Question

a. What is the referencing environment at location 1 in the program below? Use static scope rules. Include what functions the variables are declared in.

void g ( int w)

{

var b = 1; var x = 8;

        void f()

        {

varb = 3;

                      x = b + 1;

print “f: x = “ , x ;

                       ….             < - - - - - - - - - - //Used in Prob a //location 1

          }

f();

print “g: w = “ , w ;

          print “ g: b = “, b ;

          print “g: x = “, x ;

}

void main()

{

var x = 5;

       g ( 3);

       print “main: x = “, x;    

}

b. What is the referencing environment at location 1 in the program below, when the program first gets to location 1? Use dynamic scope rules. Include what functions the variables are declared in.

var a = 20;//global

var x = 17;//global

function sub1( ) {

            var x = 7;sub2();

}

function sub2 ( ) {

            var y = x +1;a = a + 1;

print “sub2: sum = “, (x +y + a) ;    

                    …                                  < - - - - - - - - - - -//location 1 (used in Problem b)

}

main( ){

    var x = 3;var a = 5;sub1(); print “main: a = “, a

}

Answer 1

Problem a:

Static Scoping Rule: If an undeclared variable is encountered, it's definition is checked in the outer block. Note that a block is referred to statements between '{' and '}'.

In the location1, there are 2 variables, b is defined whereas x is undeclared/undefined. The definition of x is checked in the outer block, that is function g. And x is accordingly updated for function g.

Problem b:

Dynamic Scoping Rule: If an undeclared variable is encountered, it's definition is checked in the block from where it is called.

In location 1, we have variables, x, y, and a used in that particular block. Variable y is defined whereas variables x and a are not defined or declared. So their definitions are checked in the block from where they are called. In this code, the calling takes place in this fashion:

main() --> sub1() --> sub2()

Hence, the definition of undefined variables are checked in sub1(). In sub1(), we found the definition of x but not of a. Hence, we will move further to the block from where sub1() is called, that is, main() block. In main block, we found the definition of a. Note that even though there is another definition of variable x in main, we are not going to use it in sub2(). The definition of variable x in sub1() will only be used in sub2().