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-Write a c program to check the validity of input string using predictive parser
Description: 
A predictive parser is a recursive descent parser with no backtracking or backup. It is a top-down parser that does not require backtracking. At each step, the choice of the rule to be expanded is made upon the next terminal symbol.

Program:-
#include <stdio.h>
#include <string.h>
char prol[7][10] = { "S", "A", "A", "B", "B", "C", "C" };
char pror[7][10] = { "A", "Bb", "Cd", "aB", "@", "Cc", "@" };
char prod[7][10] = { "S->A", "A->Bb", "A->Cd", "B->aB", "B->@", "C->Cc", "C->@" };
char first[7][10] = { "abcd", "ab", "cd", "a@", "@", "c@", "@" };
follow[7][10] = { "$", "$", "$", "a$", "b$", "c$", "d$" };
char table[5][6][10]; 
int numr(char c)
{
       switch (c)
          {
                  case 'S':
                           return 0;
                            
                                  case 'A':
                                           return 1;
                                            
                                                  case 'B':
                                                           return 2;
                                                            
                                                                  case 'C':
                                                                           return 3;
                                                                            
                                                                                  case 'a':
                                                                                           return 0;
                                                                                            
                                                                                                  case 'b':
                                                                                                           return 1;
                                                                                                            
                                                                                                                  case 'c':
                                                                                                                           return 2;
                                                                                                                            
                                                                                                                                  case 'd':
                                                                                                                                           return 3;
                                                                                                                                            
                                                                                                                                                  case '$':
                                                                                                                                                           return 4;
          }
           
              return (2);
}
 
 int main()
 {
       int i, j, k;
       for (i = 0; i < 5; i++)
             for (j = 0; j < 6; j++)
                      strcpy(table[i][j], " ");
                       
                          printf("The following grammar is used for Parsing Table:\n");
                           
                              for (i = 0; i < 7; i++)
                                    printf("%s\n", prod[i]);
                                     
                                        printf("\nPredictive parsing table:\n");
                                         
                                            fflush(stdin);
                                             
                                                for (i = 0; i < 7; i++)
                                                   {
                                                          k = strlen(first[i]);
                                                                for (j = 0; j < 10; j++)
                                                                         if (first[i][j] != '@')
                                                                                     strcpy(table[numr(prol[i][0]) + 1][numr(first[i][j]) + 1], prod[i]);
                                                   }
                                                    
                                                       for (i = 0; i < 7; i++)
                                                          {
                                                                  if (strlen(pror[i]) == 1)
                                                                        {
                                                                                     if (pror[i][0] == '@')
                                                                                              {
                                                                                                            k = strlen(follow[i]);
                                                                                                                        for (j = 0; j < k; j++)
                                                                                                                                       strcpy(table[numr(prol[i][0]) + 1][numr(follow[i][j]) + 1], prod[i]);
                                                                                              }
                                                                        }
                                                          }
                                                           
                                                              strcpy(table[0][0], " ");
                                                               
                                                                  strcpy(table[0][1], "a");
                                                                   
                                                                      strcpy(table[0][2], "b");
                                                                      strcpy(table[0][3], "c");
                                                                       
                                                                          strcpy(table[0][4], "d");
                                                                           
                                                                              strcpy(table[0][5], "$");
                                                                               
                                                                                  strcpy(table[1][0], "S");
                                                                                   
                                                                                      strcpy(table[2][0], "A");
                                                                                       
                                                                                          strcpy(table[3][0], "B");
                                                                                           
                                                                                              strcpy(table[4][0], "C");
                                                                                               
                                                                                                  printf("\n--------------------------------------------------------\n");
                                                                                                   
                                                                                                      for (i = 0; i < 5; i++)
                                                                                                            for (j = 0; j < 6; j++)
                                                                                                                  {
                                                                                                                             printf("%-10s", table[i][j]);
                                                                                                                                      if (j == 5)
                                                                                                                                                  printf("\n--------------------------------------------------------\n");
                                                                                                                  }
 }

 
                                                                                                                  }
                                                                                              }
                                                                        }
                                                          }
                                                   }
 }
          }
}