chapter 5

Question 1

What is a variable?

Answer 1

An abstraction of a memory cell.

Question 2

What are the six attributes of variables?

Answer 2

• Name
• Aliases
• Value
• Type
• Lifetime
• Scope

Question 3

What is the primary difference in scope for a local variable declared in a block {} in C++ vs. C#?

Answer 3

The identifier used to refer to the variable.

Question 4

What does ‘Address’ refer to in variable attributes?

Answer 4

The memory address associated with the variable, which can change during execution.

Question 5

What are ‘Aliases’ in relation to variables?

Answer 5

When two variable names refer to the same memory location.

Question 6

What does ‘Value’ represent in variable attributes?

Answer 6

The content stored in the memory location(s) the variable is associated with.

Question 7

What is the difference between a Keyword and a Reserved Word?

Answer 7

Keyword: Special meaning only in certain contexts. Reserved Word: Cannot be used as a user-defined name in any context (has special meaning everywhere).

Question 8

Define ‘Lifetime’ in the context of variables.

Answer 8

The time during which a variable is bound to a specific memory cell.

Question 9

Define Binding.

Answer 9

An association between an attribute (like type or value) and an entity (like a variable or function name).

Question 10

What is the difference between a Keyword and a Reserved Word?

Answer 10

Keyword: Special meaning only in certain contexts. Reserved Word: Cannot be used as a user-defined name in any context (has special meaning everywhere).

Question 11

Define Alias.

Answer 11

When two or more variable names (or references) can be used to access the same memory location.

Question 12

Why are Aliases generally considered bad for readability and reliability?

Answer 12

Readability: Hard to track changes as a value can change through multiple names. Reliability: Changes through one name can have unexpected effects when accessing through another.

Question 13

Define Binding Time.

Answer 13

The time at which a binding takes place.

Question 14

Define ‘Binding’.

Answer 14

An association between an attribute and an entity.

Question 15

What is the difference between Static Binding and Dynamic Binding?

Answer 15

Static Binding: Occurs before runtime and remains unchanged throughout execution. Dynamic Binding: Occurs during execution or can change during execution.

Question 16

List the different binding times.

Answer 16

• Language Design Time
• Language Implementation Time
• Compile Time
• Load Time
• Runtime

Question 17

What is the difference between Static Binding and Dynamic Binding?

Answer 17

Static Binding occurs before runtime and remains unchanged, while Dynamic Binding occurs during execution.

Question 18

What is a Named Constant?

Answer 18

A variable that is bound to a value only once and cannot be changed after initialization.

Question 19

What is Static Type Binding?

Answer 19

Binding that happens before runtime, usually at compile time.

Question 20

What is the advantage of Static Type Binding?

Answer 20

Writability.

Question 21

What is Type Inferencing?

Answer 21

Type is determined from the variable’s initial value or usage context.

Question 22

Define Dynamic Type Binding.

Answer 22

Binding that happens during runtime, allowing a variable to hold values of different types at different times.

Question 23

What are the disadvantages of Dynamic Type Binding?

Answer 23

• Cost (runtime type checking)
• Difficult for compiler/interpreter to detect type errors

Question 24

What is Storage Binding?

Answer 24

Associating a variable with a memory cell.

Question 25

What are the categories of variables by lifetimes?

Answer 25

* Static Variables * Stack-Dynamic Variables * Explicit Heap-Dynamic Variables * Implicit Heap-Dynamic Variables

Question 26

What is a Named Constant?

Answer 26

A variable that is bound to a value only once and cannot be changed after initialisation.

Question 27

How is the Referencing Environment determined in a language with Static Scope?

Answer 27

Back: Local variables + visible variables in all textually enclosing static scopes.

Question 28

Define Explicit Heap-Dynamic Variables.

Answer 28

The collection of all names that are visible in that statement.

Question 29

Which type of scope is generally considered more reliable and easier to read?

Answer 29

Static Scope.

Question 30

What is the difference between Static Scope and Dynamic Scope? Back: Static Scope: Based on the textual layout of the code (search outwards through enclosing blocks/functions).

Answer 30

Dynamic Scope: Based on the calling sequence of program units at runtime (search backwards through the call stack).

Question 31

Define Scope.

Answer 31

The range of statements over which a variable is visible (can be referenced).

Question 32

What is a disadvantage of Static variables (compared to Stack-Dynamic)?

Answer 32

Lack of flexibility (no recursion support), storage cannot be shared.

Question 33

What is an advantage of Stack-Dynamic variables (compared to Static)?

Answer 33

Allows recursion, conserves storage (memory can be shared).

Question 34

Name and briefly describe the four categories of variables based on their lifetimes.

Answer 34

1. Static: Bound to memory before execution, stays bound throughout. 2. Stack-Dynamic: Storage allocated when declaration is elaborated (e.g., function call), deallocated when scope exited. 3. Explicit Heap-Dynamic: Allocated/deallocated by explicit programmer code (new/delete). 4. Implicit Heap-Dynamic: Allocation/deallocation caused by assignment statements

Question 35

Define Lifetime of a variable.

Answer 35

The time during which a variable is bound to a specific memory cell.

Question 36

What is the distinction between Scope and Lifetime?

Answer 36

Scope is the textual region where a name is visible, while Lifetime is the time a variable is bound to a memory cell.

Question 37

What is a Referencing Environment?

Answer 37

The collection of all names that are visible in a statement.

Question 38

What is Type Inferencing?

Answer 38

The compiler determines a variable's type from its initial value or context, binding it statically. (Differs from dynamic binding because the type is fixed statically).

Question 39

List one advantage and one disadvantage of Dynamic Type Binding

Answer 39

Advantage: Flexibility (a variable can hold different types). Disadvantage: Less reliable (type errors caught only at runtime), Higher cost (runtime type checking).

Question 40

What is Static Type Binding?

Answer 40

The type of a variable is determined before runtime and remains fixed.

Question 41

What is Dynamic Type Binding?

Answer 41

The type of a variable is determined during runtime by assignment, and can change during execution.

Question 42

How does Prolog's scope rule work (Static or Dynamic)?

Answer 42

Static Scope. Visibility is determined by the program text structure.