Question

What does the term context mean in context-free and context-sensitive languages? Can a variable have multiple contexts? If I need to store a particular value in a particular memory address how does does affect the context of the memory address? And if I want to update a variable how does the context of the variable change?

Answer 1

A context-sensitive grammar, productions have the general form

aBc -> ab'c

where upper-case symbols are non-terminals and lower-case symbols are sequences of terminals and non-terminals.

In the above example, a and b would be the context of the non-terminal B and indicate that B may “expand” to b' only when surrounded by a and c.

In a context free grammar, the production might look like

B -> b'

i.e. B “expands” to b' regardless of whatever is around it, in other words, regardless of context.