Implementation of predictive parser in LEX.

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#define SIZE 128
#define NONE -1
#define EOS '\0'
#define NUM 257
#define KEYWORD 258
#define ID 259
#define DONE 260
#define MAX 999
char lexemes[MAX];
char buffer[SIZE];
int lastchar =  - 1;
int lastentry = 0;
int tokenval = DONE;
int lineno = 1;
int lookahead;
struct entry
{
  char *lexptr;
  int token;
} symtable[100];
struct entry keywords[] =
{
  "if", KEYWORD, "else", KEYWORD, "for", KEYWORD, "int", KEYWORD, "float",
    KEYWORD, "double", KEYWORD, "char", KEYWORD, "struct", KEYWORD,"return ",KEYWORD,0,0};

void Error_Message(char *m)
  {
    fprintf(stderr, "line %d, %s \n", lineno, m);
    exit(1);
  } int look_up(char s[])
  {
    int k;
    for (k = lastentry; k > 0; k--)
      if (strcmp(symtable[k].lexptr, s) == 0)
        return k;
    return 0;
  }
  int insert(char s[], int tok)
  {
    int len;
    len = strlen(s);
    if (lastentry + 1 >= MAX)
      Error_Message("Symbpl table is full");
    if (lastchar + len + 1 >= MAX)
      Error_Message("Lexemes array is full");
    lastentry = lastentry + 1;
    symtable[lastentry].token = tok;
    symtable[lastentry].lexptr = &lexemes[lastchar + 1];
    lastchar = lastchar + len + 1;
    strcpy(symtable[lastentry].lexptr, s);
    return lastentry;
  } /*void Initialize()
  {
  struct entry *ptr;
  for(ptr=keywords;ptr->token;ptr+1)
  insert(ptr->lexptr,ptr->token);
  }*/
  int lexer()
  {
    int t;
    int val, i = 0;
    while (1)
    {
      t = getchar();
      if (t == ' ' || t == '\t')
        ;
      else if (t == '\n')
        lineno = lineno + 1;
      else if (isdigit(t))
      {
        ungetc(t, stdin);
        scanf("%d", &tokenval);
        return NUM;
      }
      else if (isalpha(t))
      {
        while (isalnum(t))
        {
          buffer[i] = t;
          t = getchar();
          i = i + 1;
          if (i >= SIZE)
            Error_Message("Compiler error");
        }
        buffer[i] = EOS;
        if (t != EOF)
          ungetc(t, stdin);
        val = look_up(buffer);
        if (val == 0)
          val = insert(buffer, ID);
        tokenval = val;
        return symtable[val].token;
      }
      else if (t == EOF)
        return DONE;
      else
      {
        tokenval = NONE;
        return t;
      }
    }
  }
  void Match(int t)
  {
    if (lookahead == t)
      lookahead = lexer();
    else
      Error_Message("Syntax error");
  }
  void display(int t, int tval)
  {
    if (t == '+' || t == '-' || t == '*' || t == '/')
      printf("\nArithmetic Operator: %c", t);
    else if (t == NUM)
      printf("\n Number: %d", tval);
    else if (t == ID)
      printf("\n Identifier: %s", symtable[tval].lexptr);
    else
      printf("\n Token %d tokenval %d", t, tokenval);
  }
  void F()
  {
    //void E();
    switch (lookahead)
    {
      case '(':
        Match('(');
        E();
        Match(')');
        break;
      case NUM:
        display(NUM, tokenval);
        Match(NUM);
        break;
      case ID:
        display(ID, tokenval);
        Match(ID);
        break;
      default:
        Error_Message("Syntax error");
    }
  }
  void T()
  {
    int t;
    F();
    while (1)
    {
      switch (lookahead)
      {
        case '*':
          t = lookahead;
          Match(lookahead);
          F();
          display(t, NONE);
          continue;
        case '/':
          t = lookahead;
          Match(lookahead);
          display(t, NONE);
          continue;
        default:
          return ;
      }
    }
  }
  void E()
  {
    int t;
    T();
    while (1)
    {
      switch (lookahead)
      {
        case '+':
          t = lookahead;
          Match(lookahead);
          T();
          display(t, NONE);
          continue;
        case '-':
          t = lookahead;
          Match(lookahead);
          T();
          display(t, NONE);
          continue;
        default:
          return ;
      }
    }
  }
  void parser()
  {
    lookahead = lexer();
    while (lookahead != DONE)
    {
      E();
      Match(';');
    }
  }
  main()
  {
    char ans[10];
    printf("\n Program for recursive decent parsing ");
    printf("\n Enter the expression ");
    printf("And place ; at the end\n");
    printf("Press Ctrl-Z to terminate\n");
    parser();
  }