Unit 7 - Variables

Question 1

In imperative programming, a variable

Answer 1

stands for a computer memory cell (or a collection of memory cells).

Such a cell can store different values at different times and
thus it makes sense to assign a new value to a variable, overwriting the old value. It is therefore called a mutable variable.

Question 2

A variable in functional or logical programming

Answer 2

refers to a single value during its lifetime, although unknown at the time of writing. A mathematical variable cannot change its value once it is defined. It is therefore called an immutable variable.

Question 3

An imperative variable can be characterised by the
following six aspects:

Answer 3

• Name, ie an identifier used in the program;
• Scope, ie the portion of the program where one is allowed to refer to the variable
  using its name;
• Type, ie a classifying type restricting the values (optional);
• Lifetime(s), ie zero, one or more periods of time during execution when a piece of
  memory exists for storing a value of this variable;
• Reference(s), ie the address of a piece of memory where a value of the variable is
  stored; this could be either one reference for each lifetime or a relative reference
  that applies to multiple lifetimes;
• Value(s) (at each moment, there is one value for each lifetime).

Question 4

A variable declaration is a piece of program text which

Answer 4

• announces the existence of a variable with a given name,
• (optionally) states the type of the variable and
• (optionally) assigns an initial value to the variable.

Question 5

The scope of a variable is the

Answer 5

portion of the program in which the programmer can refer to the variable.

Question 6

If a scope is determined statically

Answer 6

it is usually defined by the smallest block that contains its declaration.

Question 7

Languages in which variables have no classifying type

Answer 7

Lisp, Python,
JavaScript and PHP

Question 8

Static type checking

Answer 8

The compiler tries to analyse all assignments to the variable and checks that the values assigned will always be compatible with the variable’s classifying type.

Question 9

Dynamic type checking

Answer 9

The compiler inserts a code that will check every assignment at run time.

Question 10

a classifying type is a subtype of another one if

Answer 10

its permitted values are also permitted values of the supertype.