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  1. /* Lex program to recognize valid arithmetic expression
  2. 		and identify the identifiers and operators */
  3. %{
  4. #include <stdio.h>
  5. #include <string.h>
  6. 	int operators_count = 0, operands_count = 0, valid = 1, top = -1, l = 0, j = 0;
  7. 	char operands[10][10], operators[10][10], stack[100];
  8. %}
  9. %%
  10. "(" {
  11. 	top++;
  12. 	stack[top] = '(';
  13. }
  14. "{" {
  15. 	top++;
  16. 	stack[top] = '{';
  17. }
  18. "[" {
  19. 	top++;
  20. 	stack[top] = '[';
  21. }
  22. ")" {
  23. 	if (stack[top] != '(') {
  24. 		valid = 0;	
  25. 	}
  26. 	else if(operands_count>0 && (operands_count-operators_count)!=1){
  27. 		valid=0;
  28. 	}
  29. 	else{
  30. 		top--;
  31. 		operands_count=1;
  32. 		operators_count=0;
  33. 	}
  34. }
  35. "}" {
  36. 	if (stack[top] != '{') {
  37. 		valid = 0;	
  38. 	}
  39. 	else if(operands_count>0 && (operands_count-operators_count)!=1){
  40. 		valid=0;
  41. 	}
  42. 	else{
  43. 		top--;
  44. 		operands_count=1;
  45. 		operators_count=0;
  46. 	}
  47. }
  48. "]" {
  49. 	if (stack[top] != '[') {
  50. 		valid = 0;
  51. 	}
  52. 	else if(operands_count>0 && (operands_count-operators_count)!=1){
  53. 		valid=0;
  54. 	}
  55. 	else{
  56. 		top--;
  57. 		operands_count=1;
  58. 		operators_count=0;
  59. 	}
  60.  
  61. }
  62. "+"|"-"|"*"|"/" {
  63. 	operators_count++;
  64. 	strcpy(operators[l], yytext);
  65. 	l++;
  66. }
  67. [0-9]+|[a-zA-Z][a-zA-Z0-9_]* {
  68. 	operands_count++;
  69. 	strcpy(operands[j], yytext);
  70. 	j++;
  71. }
  72. %%
  73.  
  74.  
  75. int yywrap()
  76. {
  77. 	return 1;
  78. }
  79. int main()
  80. {
  81. 	int k;
  82. 	printf("Enter the arithmetic expression: ");
  83. 	yylex();
  84.  
  85. 	if (valid == 1 && top == -1) {
  86. 		printf("\nValid Expression\n");
  87. 	}
  88. 	else
  89. 		printf("\nInvalid Expression\n");
  90.  
  91. 	return 0;
  92. }
  93.  
Success #stdin #stdout #stderr 0.02s 6916KB
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stdin
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Standard input is empty
stdout
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Standard output is empty
stderr
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ERROR: /home/3XdIJ9/prog:92:1: Syntax error: Unexpected end of file
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? EOF: exit
https://ideone.com/AWEe6t
language:
Prolog (swi 7.6.4)
created:
1 year ago
visibility:
public

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