How to build a predictive parser in c?

[radha gogia]

Below is the code for doing the syntax analysis of an expression using Predictive parser in c and it is neither displaying any output nor is it working correctly ,it simply crashes after running , so please clarify it .

#include<stdio.h>
#include<string.h>
#include<conio.h>
char a[10];
int top=-1,i;
void error(){
printf("Syntax Error");
}
void push(char k[]) //Pushes The Set Of Characters on to the Stack
{
  for(i=0;k[i]!='\0';i++)
  {
    if(top<9)
    a[++top]=k[i];
  }
}
char TOS()        //Returns TOP of the Stack
{
  return a[top];
}
void pop()       //Pops 1 element from the Stack
{
  if(top>=0)
    a[top--]='\0';
}
void display()  //Displays Elements Of Stack
{
  for(i=0;i<=top;i++)
    printf("%c",a[i]);
}
void display1(char p[],int m) //Displays The Present Input String
{
  int l;
  printf("\t");
  for(l=m;p[l]!='\0';l++)
    printf("%c",p[l]);
}
char* stack(){
return a;
}
int main()
{
  char ip[20],r[20],st,an;
  int ir,ic,j=0,k;
  char t[5][6][10]={"$","$","TH","$","TH","$",
           "+TH","$","e","e","$","e",
           "$","$","FU","$","FU","$",
           "e","*FU","e","e","$","e",
           "$","$","(E)","$","i","$"};
  printf("\nEnter any String(Append with $)");
  gets(ip);
  printf("Stack\tInput\tOutput\n\n");
  push("$E");
  display();
  printf("\t%s\n",ip);
  for(j=0;ip[j]!='\0';)
  {
  if(TOS()==an)
      {
    pop();
    display();
    display1(ip,j+1);
    printf("\tPOP\n");
    j++;
      }
    an=ip[j];
    st=TOS();
      if(st=='E')ir=0;
      else if(st=='H')ir=1;
      else if(st=='T')ir=2;
      else if(st=='U')ir=3;
      else if(st=='F')ir=4;
      else {
        error();
        break;
        }
      if(an=='+')ic=0;
      else if(an=='*')ic=1;
      else if(an=='(')ic=2;
      else if(an==')')ic=3;
      else if((an>='a'&&an<='z')||(an>='A'&&an<='Z')){ic=4;an='i';}
      else if(an=='$')ic=5;
      strcpy(r,strrev(t[ir][ic]));
      strrev(t[ir][ic]);
      pop();
      push(r);
      if(TOS()=='e')
      {
    pop();
    display();
    display1(ip,j);
    printf("\t%c->%c\n",st,238);
      }
      else{
      display();
      display1(ip,j);
      printf("\t%c->%s\n",st,t[ir][ic]);
      }
      if(TOS()=='$'&&an=='$')
      break;
      if(TOS()=='$'){
    error();
    break;
    }
      }
      k=strcmp(stack(),"$");
      if(k==0 && i==strlen(ip))
    printf("\n Given String is accepted");
    else
    printf("\n Given String is not accepted");
  return 0;
}

[donbock]

Here are some general comments from a quick scan of your program.

1. The size of the a array appears on lines 4 and 13. Program is correct as is, but it would be more maintainable if array size could be altered by changing only a single line rather than two.
2. stdio typically uses buffered I/O. Thus you might want to terminate output strings with a newline to insure they print right away.
3. Function TOS() will fail if the stack is empty (top < 0). Perhaps return '\0' if the stack is empty.
4. an is uninitialized at line 58 the first time through the loop.
5. Are you sure display1() doesn't need to trap error cases: m indexes past end of p[]; or p[] is not null-terminated?