1. Chapter 4 Variables & Binding
Sung-Dong Kim
Dept. of Computer Engineering,
Hansung University

2. Abstract Identifier Variable Declaration Binding Assignment Statement
Alias
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3. Abstract Scope Initialization Reference Environment
Memory binding & Variable types
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4. Abstract Basic features of variables  attribute (속성) Address Value
Type
Scope
Lifetime
Type checking
Initialization
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5. Abstract Binding variable feature_value_1 feature_value_2 …
feature_value_n
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6. Identifier
Character string for identifying program’s elements or objects
Consideration
Maximum length
Character set
Case sensitiveness
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7. Variable (1) A location in memory Attributes Store data value Name
Address: l-value
Scope
Lifetime
Value: r-value
Type
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8. <D.W.Barron’s variable definition>
Example
var sum: integer;
sum := 0;
이름: sum
유형: 정수
값: 0
주소(참조)
<D.W.Barron’s variable definition>
속성 (attribute) 값
속성 결정 시간
이름 (name)
sum
컴파일 시간
유형 (type)
정수형
주소 (address) ?
Loading time
값 (value)
실행 시간
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9. 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
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10. Declaration (1) Declaration Example: int a[10];
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
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11. Declaration (2) Role Efficient memory management Static type checking
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12. Declaration (3) Implicit declaration
FORTRAN: variable name beginning with I ~ N  integer
BASIC
Integer variable: ends with %
Character variable: ends with $
Perl:
$: scalar
@: array
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13. Binding (1) Binding Binding time Assign attribute value Design time
Implementation time
Translation time
Execution time
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14. Binding (2) Example: min := min + 5; binding 되는 속성 binding time
변수 min의 유형 (type)
변수 min이 가질 수 있는 값의 범위
변수 min의 주소 값
변수 min의 값
연산자 +의 의미의 집합
연산자 +의 연산 종류
5의 내부적 표현
언어 설계 시간
컴파일 시간
언어 설계시간 or 구현시간
프로그램 적재 시간
언어 실행 시간
언어 구현 시간
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15. Binding (3) Types Dynamic binding Runtime binding Easy programming
Program’s flexibility
Interpreter languages: LISP, Perl, Prolog, …
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16. Binding (4) Static binding Compile time binding
Explicit declaration for variables
Determine attributes in compile time
Efficient code
Compile languages: FORTRAN, Pascal, …
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17. Binding (5) Example of flexibility Perl $price = 1000;
$price = $price + 100;
$price = “Very expensive”;
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18. Assignment statement (1)
Change variable’s content (value)
Perl: ($a, $b) = ($x, $y);
l-value
Address
r-value
Value
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19. 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
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20. int a = 1, b = 2; a = b; Case 1 a: 1 a: 2 b: 2 b: 2
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21. Alias (1) Alias More than two names for one variable
More than two names for one memory location
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22. Alias (2) Alias generation Pointer variable in C
EQUIVALENCE, COMMON in FORTRAN
Parameters
void main() {
int *ip, i = 3;
ip = &i;
*ip = 10;
printf(“%d”, i); }
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23. Scope (1)
Scope
Range of statements where the variables can be accessed
Visible: variable can be access (referenced)
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24. Scope (2) Example 1 Area A Area B int a, b float c, d int a char b, c
float d
float a, b, c, d;
void main() {
int a, b;
a = b = 12; …
{ char b, c; }
Area A
Area B
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25. Scope (3) Example 2 main: a, b, Large Large: a, b, x, y, Large, sub1
sub1: a, b, x, y, z, Large, sub1
program main;
var a, b : integer;
procedure Large;
var x, y : integer;
procedure sub1;
var z : integer;
begin
{ sub1 }
end;
{ Large }
{ main }
end.
main
Large
sub1
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26. 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, …
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27. Scope (5) Dynamic scope rule Function call order
Determine scope at runtime
Non-local variable: search variables in calling functions
APL, SNOBOL4, …
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28. Scope (6) Example Static scope rule Dynamic scope rule print(x): 2
program scope_rule;
var x: integer;
procedure sub1;
begin
print(x)
end;
procedure sub2;
x := 4;
sub1
x := 2;
sub2
end.
scope_rule
sub1
sub2
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29. 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
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30. Scope (8) Example 1 Scope and lifetime of temp: if { }
if (a[j] > a[k]) {
int temp;
temp = a[j];
a[j] = a[k];
a[k] = temp; }
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31. Scope (9) Example 2 void sub() { static int j = 1; int k = 1;
printf(“j = %d k = %d\n”, j, k);
j++; k++; }
void main() {
sub();
j = 1 k = 1
j = 2 k = 1
j = 3 k = 1
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32. Initialization Assign value when declaring the variable
Memory binding time = value binding time
Pascal, Modula-2: no variable initialization
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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
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34. sub1 sub2 sub3 program example; var a, b : integer; … procedure sub1;
var x, y : integer;
begin { sub1 }
end; { sub1 }
procedure sub2;
var x : integer;
procedure sub3;
begin { sub3 }
end; { sub3}
begin { sub2 }
end; { sub2 }
begin { example }
end. { example }
example
sub1
sub2
sub3 1
1: sub1의 x, y, example의 a, b
2: sub3의 x, example의 a, b
3: sub2의 x, example의 a, b
4: example의 a, b 2 3 4
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35. void sub1() { int a, b; … } void sub2() { 1 int b, c; sub1();
void main() {
int c, d;
sub2(); 1 2 3
1: sub1의 a, b, sub2의 c, main의 d
2: sub2의 b, c, main의 d
3: main의 c, d
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36. 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
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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
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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
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