0

Using (Dynamic scoping)

procedure Main is
     X, Y, Z : Integer;
     procedure Suba is
        A, Y, X : Integer;
        begin
          -- Suba body
        end;
     procedure Subb is
        A, B, Z : Integer;
        begin
        -- Subb body
        end;
     procedure Subc
        A, X, W : Integer;
        begin
        --- Subc body
        end;
        begin
        --- Main body
        end;

The question is: For the calling sequence, state which variables are visible during the execution of the last procedure Main calls Suba; Suba calls Subb; Subb calls Subc

Why is the answer: Suba: A, X, W Subb: B, Z Subc: Y

I tried working through it and I just don't see how that is the answer can anyone provide an explanation

Achim Zeileis
  • 15,710
  • 1
  • 39
  • 49
complexityyy
  • 123
  • 4

1 Answers1

0

Each call "inherits" the variables visible by its caller, and introduces its own variables. If a call introduces a variable with the same name as one in the caller, the caller's variable is shadowed. Picture the layers like this, as visible from inside Subc:

Main                       X   Y   Z
Suba         A                 Y   Z
Subb         A   B                 Z
Subc         A          W  X

The variables you can see are the first ones in each column looking up from the bottom. This would suggest that from inside the call to Subc, you can see the following:

* `A`, `W`, and `X` as defined by `Subc`.
* `B` and `Z` as defined by `Subb`.
* `Y` as defined by `Suba`.

This reverses the rolls of Suba and Subc as you mention in the question. That answer doesn't really make sense, because Suba doesn't define W at all.

chepner
  • 497,756
  • 71
  • 530
  • 681