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PPL: Local referencing environments

A LECTURE VIDEO ON LOCAL REFERENCING ENVIRONMENTS

LOCAL REFERENCING ENVIRONMENTS

Local Variables
  • Vars that are defined inside subprograms are called local vars.
  • Local vars can be either static or stack dynamic.
  • By default an variable in stack dynamic.
Advantages of using stack dynamic variable:
a. Support for recursion.
b. Storage for locals is shared among some subprograms.

Disadvantages of using stack dynamic variable:
a. Allocation/deallocation time required.
b. Indirect addressing “only determined during execution.”
c. Subprograms cannot be history sensitive “can’t retain data values of local vars
between calls.”

Advantages of using static variable:
a. Static local vars can be accessed faster because there is no indirection.
b. No run-time overhead for allocation and deallocation.
c. Allow subprograms to be history sensitive.

Disadvantages of using static variable:
a. Inability to support recursion.
b. Their storage can’t be shared with the local vars of other inactive subprograms. 

Example for  stack dynamic variable:
int addition(int a, int b) {
int sum = 0;
sum = a+b;
return sum;
}

Example for  static variable:
//static variable uses static keyword
int addition(int a, int b) {
static int sum = 0;
sum = a+b;
return sum;
}

Nested Subprograms

  • Residing of a subprogram under another subprogram is known as nested subprogram.
  • When a subprogram is placed within another subprogram,it gets hide from the rest of the program.
  • Algol 68, Pascal, Ada, JavaScript, Python, Ruby allows nesting of subprogram.
  • C, do not allow subprogram nesting.

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