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| == The Problem ==
| | #REDIRECT [[ist:Introduction to Syntax/Exercise 1]] |
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| Consider the following grammar:
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| bexpr -> bexpr or bexpr | bterm
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| bterm -> bterm and bterm | bfactor
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| bfactor -> not bfactor | ( bexpr ) | true | false
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| # Identify the terminal and non-terminal symbols of the grammar.
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| # Show that the grammar is ambiguous by deriving two different trees for the same input sequence.
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| # Write a non-ambiguous grammar for the same language.
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| # Build the tree corresponding to the analysis of the following input sequence: '''not ( true or false and true )''' | |
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| == Solution ==
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| 1. The terminal symbols are all the symbols not defined by a rule: '''<tt>or</tt>''', '''<tt>and</tt>''', '''<tt>not</tt>''', '''<tt>(</tt>''', '''<tt>)</tt>''', '''<tt>true</tt>''', '''<tt>false</tt>'''.
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| 2. The following trees are possible for '''true and false and true''' (an analogous analysis could be done for '''or'''). Terminals are in blue and non-terminals in black.
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| <graph>
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| graph nfa {
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| node [shape=plaintext]
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| 0 [id=0,label="bexpr"];
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| 1 [id=1,label="bterm"];
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| 2 [id=2,label="bterm"];
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| 3 [id=3,label="bterm"];
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| 4 [id=4,label="bfactor"];
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| 5 [id=5,label="true", fontcolor=blue];
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| 6 [id=6,label="and", fontcolor=blue];
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| 7 [id=7,label="bterm"];
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| 8 [id=8,label="bfactor"];
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| 9 [id=9,label="false", fontcolor=blue];
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| 10 [id=10,label="and", fontcolor=blue];
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| 11 [id=11,label="bterm"];
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| 12 [id=12,label="bfactor"];
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| 13 [id=13,label="true", fontcolor=blue];
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|
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| 20 [id=20,label="bexpr"];
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| 21 [id=21,label="bterm"];
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| 22 [id=22,label="bterm"];
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| 23 [id=23,label="bfactor"];
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| 24 [id=24,label="true", fontcolor=blue];
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| 25 [id=25,label="and", fontcolor=blue];
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| 26 [id=26,label="bterm"];
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| 27 [id=27,label="bterm"];
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| 28 [id=28,label="bfactor"];
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| 29 [id=29,label="false", fontcolor=blue];
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| 30 [id=30,label="and", fontcolor=blue];
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| 31 [id=31,label="bterm"];
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| 32 [id=32,label="bfactor"];
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| 33 [id=33,label="true", fontcolor=blue];
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| { rank = same; 5; 6; 9; 10; 13; 24; 25; 29; 30; 33 }
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| { rank = same; 4; 8; 12; 23; 28; 32 }
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| 0 -- 1
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| 1-- 2
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| 1-- 10
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| 1 -- 11
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| 2 -- 3 -- 4 -- 5
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| 2 -- 6
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| 2 -- 7 -- 8 -- 9
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| 11 -- 12 -- 13
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| 20 -- 21
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| 21-- 22
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| 21-- 25
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| 21-- 26
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| 22 -- 23 -- 24
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| 26 -- 27 -- 28 -- 29
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| 26 -- 30
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| 26 -- 31 -- 32 -- 33
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| }
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| </graph>
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| 3. The following grammar selects left associativity for the binary operators ('''and''' and '''or'''):
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| bexpr -> bexpr or bterm | bterm
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| bterm -> bterm and bfactor | bfactor
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| bfactor -> not bfactor | ( bexpr ) | true | false
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| 4. The following analysis considers the grammar defined in 3.
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| <graph>
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| graph nfa {
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| node [shape=plaintext]
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| 0 [id=0,label="bexpr"];
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| 1 [id=1,label="bterm"];
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| 2 [id=2,label="bfactor"];
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| 3 [id=3,label="not",fontcolor=blue];
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| 4 [id=4,label="bfactor"];
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| 5 [id=5,label="(", fontcolor=blue];
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| 6 [id=6,label="bexpr"];
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| 7 [id=7,label="bexpr"];
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| 8 [id=8,label="bterm"];
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| 9 [id=9,label="bfactor"];
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| 10 [id=10,label="true", fontcolor=blue];
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| 11 [id=11,label="or", fontcolor=blue];
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| 12 [id=12,label="bterm"];
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| 13 [id=13,label="bterm"];
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| 14 [id=14,label="bfactor"];
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| 15 [id=15,label="false", fontcolor=blue];
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| 16 [id=16,label="and", fontcolor=blue];
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| 17 [id=17,label="bfactor"];
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| 18 [id=18,label="true", fontcolor=blue];
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| 19 [id=19,label=")", fontcolor=blue];
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| { rank = same; 3; 5; 10; 11; 15; 16; 18; 19 }
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| { rank = same; 9; 14; 17 }
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| 0 -- 1 -- 2
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| 2 -- 3
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| 2 -- 4
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| 4 -- 5
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| 4 -- 6
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| 6 -- 7 -- 8 -- 9 -- 10
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| 6 -- 11
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| 6 -- 12
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| 12 -- 13 -- 14 -- 15
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| 12 -- 16
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| 12 -- 17 -- 18
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| 4 -- 19
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| }
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| </graph>
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| [[category:Compilers]]
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| [[category:Teaching]]
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