Call by binding [was Re: [Tutor] beginning to code]

[Marko Rauhamaa]

Antoon Pardon <antoon.pardon at vub.be>:

> Op 25-09-17 om 13:32 schreef Marko Rauhamaa:
>> In Python, assignment "mutates the target" as well. It's only that in
>> Python, the target is always a pointer.
>
> Fine if you want to word it like that, the assignments in Pascal and
> Python are still sufficiently different.
>
> If you do A := B in Pascal, you have two different entities with equal
> values.
>
> If you do A = B in Python, you have one entity that is refered to by
> two variables/names.

Python only operates with pointers. You can operate with pointers in
Pascal as well.

Say I have (in Pascal):

   new(A);
   B := A

Then, in Pascal, you have one entity that is referred to by two
variables/names.

> The difference becomes clear if you later mutate A or B. In the case
> of Pascal, you will have mutated one of two entities and the other
> entity remains the same. In the case of Python, you will have mutated
> the one entity that both A and B refer to and so the mutations will be
> visible through the other variable/name.

Continuing the Pascal example above:

   new(A);
   B := A;     { one entity, two names }
   A^ := 3;    { mutate the one entity }
   writeln(B);
   A^ := 7;    { mutate the one entity again }
   writeln(B)

will output

   3
   7

(although I haven't programmed anything in Pascal for than 30 years so
my syntax may be off).


Marko