Postfix and Prefix

Hello guys,

#include<stdio.h>
void prefix();
void postfix();
char stack[50];
int s[50];
char post[50],pre[50];
int p = -1;
int top = -1;
int rtop = -1;
int xtop=-1;      
 
xpush(int elem)
{     
 s[++xtop]=elem;                
}
 
int xpop()
{                      
 return(s[xtop--]);
}



void push(char x)
{
        stack[++top] = x;
}
 
char pop()
{
        if(top == -1)
                return -1;
        else
            return stack[top--];
}
 
int priority(char x)
{
    if(x == '(')
        return 0;
    if(x == '+' || x == '-')
        return 1;
    if(x == '*' || x == '/')
        return 2;
}


void main()
{
 int choice;
 do
 {
  printf("\n\n----MENU----\n1.INFIX - POSTFIX\n2.INFIX - PREFIX\n3.EXIT\n");
  printf("Enter your choice\n");
  scanf("%d",&choice);
  switch(choice)
  {
   case 1 : postfix();break;
   case 2 : prefix();break;
   case 3 : printf("~~~~~ Thank You ~~~~~\n");break;
   default: printf("INVALID INPUT !!!!\n");
  }
 }while(choice!=3);
}

void postfix()
{
  p=-1;
     char exp[20];
        char *e, x;
        printf("Enter the expression :: ");
        scanf("%s",exp);
        printf("\nPostfix expression :: ");
        e = exp;
        while(*e != '\0')
        {
                if(isalnum(*e))
                {
     printf("%c",*e);
                    post[++p]=*e;
                }
                else if(*e == '(')
              push(*e);
              
          else if(*e == ')')
                {
                    while((x = pop()) != '(')
                 {
      printf("%c", x);
                 post[++p]=x;
                 }
                }
         else
         {
             while(priority(stack[top]) >= priority(*e))
             {
              char a;
              a= pop();
        printf("%c",a);
                 post[++p]=a;
             }
             push(*e);
         }
                e++;
        }
    while(top != -1)
    {
     char b;
     b=pop();
        printf("%c",b);
        post[++p]=b;
    }
        
 int i=0,op1,op2; /*Evaluation*/
 char ch;
 while(i != (p+1))
 {
  ch=post[i];
  i++;
  if(isdigit(ch))
  xpush(ch-'0'); 
  else
  {        
   op2=xpop();
   op1=xpop();
   switch(ch)
   {
   case '+':xpush(op1+op2);break;
   case '-':xpush(op1-op2);break;
   case '*':xpush(op1*op2);break;
   case '/':xpush(op1/op2);break;
   }
  }
 }
  printf("\n\nResult after Evaluation: %d\n",s[xtop]);
    

}

void prefix()
{
 xtop=-1;rtop=-1;p=-1;
 int i=0,j=0;
 char exp[50],rexp[50];
    char *e,*r, x;
    printf("Enter the expression :: ");
    scanf("%s",exp);
    while(exp[i]!='\0')
    i++;
    i--;
    while(i>=0)
    {
     if(exp[i]==')')
     {
      rexp[j++]='(';
      i--;
  }
  else if(exp[i]=='(')
  {
   rexp[j++]=')';
   i--;
  }
  else
  rexp[j++]=exp[i--];
    
 }
 
 rexp[j]='\0'; 
 
 r=rexp;
 while(*r != '\0')
 {
  if(isalnum(*r))
        {
            pre[++p]=*r;
        }
        else if(*r=='(')
        {
         push(*r);
  }
  else if(*r==')')
  {
   while((x=pop())!='(')
   pre[++p]=x;
  }
  else
  {
   while(priority(stack[top])>=priority(*r))
   pre[++p]=pop();
   push(*r);
  }
  r++;
 }
 while(top != -1)
    {
        pre[++p]=pop();
    }
    int print=p;
    printf("\nPrefix expression :: ");
    while(print!=-1)
    printf("%c",pre[print--]);
    /*evaluation*/
 i=0;
 j=0;
    int op1,op2;
    char ch;
    while(i!=(p+1))
    {
     i++;
     ch=pre[j++];
     if(isdigit(ch))
      xpush(ch-'0');
     else
     {
      op1=xpop();
      op2=xpop();
      switch(ch)
      {
       case '+':xpush(op1+op2);break;
      case '-':xpush(op1-op2);break;
      case '*':xpush(op1*op2);break;
      case '/':xpush(op1/op2);break;
   }
  }
 }
 printf("\n\nResult after Evaluation: %d\n",s[xtop]);
}

/*
OUTPUT

----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
1
Enter the expression :: 8*5*(10-3)

Postfix expression :: 85*103-*

Result after Evaluation: 280


----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
2
Enter the expression :: 8*8*(10-3)

Prefix expression :: *8*8-103

Result after Evaluation: 448


----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
1
Enter the expression :: 5*5-4*(4-3)

Postfix expression :: 55*443-*-

Result after Evaluation: 21


----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
2
Enter the expression :: 5*5-4*(4-3)

Prefix expression :: -*55*4-43

Result after Evaluation: 21


----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
10
INVALID INPUT !!!!


----MENU----
1.INFIX - POSTFIX
2.INFIX - PREFIX
3.EXIT
Enter your choice
3
~~~~~ Thank You ~~~~~

*/