Simple Parser in C - operator precedence parser

Simple Parser in C - operator precedence parser

#include <iostream.h>
#include <conio.h>
#include <stdlib.h>

struct node
{
char symbol;
struct node *left;
struct node *right;
};
typedef struct node node;

struct stack
{
char op;
node *op_pointer;
};
typedef struct stack stack;

int TOS=-1;
stack s[10];

/* This is the operator precedence table stored as it is in the
form of matrix and operators are assigned values as follows:
NULL = 0, Equal (=) = 1, less than (<) = 2, greater than (>) = 3. */

// +,*,(,),<>,-,/,^
int m[8][8]={ {3,2,2,3,3,3,2,2}, //+
{3,3,2,3,3,3,2,2}, //*
{2,2,2,1,0,2,2,2}, //(
{3,3,0,3,3,3,3,3}, //)
{2,2,2,0,1,2,2,2}, //><
{3,2,2,3,3,3,2,2}, //-
{3,3,3,3,3,3,3,2}, ///
{3,3,3,3,3,3,3,2}}; //^

void main()
{
clrscr();

int comp(char );
void push(char);
void pop();
void display(node *);
node *temp;

char str[20];
int i=0;

cout << "Enter the string : ";
cin >> str;
push('<');
i++;
while(str[i]!='\0')
{
if((str[i]>='a'&&str[i]<='z'))
{
temp = new node;
temp->symbol=str[i];
temp->left=NULL;
temp->right=NULL;
s[TOS].op_pointer = temp;
}
else
{
int p_index,q_index;
p_index = comp(s[TOS].op);
q_index = comp(str[i]);
while(m[p_index][q_index]==3)
{
temp = new node;
temp->symbol=s[TOS].op;
temp->left=s[TOS-1].op_pointer;
temp->right=s[TOS].op_pointer;
pop();
s[TOS].op_pointer=temp;
p_index = comp(s[TOS].op);
q_index = comp(str[i]);
}

if(m[p_index][q_index]==2)
{
push(str[i]);
}
if(m[p_index][q_index]==1)
{
if(comp(s[TOS].op)==4)
break;
if(comp(s[TOS].op)==2)
{
temp=s[TOS].op_pointer;
pop();
s[TOS].op_pointer=temp;
}
}
if(m[p_index][q_index]==0)
{
cout << "Invalid string.";
getch();
exit(1);
}
}
i++;
}
display(s[TOS].op_pointer);
getch();
}
// function to get the index of the operator that comes in the TOS
// and also for the current operator that come in the string.
int comp(char x)
{
int y;
switch(x)
{
case '+':
y=0;
break;
case '*':
y=1;
break;
case '(':
y=2;
break;
case ')':
y=3;
break;
case '>':
case '<':
y=4;
break;
case '-':
y=5;
break;
case '/':
y=6;
break;
case '^':
y=7;
break;
}
return y;
}

void push(char x)
{
TOS++;
s[TOS].op=x;
}

void pop()
{
TOS--;
}

void display(node *s)
{
if(s==NULL)
return;
if(s->left!=NULL)
display(s->left);
if(s->right!=NULL)
display(s->right);
cout << s->symbol;
}


/*

INPUT:
Enter the string : <b*(c+d)*e/f>
OUTPUT:
bcd+*ef/*

*/