Question

(8) (3 marks) Write BNF grammar rules for a language that is comprised only of sentences described as follows: symbol, fol symbols orjust onéx symbol, A sequence of one or more occurrences of an a lowed by either zero or morez followed by a sequence of one or more b symbols. (9) (1 mark) The following fragment of grammar describes the syntax of the exponentiation operator. What is the associativity ot the operator as detined here? factor | expr factor expr (10) (1 mark) What are derivation order do they attempt to construct for the sentence being parsed? the two major styles of parser? In each case, what (11) (1 mark) Name a variable attribute which is bound at (a) compile time (b) link time (c) run time (12) (1 mark) Define what is meant by the lifetime of a variable. (13) (1mark) Describe one of the common programming errors which can arise when using pointers. (14) (2 marks) Consider the array x: Ita double occupies 8 bytes, and accessis in row major order, what is the byte ottset (the number of bytes from the beginning of the array, starting trom zero) ot the element x[7,6]? array [0..10, 5..10] of double (15) (1 mark) What advantage is gained by providing ser-located loop control statements in a programming language? (16) (2 marks) Using an example, explain how operand evaluation order can attect the result of expression evaluation when a functional side ettect is present

Answer 1

(8) BNF

A sequence is

• one or more occurrences of an a symbol, followed by
• either 0 or more z symbols or just one x symbol, followed by
• a sequence is one or more occurrences of b symbol

<sequence> :: = <a_symbol>+ ( <z_symbol>* | <x_symbol> ) <b_symbol>+

<a_symbol> :: = a

<z_symbol> :: = z

<x_symbol> :: = x

<b_symbol> :: = b

OR

<sequence> :: = a+ ( z* | x ) b+

where,

• + is used to indicate one or more occurrences,
• * is used to indicate 0 or more occurrences,
• | is used to indicate or.

(9) Associativity

factor --> expr ** factor | expr

'factor' is to the right side of the operator => right associativity

The factor has to be expanded first to evaluate the expression - >right associative

(10) Major styles of parser

• Top - down parser

Starts with start symbol and transforms it into input string.

Derivation order : Leftmost derivation order

• Bottom - up parser

Starts with input string and goes up to the start symbol.

Derivation order : Rightmost derivation in reverse order

(11) Variable bound at

• Compile time : binding to a datatype
• Link time : binding static variable to memory
• Run time : binding non static local variable to memory

(12) Lifetime of a program variable:

Indicates which part of program the variable is active (valid memory)