1. Scope

2. Scope
Variable scope defines the range of statements over which a variable is visible
Local variables of a program unit X are variables that are declared within X and therefore visible within X
Non-local variables of a program unit X are variables that are visible but not declared within X

3. Static Scope
Static scope indicates that the scope of a variable can be determined before execution
How?
Search local declarations first, then search in increasingly larger enclosing scopes, until a valid declaration is found
Variable x becomes hidden
Not valid in Java or C#
main() { /** C **/
int x = 5;
printf("%d\n", x); {
int x = 10;
int y = 20;
int x = 15;
int z = 20; }

4. Static Scope
Static scope indicates that the scope of a variable can be determined before execution
JavaScript and PHP do not support nested static scopes
But JavaScript and PHP do support function-level scope
var abc = false;
if ( s == "HELLO" ) {
abc = true;
var abc = 500;
document.write("abc is " + abc) }
document.write("abc is still " + abc)

5. Dynamic Scoping
Dynamic scoping defines a variable’s scope based on the calling sequences of subprograms
Used in APL, SNOBOL4, early versions of LISP
procedure Main is
X : Integer;
procedure Sub1 is
begin -- of Sub1
... X ...
end; -- of Sub1
procedure Sub2 is
begin -- of Sub2
...
end; -- of Sub2
begin -- of Main
end; -- of Main
Call Main, which calls Sub1
Call Main, which calls Sub2, which then calls Sub1
scope of X can only be
determined at run time

6. repeat using dynamic scoping
Problem 8
Given this Ada code:
Assume execution of this code is Main calls Sub1 Sub1 calls Sub2 Sub2 calls Sub3
Using static scoping, which X is referenced in Sub1? In Sub2? In Sub3?
procedure Main is
X : Integer;
procedure Sub3; -- allows Sub1 to
-- to call Sub3
procedure Sub1 is
procedure Sub2 is
begin -- of Sub2
...
end; -- of Sub2
begin -- of Sub1
end; -- of Sub1
procedure Sub3 is
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main
repeat using dynamic scoping

7. SOLUTIONS Problem 8 Given this Ada code:
Assume execution of this code is Main calls Sub1 Sub1 calls Sub2 Sub2 calls Sub3
Using static scoping
Sub1 references Sub1.X
Sub2 references Sub1.X
Sub3 references Main.X
procedure Main is
X : Integer;
procedure Sub3; -- allows Sub1 to
-- to call Sub3
procedure Sub1 is
procedure Sub2 is
begin -- of Sub2
...
end; -- of Sub2
begin -- of Sub1
end; -- of Sub1
procedure Sub3 is
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main

8. SOLUTIONS Problem 8 Given this Ada code:
Assume execution of this code is Main calls Sub1 Sub1 calls Sub2 Sub2 calls Sub3
Using dynamic scoping
Sub1 references Sub1.X
Sub2 references Sub1.X
Sub3 references Sub1.X
procedure Main is
X : Integer;
procedure Sub3; -- allows Sub1 to
-- to call Sub3
procedure Sub1 is
procedure Sub2 is
begin -- of Sub2
...
end; -- of Sub2
begin -- of Sub1
end; -- of Sub1
procedure Sub3 is
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main

9. Problem 9 Given this Ada code:
Assume program execution using static-scoping rules
What is the output of this program?
What is the output using dynamic-scoping rules?
procedure Main is
X : Integer;
procedure SubA is
begin -- of SubA
Put(X); -- i.e. output X
end; -- of SubA
procedure SubB is
begin -- of SubB
X := 10;
SubA
end; -- of SubB
begin -- of Main
X := 5;
SubB
end; -- of Main

10. SOLUTIONS Problem 9 Given this Ada code:
Assume program execution using static-scoping rules
What is the output of this program? 5
What is the output using dynamic-scoping? 10
procedure Main is
X : Integer;
procedure SubA is
begin -- of SubA
Put(X); -- i.e. output X
end; -- of SubA
procedure SubB is
begin -- of SubB
X := 10;
SubA
end; -- of SubB
begin -- of Main
X := 5;
SubB
end; -- of Main

11. Problem 10 Given this Ada code:
procedure Main is
X, Y, Z : Integer;
procedure Sub1 is
A, Y, Z : Integer;
procedure Sub2 is
A, B, Z : Integer;
begin -- of Sub2
...
end; -- of Sub2
begin -- of Sub1
end; -- of Sub1
procedure Sub3 is
A, X, W : Integer;
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main
Given this Ada code:
Assume program execution using static-scoping rules
For Sub1, Sub2, and Sub3, list the visible variables and the program units within which they are declared

12. SOLUTIONS Problem 10 Given this Ada code:
procedure Main is
X, Y, Z : Integer;
procedure Sub1 is
A, Y, Z : Integer;
procedure Sub2 is
A, B, Z : Integer;
begin -- of Sub2
...
end; -- of Sub2
begin -- of Sub1
end; -- of Sub1
procedure Sub3 is
A, X, W : Integer;
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main
Given this Ada code:
Using static-scoping rules, list visible variables and the program units in which they are declared
Sub1: A (Sub1), Y (Sub1), Z (Sub1), X (Main)
Sub2: A (Sub2), B (Sub2), Z (Sub2), Y (Sub1), X (Main)
Sub3: A (Sub3), X (Sub3), W (Sub3), Y (Main), Z (Main)

13. Problem 11 Given this Ada code:
procedure Main is
X, Y, Z : Integer;
procedure Sub1 is
A, Y, Z : Integer;
begin -- of Sub1
...
end; -- of Sub1
procedure Sub2 is
A, X, W : Integer;
procedure Sub3 is
A, B, Z : Integer;
begin -- of Sub3
end; -- of Sub3
begin -- of Sub2
end; -- of Sub2
begin -- of Main
end; -- of Main
Given this Ada code:
Assume program execution using static-scoping rules
For Sub1, Sub2, and Sub3, list the visible variables and the program units within which they are declared

14. SOLUTIONS Problem 11 Given this Ada code:
procedure Main is
X, Y, Z : Integer;
procedure Sub1 is
A, Y, Z : Integer;
begin -- of Sub1
...
end; -- of Sub1
procedure Sub2 is
A, X, W : Integer;
procedure Sub3 is
A, B, Z : Integer;
begin -- of Sub3
end; -- of Sub3
begin -- of Sub2
end; -- of Sub2
begin -- of Main
end; -- of Main
Given this Ada code:
Using static-scoping rules, list visible variables and the program units in which they are declared
Sub1: A (Sub1), Y (Sub1), Z (Sub1), X(Main)
Sub2: A (Sub2), X (Sub2), W (Sub2), Z (Main), Y (Main)
Sub3: A (Sub3), B (Sub3), Z (Sub3), X (Sub2), W (Sub2), Y (Main)

15. Problem 12 Given this C code:
For each of the marked points A, B, C, and D, list each visible variable and the number of the declaration statement
void doSomething() {
int a, b, c; /** 1 **/
...
while ( ... )
int b, c, d; /** 2 **/
Point A
int c, d, e; /** 3 **/
Point B }
Point C
Point D

16. SOLUTIONS Problem 12 Given this C code:
For points A, B, C, and D, list each visible variable and the number of the declaration statement
A: a (1), b (2), c (2), d (2)
B: a (1), b (2), c (3), d (3), e (3)
C: a (1), b (2), c (2), d (2)
D: a (1), b (1), c (1)
void doSomething() {
int a, b, c; /** 1 **/
...
while ( ... )
int b, c, d; /** 2 **/
Point A
int c, d, e; /** 3 **/
Point B }
Point C
Point D

17. Example Given this C-like code: Assume dynamic-scoping rules
Given the following call sequences, what variables are visible during execution of the last function called ?
Also specify the name of the function in which each variable was defined
main() calls f1(), which calls f3()
main() calls f2(), which calls f3(), which calls f1()
main() calls f3(), which calls f1()
void main() {
int a, b, c;
... }
void f1() {
int b, c, d;
void f2() {
int c, d, e;
void f3() {
int d, e, f;

18. SOLUTIONS Example Given this C-like code: Assume dynamic-scoping rules
Given the following call sequences, what variables are visible during execution of the last function called ?
main() calls f1(), which calls f3()
In f3(): d, e, f declared in f3() b, c declared in f1() a declared in main()
void main() {
int a, b, c;
... }
void f1() {
int b, c, d;
void f2() {
int c, d, e;
void f3() {
int d, e, f;

19. SOLUTIONS Example Given this C-like code: Assume dynamic-scoping rules
Given the following call sequences, what variables are visible during execution of the last function called ?
main() calls f2(), which calls f3(), which calls f1()
In f1(): b, c, d declared in f1() e, f declared in f3() a declared in main()
void main() {
int a, b, c;
... }
void f1() {
int b, c, d;
void f2() {
int c, d, e;
void f3() {
int d, e, f;

20. SOLUTIONS Example Given this C-like code: Assume dynamic-scoping rules
Given the following call sequences, what variables are visible during execution of the last function called ?
main() calls f3(), which calls f1()
In f1(): b, c, d declared in f1() e, f declared in f3() a declared in main()
void main() {
int a, b, c;
... }
void f1() {
int b, c, d;
void f2() {
int c, d, e;
void f3() {
int d, e, f;

21. Problem 14 Given this Ada code: Assume dynamic-scoping rules
procedure Main is
X, Y, Z : Integer;
procedure Sub1 is
A, Y, Z : Integer;
begin -- of Sub1
...
end; -- of Sub1
procedure Sub2 is
A, B, Z : Integer;
begin -- of Sub2
end; -- of Sub2
procedure Sub3 is
A, X, W : Integer;
begin -- of Sub3
end; -- of Sub3
begin -- of Main
end; -- of Main
Given this Ada code:
Assume dynamic-scoping rules
Given the following call sequences, what variables are visible during execution of the last function called ?
Also specify the name of the function in which each variable was defined
main() calls Sub1(), which calls
Sub2(), which calls Sub3()
main() calls Sub1() , which calls
Sub3()
main() calls Sub2(), which calls
Sub3() , which calls Sub3()