example: class PrintStream print, println defined for: boolean, char, int, long, float, double, char[ ] String, Object 42

43 SEMANTICS 43

44 MOTIVATION To provide an authoritative definition of the meaning of all language constructs for: Programmers Compiler writers Standards developers A programming language is complete only when its syntax, type system, and semantics are well-defined. 44

45 MOTIVATION (CONT D) Semantics is a precise definition of the meaning of a syntactically and type-wise correct program. Ideas of meaning : Operational Semantics: The meaning attached by compiling using compiler C and executing using machine M. Ex: Fortran on IBM 709. Axiomatic Semantics: Program Verification Denotational Semantics: Statements as state transforming functions High level mathematic precision 45

46 EXPRESSION SEMANTICS Expression Semantics: Operators + associativity + precedence Evaluation orders Pure vs. Impure 46

47 AN EXAMPLE Infix (a + b) (c * d) Infix uses associativity and precedence to disambiguate. Polish Prefix: - + a b * c d Polish Postfix: a b + c d * - Same symbol can t be used for operations of different arities Cambridge Polish: (- (+ a b) (* c d)) 47

48 SHORT CIRCUIT EVALUATION Traditionally, a op b is evaluated as eval (a) and eval (b) and then op Optimization: a and b evaluated as: if a then b else false a or b evaluated as: if a then true else b Also known as lazy evaluation: b can be undefined 48

49 SIDE EFFECT A change to any non-local variable or I/O. What is the value of: i = 2; b = 2; c = 5; a = b * i++ + c * i; Depends on which one is evaluated first: b * i++ or c * I Side Effects make a language impure and should be avoided if possible! 49

50 PROGRAM STATE (IMPERATIVE & OO) The state of a program is the bindings of all active variables and their current values. Maps: the pairing of active variable names with specific memory locations environment the pairing of active memory locations with their current values memory state = memory environment 50

51 PROGRAM STATE (CONT D) The current statement (portion of an abstract syntax tree) to be executed in a program is interpreted relative to the current state. The individual steps that occur during a program run can be viewed as a series of state transformations. For the purposes of this lecture, use only a map from a variable to its value; like a debugger watch window, tied to a particular statement. 51

52 THE FACTORIAL PROGRAM // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } 52

53 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f undef undef undef 53

54 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f 3 undef undef 54

55 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f 3 1 undef 55

56 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f

57 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f

58 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f

59 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f

60 // compute the factorial of n 1 void main ( ) { 2 int n, i, f; 3 n = 3; 4 i = 1; 5 f = 1; 6 while (i < n) { 7 i = i + 1; 8 f = f * i; 9 } 10 } n i f

61 ASSIGNMENT SEMANTICS Fundamental to imperative and object-oriented programming Issues Multiple assignment Assignment statement vs. expression Copy vs. reference semantics Semantics of assignment: Evaluate the source expression a value Replace the value of the target variable a state 61

62 COPY VS. REFERENCE SEMANTICS Copy: a = b; a, b have same value. Changes to either have no effect on other. Used in imperative languages. Reference a, b point to the same object. A change in object state affects both Used by many object-oriented languages. 62

63 COPY VS. REFERENCE SEMANTICS public void add (Object word, Object number) { Vector set = (Vector) dict.get(word); if (set == null) { // not in Concordance set = new Vector( ); dict.put(word, set); } if (allowdupl!set.contains(number)) set.addelement(number); } //Under copy semantics, number will not be added to the dictionary, because set is an object referenced from dict. 63

64 CONTROL FLOW SEMANTICS To be complete, an imperative language needs: Statement sequencing Conditional statement Looping statement 64

65 SEQUENCE s1 s2 Semantics: in the absence of a branch (return, break, continue, etc.): First execute s1 Then execute s2 Output state of s1 is the input state of s2 65

66 CONDITIONAL IfStatement if ( Expresion ) Statement [ else Statement ] Example: if (a > b) else z = a; z = b; If the test expression is true then the output state of the conditional is the output state of the then branch Else: the output state of the conditional is the output state of the else branch 66

67 LOOPS WhileStatement while ( Expression ) Statement The expression is evaluated. If it is true, first the statement is executed, and then the loop is executed again. Otherwise the loop terminates. Foreach loop: An iterator is any finite set of values over which a loop can be repeated. 67

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