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Erik Jonsson School of Engineering and Computer Science

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### Context-free Languages

### Pushdown Automata

### PUMPING LEMMA

### Ambiguity

### TWO’S COMPLEMENTS

### PRACTICE

Erik Jonsson School of Engineering

and Computer Science

CS 4384– 001

Automata Theory

http://www.utdallas.edu/~pervin

Tuesday: Context-Free Languages

Chapter Three – Review All

Thursday4-03-14

FEARLESS Engineering

A search of Albert Einstein's archived manuscripts last year turned up a draft of an early unpublished paper by the famed physicist that concluded, mistakenly, that new matter, such as stars and galaxies, would appear to fill the expanding universe. Einstein was attempting to make sense of Edwin Hubble's observation that the universe was expanding, but he made a mathematical error. When he discovered his mistake, he set aside the paper, which is now in a digital archive of his papers at the Hebrew University in Jerusalem. Waterford Institute of Technology physicist Cormac O'Raifeartaigh and colleagues found the paper while scouring the archive.

Chapter Three

Left-recursion

3-04-14

Many similar definitions

Context-Free

NOTE: CFLs are not closed under intersection (or complement)!

Read and push a’s onto the stack; read b’s and match them against the stack until empty; read any additional b’s and push them onto the stack; read and match c’s with stack exactly.

For each a read, non-deterministically push ONE or TWO a’s on the stack; then match b’s read exactly with stack.

Dangling ELSE Problem

“An ELSE belongs to the last preceding IF for which there is no ELSE already”

End of File (EOF):

UNIX Control-D (ASCII 04)

Windows Control-Z (ASCII 26)

Java’s Buffered Reader Null (ASCII 0)

Empty Stack ($), (Zo)

End of File; Empty StackS -> aSb | ab => S -> X_aY | X_aX_b ; Y -> SX_b;

X_a -> a ; X_b -> b

An automaton whose output response is limited to a simple “yes” or “no” is called an accepter. Presented with an input string, an accepter either accepts (recognizes) the string or rejects it.

- A more general automaton, capable of producing strings of symbols as output, is called a transducer.

Linz, P. 26 (Pushdown Transducer)

CHAPTER_ONE

Parsing

LL(k) Grammars

CFG+PDA+CFL+PL

(Nothing “useless”)

Example 3.4.1 on P. 105ff M&S

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