Concept of Progamming Languages 10th Edition (Robert W. Sebesta) International E

Question

Concept of Progamming Languages 10th Edition (Robert W. Sebesta) International Edition
CH5 => 6PS (Problem Set)

6. Consider the following JavaScript skeletal program:

//The main program
function sub1(){
var x;
   function sub2(){
...
   }
}
function sub3(){
....
}

Assume that the execution of this program is in the following unit order:
main calls sub1
sub1 calls sub2
sub2 calls sub3

Question:
a. assuming static scoping, in the following, which declaration of x is the correct one for reference to x?
i. sub1
   ii. sub2
   iii. sub3

b. Repeat part a, but assume dynamic scoping.

Explanation / Answer

Static Scoping –     Variables can be bound at compile time without regards to calling code.

Dynamic Scoping – Variable binding (context) can only be determined at the moment code is executed.

C, C++, C#, and JavaScript all use static scoping, whereas Perl and Logo allow dynamic scoping though their local keyword.

Examples:

Static Scoping

const int b=5;

int foo()

{ int a=b+5;

return a;}

int bar() {

int b=2;

return foo(); }

int main() {

foo(); //returns 10

bar();   //returns 10

return 0;

}

Dynamic Scoping

const int b=5;

int foo()

{ int a=b+5;

return a;}

int bar() {

int b=2;

return foo(); }

int main() {

foo(); //returns 10

bar();   //returns 7

return 0;

}

Now Consider the following JavaScript skeletal program:

//The main program
function sub1(){
var x;
   function sub2(){
...
   }
}
function sub3(){
....
}

In case of static scoping---

Here in case of function sub1 and sub2, the concept of nested scope will be applied i.e. the direct scope of x in function sub1 but it is also accessible in the nested scope function sub2, so for function sub1 and sub2 , the declaration of x in function sub1 is correct

but in case of function sub3 , the variable x would not be accessible , so for function sub3 the declaration of x in function sub1 is incorrect , it should be in main function

i.e. sub1: sub1

     sub2: sub1

     sub3: main

In case of dynamic scoping---

As for function sub1 and function sub2 , the concept of nested scope is applicable, so the result also remains the same i.e. for function sub1 and sub2 , the declaration of x in function sub1 is correct.

but for function sub3 in case of dynamic scoping, the correct declaration of x should be in function sub1 because here we are calling function sub3 from function sub2 , so function sub3 would inherit scope from function sub2 and function sub2 inheriting scope (nested scope) from function sub1 so ultimately function sub3 inheriting scope from function sub1

i.e. sub1: sub1.

     sub2: sub1

     sub3: sub1

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