Question

1B. Explain the pointers used in the buffer pairs. Also explain how these pointers in the 2M buffer pair overcome the drawbacks of single input buffer with respect to the following expression. pos-i*

Answer 1

Two pointers are used in buffer pairs:

lexemeBegin Pointer:

It marks the beginning of the current lexeme, whose extent you are attempting to determine.

forward Pointer:

Scans until a pattern match is identified(found).

Once the next lexeme is determined, forward is set to the character at its right end.

After the lexeme is recorded as an attribute value a token returned to the parser, lexemeBegin is set to the character immediately after the lexeme that is identified.

A buffer is divided into two N-character halves

Each buffer is of the same size N. N is the number of characters on one block(1024 or 4096 bytes).

Using one system read command we can read N characters into a buffer.

If fewer than N characters remain in the input file, then a special character, represented by eof, marks the end of the source.

Because the amount of time taken to process characters and the larger no. of. characters that are done during the compilation of a large source program, specialized buffering techniques have been developed to reduce the amount of overhead required to process a single input character.

The unary, postfix operator + represents the positive closure of a regular expression and its language.

If r is a regular expression, then ( r ) + denotes the language (L(r)) + .

The operator + has the same precedence and associativity as the operator *.

Two useful algebraic laws, r* = r+1e and r+ = rr* = r*r relate the Kleene closure and positive closure.

You can do by quoting the character within a string of length one or more than that.

For example,the regular expression "**" matches the string **.

We can also get the meaning of an operator character by preceding it by a /.

So, the regular expression 1 * 1 * also matches the string **