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  1. #include <string>
  2. #include <iostream>
  3.  
  4. using namespace std;
  5.  
  6. enum lex_t {
  7. 	INVALID,
  8. 	THE,
  9. 	NOUN,
  10. 	VERB,
  11. 	CONJ,
  12. 	DOT,
  13. };
  14.  
  15. string word;
  16. lex_t token;
  17.  
  18. lex_t word_token( const string& w )
  19. {
  20. 	if( w == "the" ) {
  21. 		return THE;
  22. 	}
  23.  
  24. 	if( w == "birds" ||
  25. 		w == "fish" ||
  26. 		w == "C++"
  27. 	) {
  28. 		return NOUN;
  29. 	}
  30.  
  31. 	if( w == "fly" ||
  32. 		w == "rules" ||
  33. 		w == "swim" 
  34. 	) {
  35. 		return VERB;
  36. 	}
  37.  
  38. 	if( w == "and" ||
  39. 		w == "or" ||
  40. 		w == "but" 
  41. 	) {
  42. 		return CONJ;
  43. 	}
  44.  
  45. 	if( w == "." ) {
  46. 		return DOT;
  47. 	}
  48.  
  49. 	return INVALID;
  50. }
  51.  
  52. void next_word()
  53. {
  54. 	cin >> word;
  55. 	token = word_token( word );
  56. }
  57.  
  58. bool maybe( lex_t t )
  59. {
  60. 	if( token == t ) {
  61. 		next_word();
  62. 	}
  63. 	return true;
  64. }
  65.  
  66. bool match( lex_t t )
  67. {
  68. 	if( token == t ) {
  69. 		next_word();
  70. 		return true;
  71. 	}
  72. 	return false;
  73. }
  74.  
  75. void skip( lex_t t )
  76. {
  77. 	while( !match( t ) ) {
  78. 		next_word();
  79. 	}
  80. }
  81.  
  82. bool sentence()
  83. {
  84. 	if( maybe( THE ) && match( NOUN ) && match( VERB ) ) {
  85. 		if( match( CONJ ) ) {
  86. 			return sentence();
  87. 		}
  88. 		if( match( DOT ) ) {
  89. 			return true;
  90. 		}
  91. 		return false;
  92. 	}
  93. 	skip( DOT );
  94. 	return false;
  95. }
  96.  
  97. int main() {
  98. 	next_word();
  99. 	while( cin ) {
  100. 		cout << boolalpha << sentence() << endl;
  101. 	}
  102. }
Success #stdin #stdout 0s 15240KB
comments (?)
stdin 
birds fly but fish swim .
the fish fly .
the the C++ rules .
birds fly
stdout 
true
true
false
false
https://ideone.com/X26IdT
LL(1) demo
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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