CHAPTER 4 VARIABLES & BINDING SUNG-DONG KIM DEPT. OF COMPUTER ENGINEERING, HANSUNG UNIVERSITY

ABSTRACT  Identifier  Variable  Declaration  Binding  Assignment Statement  Alias (2013-1) Understanding of Programming Languages 2

ABSTRACT  Scope  Initialization  Reference Environment  Memory binding & Variable types (2013-1) Understanding of Programming Languages 3

ABSTRACT  Basic features of variables  attribute ( 속성 ) Address Value Type Scope Lifetime Type checking Initialization (2013-1) Understanding of Programming Languages 4

ABSTRACT  Binding (2013-1) Understanding of Programming Languages 5 variable feature_value_1 feature_value_2 … feature_value_n

IDENTIFIER  Character string for identifying program’s elements or objects  Consideration Maximum length Character set Case sensitiveness (2013-1) Understanding of Programming Languages 6

VARIABLE (1)  A location in memory Store data value  Attributes Name Address: l-value Scope Lifetime Value: r-value Type (2013-1) Understanding of Programming Languages 7

VARIABLE (2)  Example (2013-1) Understanding of Programming Languages 8 var sum: integer; sum := 0; 이름 : sum 유형 : 정수 값 : 0 주소 ( 참조 ) 속성 (attribute) 값속성 결정 시간 이름 (name) sum 컴파일 시간 유형 (type) 정수형컴파일 시간 주소 (address) ?Loading time 값 (value) 0 실행 시간 Binding Binding time

VARIABLE (3)  Other attributes Internal representation for integer 16 bit, 32 bit, … 1’s complement, 2’s complement Maximum/minimum value of integer type variable FORTRAN: -32,768 ~ 32,767 Implementation time, design(definition) time All possible operations for integer type variable Definition(design) time (2013-1) Understanding of Programming Languages 9

DECLARATION (1)  Declaration Let compiler know the information about data attributes  Example: int a[10]; Array’s name: a Type of array a: integer Dimension: 1 # of elements in array: 10 Index range: 0 ~ 9 (2013-1) Understanding of Programming Languages 10

DECLARATION (2)  Role Efficient memory management Static type checking (2013-1) Understanding of Programming Languages 11

DECLARATION (3)  Implicit declaration FORTRAN: variable name beginning with I ~ N  integer BASIC Integer variable: ends with % Character variable: ends with $ Perl: $: array (2013-1) Understanding of Programming Languages 12

BINDING (1)  Binding Assign attribute value  Binding time Design time Implementation time Translation time Execution time (2013-1) Understanding of Programming Languages 13

BINDING (2)  Example: min := min + 5; (2013-1) Understanding of Programming Languages 14 binding 되는 속성 binding time 변수 min 의 가능한 변수 유형 변수 min 의 유형 (type) 변수 min 이 가질 수 있는 값의 범위 변수 min 의 주소 값 변수 min 의 값 연산자 + 의 의미의 집합 연산자 + 의 연산 종류 5 의 내부적 표현 언어 설계 시간 컴파일 시간 언어 설계시간 or 구현시간 프로그램 적재 시간 언어 실행 시간 언어 설계 시간 컴파일 시간 언어 구현 시간

BINDING (3)  Types Dynamic binding Runtime binding Easy programming Program’s flexibility Interpreter languages: LISP, Perl, Prolog, … (2013-1) Understanding of Programming Languages 15

BINDING (4) Static binding Compile time binding Explicit declaration for variables Determine attributes in compile time Efficient code Compile languages: FORTRAN, Pascal, … (2013-1) Understanding of Programming Languages 16

BINDING (5)  Example of flexibility Perl (2013-1) Understanding of Programming Languages 17 $price = 1000; $price = $price + 100; $price = “Very expensive”;

ASSIGNMENT STATEMENT (1)  Assignment statement Change variable’s content (value) Perl: ($a, $b) = ($x, $y);  l-value Address  r-value Value (2013-1) Understanding of Programming Languages 18

ASSIGNMENT STATEMENT (2)  Features All variables have l-value and r-value Constant: only r-value Pointer variable: int *ip l-value: ip’s address r-value: other variable’s address (l-value) A[i] l-value: address of A’s ith element r-value: value (2013-1) Understanding of Programming Languages 19

(2013-1) Understanding of Programming Languages 20 int a = 1, b = 2; a = b; Case a: b: 2 2 a: b: int *a = 1, *b = 2; a = b; Case 2 a: b: 1 2 a: b: 2 int *a = 1, *b = 2; *a = *b; Case 3 a: b: 1 2 a: b: 2 2

(2013-1) Understanding of Programming Languages a: b: p1: p2: 1 2 :a :b p1: p2: 1 2 :a :b

ALIAS (1)  Alias More than two names for one variable More than two names for one memory location (2013-1) Understanding of Programming Languages 22

ALIAS (2)  Alias generation Pointer variable in C EQUIVALENCE, COMMON in FORTRAN Parameters (2013-1) Understanding of Programming Languages 23 void main() { int *ip, i = 3; ip = &i; *ip = 10; printf(“%d”, i); }

SCOPE (1)  Scope Range of statements where the variables can be accessed Visible: variable can be access (referenced) (2013-1) Understanding of Programming Languages 24

SCOPE (2)  Example 1 Area A int a, b float c, d Area B int a char b, c float d (2013-1) Understanding of Programming Languages 25 float a, b, c, d; void main() { int a, b; a = b = 12; … { char b, c; … } … } Area A Area B Area A

SCOPE (3)  Example 2 main: a, b, Large Large: a, b, x, y, Large, sub1 sub1: a, b, x, y, z, Large, sub1 (2013-1) Understanding of Programming Languages 26 program main; var a, b : integer; procedure Large; var x, y : integer; procedure sub1; var z : integer; begin { sub1 } end; begin { Large } end; begin { main } end. main Large sub1

SCOPE (4)  Static scope rule Scope: nesting relation between blocks Non-local variables: variables declared in nearest nesting block Determine scope at compile time C, C++, Pascal, … (2013-1) Understanding of Programming Languages 27

SCOPE (5)  Dynamic scope rule Function call order Determine scope at runtime Non-local variable: search variables in calling functions APL, SNOBOL4, … (2013-1) Understanding of Programming Languages 28

SCOPE (6)  Example Static scope rule print(x): 2 Dynamic scope rule print(x): 4 (2013-1) Understanding of Programming Languages 29 program scope_rule; var x: integer; procedure sub1; begin print(x) end; procedure sub2; var x: integer; begin x := 4; sub1 end; begin x := 2; sub2 end. scope_rule sub1 sub2

SCOPE (7)  Scope & Lifetime Lifetime Memory allocation time ~ memory return time Block structure language: scope = lifetime static in C, C++ Scope: static, local to function Lifetime: until the program ends (2013-1) Understanding of Programming Languages 30

SCOPE (8) Example 1 Scope and lifetime of temp: if { } (2013-1) Understanding of Programming Languages 31 if (a[j] > a[k]) { int temp; temp = a[j]; a[j] = a[k]; a[k] = temp; }

SCOPE (9) Example 2 (2013-1) Understanding of Programming Languages 32 void sub() { static int j = 1; int k = 1; printf(“j = %d k = %d\n”, j, k); j++; k++; } void main() { sub(); } j = 1k = 1 j = 2k = 1 j = 3 k = 1

INITIALIZATION  Assign value when declaring the variable  Memory binding time = value binding time  Pascal, Modula-2: no variable initialization (2013-1) Understanding of Programming Languages 33

REFERENTIAL ENVIRONMENT  All visible (usable) names(data, variables, functions)  Static scope language Variables in local area + visible variables in parent area  Dynamic scope language Variables in local area + variables in other currently activating subprograms (2013-1) Understanding of Programming Languages 34

(2013-1) Understanding of Programming Languages 35 program example; var a, b : integer; … procedure sub1; var x, y : integer; begin { sub1 } … end; { sub1 } procedure sub2; var x : integer; … procedure sub3; var x : integer; begin { sub3 } … end; { sub3} begin { sub2 } … end; { sub2 } begin { example } … end. { example } example sub1 sub2 sub : sub1 의 x, y, example 의 a, b 2: sub3 의 x, example 의 a, b 3: sub2 의 x, example 의 a, b 4: example 의 a, b

(2013-1) Understanding of Programming Languages 36 void sub1() { int a, b; … } void sub2() { int b, c; … sub1(); } void main() { int c, d; … sub2(); } : sub1 의 a, b, sub2 의 c, main 의 d 2: sub2 의 b, c, main 의 d 3: main 의 c, d

MEMORY BINDING & VARIABLE TYPES (1)  Static variables Binding occurs before execution Binding remains until program termination Global variables Efficiency Disadvantages Low flexibility  no recursion Variables can’t share memory location C, C++, Java: static (2013-1) Understanding of Programming Languages 37

MEMORY BINDING & VARIABLE TYPES (2)  Stack-dynamic variables Binding on the declaration statement Type: static binding Runtime stack Recursive program Subprogram requires its own memory for its local variables Less efficient: memory allocation/free C, C++: local variables (2013-1) Understanding of Programming Languages 38

MEMORY BINDING & VARIABLE TYPES (3)  Explicit heap-dynamic variables Explicit runtime instruction Heap Access by only pointer, reference variables C: malloc(), free() function C++: new, delete operator (2013-1) Understanding of Programming Languages 39