Context-Free Languages

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Context-Free Languages

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Regular Languages
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Context-Free Languages
Regular Languages
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Context-Free Languages
Context-Free Grammars
Pushdown Automata
stack
automaton
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Context-Free Grammars

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Example

A context-free grammar
A derivation
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A context-free grammar
Another derivation
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(((( ))))
Describes parentheses
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Example
A context-free grammar
A derivation
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A context-free grammar
Another derivation
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Example
A context-free grammar
A derivation
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A context-free grammar
A derivation
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Describes matched parentheses
() ((( ))) (( ))
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Definition Context-Free Grammars
Grammar
Variables
Terminal symbols
Start variable
Productions of the form
String of variables and terminals
Variable
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Definition Context-Free Languages

• A language is context-free
• if and only if
• there is a context-free grammar
• with

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Derivation Order

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Derivation Trees

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Derivation Tree
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Derivation Tree
yield
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Partial Derivation Trees

Partial derivation tree
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Partial derivation tree
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sentential form

Partial derivation tree
yield
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Sometimes, derivation order doesnt matter
Leftmost

Rightmost
Same derivation tree
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So far, we concentrated on generative aspect of
grammars.How about analytical aspect?Parsing..

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Parser
input string
derivation
grammar
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Example
Parser
derivation
input
?
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Exhaustive Search
Phase 1
Find derivation of
All possible derivations of length 1
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Phase 2
Phase 1
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Phase 2
Phase 3
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Final result of exhaustive search
(top-down parsing)
Parser
input
derivation
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Time complexity of exhaustive search
Suppose there are no productions of the form
Number of phases for string is Why?
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For grammar with rules
Time for phase 1
possible derivations
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Time for phase 2
possible derivations
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Time for phase
possible derivations which is exponential in the
length of w
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Total time needed for parsing
phase 1
phase 2w
phase 2
Extremely bad!!!
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There exist faster algorithms for specialized
grammars
S-grammar
symbol
string of variables
appears once
Pair
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S-grammar example
Each string has a unique derivation
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For S-grammars
In the exhaustive search parsing there is only
one choice in each phase
Time for a phase
Total time for parsing string
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For general context-free grammars
There exists a parsing algorithm that parses a
string in time
(we will show it in the next class)
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Ambiguity

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leftmost derivation
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leftmost derivation
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The grammar
is ambiguous
since there is a string, namely
, which has two derivation trees
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The grammar
is ambiguous
since string
has two leftmost derivations
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Definition
A context-free grammar is ambiguous if
some string has two or more
distinct derivation trees
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Alternatively we may say
Ambiguity of a grammar implies the existence
of two or more leftmost (or rightmost)
derivations
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Why do we care about ambiguity?
take
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Correct result
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• Ambiguity is bad for programming languages

• We want to remove ambiguity

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We may be able to fix the ambiguity
New non-ambiguous grammar
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Unique derivation tree
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The grammar
is non-ambiguous
Every string has a unique
derivation tree
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Another Ambiguous Grammar
IF_STMT
if EXPR then STMT
if EXPR then STMT else STMT
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if expr1 then if expr2 then stmt1 else stmt2
IF_STMT
if
expr1
then
STMT
else
if
expr2
then
stmt1
stmt2
IF_STMT
if
expr1
then
else
STMT
stmt2
if
expr2
then
stmt1
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Inherent Ambiguity

• Some context-free languages
• have only ambiguous grammars

Example
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The string
has two derivation trees
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Compilers

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Machine Code
Program
Add v,v,0 cmp v,5 jmplt ELSE THEN add x,
12,v ELSE WHILE cmp x,3 ...
v 5 if (vgt5) x 12 v while (x !3)
x x - 3 v 10 ......
Compiler
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Compiler
Lexical analyzer
parser
input
output
machine code
program
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A parser knows the grammar of the programming
language
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Parser
PROGRAM STMT_LIST STMT_LIST STMT
STMT_LIST STMT STMT EXPR IF_STMT
WHILE_STMT
STMT_LIST EXPR EXPR EXPR EXPR - EXPR
ID IF_STMT if (EXPR) then STMT
if (EXPR) then STMT else STMT WHILE_STMT
while (EXPR) do STMT
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The parser finds the derivation of a particular
input
derivation
Parser
input
E gt E E gt E E E gt 10 EE gt
10 2 E gt 10 2 5
E -gt E E E E INT
10 2 5
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derivation tree
derivation
E
E gt E E gt E E E gt 10 EE gt
10 2 E gt 10 2 5

E
E
10
E
E

2
5
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derivation tree
E
machine code

E
E
mult a, 2, 5 add b, 10, a
10
E
E

2
5