Scope

1. Scope

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

3. Static Scope • Static scope indicates that the scope of a variablecan be determined before execution • How? • Search local declarations first,then search in increasinglylarger enclosing scopes, untila 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; printf("%d\n", x); { int x = 15; int z = 20; printf("%d\n", x); } } }

4. Static Scope • Static scope indicates that the scope of a variablecan 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. scope of X can only be determined at run time Dynamic Scoping • Dynamic scoping defines a variable’s scope basedon 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 X : Integer; 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

6. Problem 8 • Given this Ada code: • Assume execution ofthis code isMain calls Sub1Sub1 calls Sub2Sub2 calls Sub3 • Using static scoping,which X is referencedin Sub1? In Sub2?In Sub3? procedure Main is X : Integer; procedure Sub3; -- allows Sub1 to -- to call Sub3 procedure Sub1 is X : Integer; 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 usingdynamic scoping

7. Problem 8 SOLUTIONS • Given this Ada code: • Assume execution ofthis code isMain calls Sub1Sub1 calls Sub2Sub2 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 X : Integer; 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. Problem 8 SOLUTIONS • Given this Ada code: • Assume execution ofthis code isMain calls Sub1Sub1 calls Sub2Sub2 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 X : Integer; 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 programexecution usingstatic-scoping rules • What is the outputof this program? • What is the outputusing dynamic-scopingrules? procedure Main is X : Integer; procedure SubA is begin -- of SubA Put(X); -- i.e. output X end; -- of SubA procedure SubB is X : Integer; 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 programexecution usingstatic-scoping rules • What is the outputof this program? • 5 • What is the outputusing 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 X : Integer; 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: • Assume program executionusing static-scoping rules • For Sub1, Sub2, and Sub3,list the visible variables andthe program units within whichthey are declared 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

12. SOLUTIONS Problem 10 • Given this Ada code: • Using static-scoping rules, listvisible variables and the programunits 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) 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

13. Problem 11 • Given this Ada code: • Assume program executionusing static-scoping rules • For Sub1, Sub2, and Sub3,list the visible variables andthe program units within whichthey are declared 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

14. SOLUTIONS Problem 11 • Given this Ada code: • Using static-scoping rules, listvisible variables and the programunits 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) 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

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

16. Problem 12 SOLUTIONS • Given this C code: • For points A, B, C, and D, list each visible variableand the number of thedeclaration 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 while ( ... ) { 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 duringexecution of the last function called? • Also specify the name of the functionin 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. Example SOLUTIONS • Given this C-like code: • Assume dynamic-scoping rules • Given the following call sequences,what variables are visible duringexecution 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. Example SOLUTIONS • Given this C-like code: • Assume dynamic-scoping rules • Given the following call sequences,what variables are visible duringexecution 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. Example SOLUTIONS • Given this C-like code: • Assume dynamic-scoping rules • Given the following call sequences,what variables are visible duringexecution 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 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 duringexecution of the last function called? • Also specify the name of the functionin 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()