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[vIp3rS]

#include <stdio.h>
#include <conio.h>
#include <ctype.h>

#define MAX 100
#define EMPTY -1

struct stack
{
		  int data[MAX];
		  int top;
};


int isempty(struct stack *s)
{
		  return (s->top == EMPTY) ? 1 : 0;
}

void emptystack(struct stack* s)
{
		  s->top = EMPTY;
}

void push(struct stack* s,int item)
{
		  if(s->top == (MAX-1))
		  {
					 printf("\nSTACK FULL");
		  }
		  else
		  {
					 ++s->top;
					 s->data[s->top]=item;
		  }
}

int pop(struct stack* s)
{
		  int ret=EMPTY;
		  if(s->top != EMPTY)
		  {
					 ret= s->data[s->top];
					 --s->top;
		  }
		  return ret;
}

void display(struct stack s)
{
		  while(s.top != EMPTY)
		  {
					 printf("\n%d",s.data[s.top]);
					 s.top--;
		  }
}


int isoperator(char e)
{
		  if(e == '+' || e == '-' || e == '*' || e == '/' || e == '%')
					 return 1;
		  else
					 return 0;
}


int priority(char e)
{
		  int pri = 0;

		  if(e == '*' || e == '/' || e =='%')
					 pri = 2;
		  else
		  {
					 if(e == '+' || e == '-')
								pri = 1;
		  }
		  return pri;
}

int evaluate(char *postfix)
{
		  char *p;
		  struct stack stk;
		  int op1,op2,result;

		  emptystack(&stk);
		  p = &postfix[0];

		  while(*p != '\0')
		  {
					 /* removes tabs and spaces */
					 while(*p == ' ' || *p == '\t' )
					 {
								p++;
					 }
					 if(*p == '\0')
								break;
			 /* if is digit */
					 if(isdigit(*p))
					 {
								push(&stk,*p - 48);
					 }
					 else
					 {
						 /* it is an operator */
								op1 = pop(&stk);
								op2 = pop(&stk);

								switch(*p)
								{
										  case '+':
													 result = op2 + op1;
													 break;

										  case '-':
													 result = op2 - op1;
													 break;

										  case '/':
													 result = op2 / op1;
													 break;

										  case '*':
													 result = op2 * op1;
													 break;

										  case '%':
													 result = op2 % op1;
													 break;

										  default:
													 printf("\nInvalid Operator");
													 return 0;
								}
								push(&stk,result);
					 }
					 p++;
		  }
		  result = pop(&stk);
		  return result;
}


void infix2postfix(char* infix, char * postfix, int insertspace)
{
		  char *i,*p;
		  struct stack X;
		  char n1;
		  emptystack(&X);
		  i = &infix[0];
		  p = &postfix[0];

		  while(*i)
		  {
					 while(*i == ' ' || *i == '\t')
					 {
								i++;
					 }

					 if( isdigit(*i) || isalpha(*i) )
					 {
								while( isdigit(*i) || isalpha(*i))
								{
										  *p = *i;
										  p++;
										  i++;
								}
								/*SPACE CODE*/
								if(insertspace)
								{
										  *p = ' ';
										  p++;
								}
								/*END SPACE CODE*/
					 }

					 if( *i == '(' )
					 {
								push(&X,*i);
								i++;
					 }

					 if( *i == ')')
					 {
								n1 = pop(&X);
								while( n1 != '(' )
								{
										  *p = n1;
										  p++;
										  /*SPACE CODE*/
										  if(insertspace)
										  {
													 *p = ' ';
													 p++;
										  }
										  /*END SPACE CODE*/
										  n1 = pop(&X);
								}
								i++;
					 }

					 if( isoperator(*i) )
					 {
								if(isempty(&X))
										  push(&X,*i);
								else
								{
										  n1 = pop(&X);
										  while(priority(n1) >= priority(*i))
										  {
													 *p = n1;
													 p++;
													 /*SPACE CODE*/
													 if(insertspace)
													 {
																*p = ' ';
																p++;
													 }
													 /*END SPACE CODE*/
													 n1 = pop(&X);
										  }
										  if(n1!=-1)
										  push(&X,n1);
										  push(&X,*i);
								}
								i++;
					 }
		  }
		  while(!isempty(&X))
		  {
					 n1 = pop(&X);
					 *p = n1;
					 p++;
					 /*SPACE CODE*/
					 if(insertspace)
					 {
								*p = ' ';
								p++;
					 }
					 /*END SPACE CODE*/
		  }
		  *p = '\0';
}

int main(void)
{
	char infix[MAX], postfix[MAX], choice, option;
	printf("\t\tMISSION-RIVERA's CALCULATOR PROGRAM!");
	printf("\n\nDo you want to use our calculator program?<Y/N>");
	scanf("%c",&option);
	if(option=='y' || option=='Y')
	{
		do
		{
			clrscr();
			printf("Enter expression: ");
			fflush(stdin);
			gets(infix);
			infix2postfix(&infix[0],&postfix[0],1);
			printf("\n\nPostfix expression is: %s",&postfix[0]);
			printf("\n\n\t\tThe answer to this expression %s is %d.",&infix[0],evaluate(&postfix[0]));
			printf("\n\nDo you want to continue?<Y/N>");
			scanf("%c",&choice);
		 }while(choice=='y' || choice=='Y');
	printf("\n\t\t\tTHANK YOU FOR USING!!!");
	printf("\n\nPress any key to exit!");
	getch();
	}
	else
	{
		printf("\n\t\t\tTHANK YOU FOR USING!!!");
		printf("\n\nPress any key to exit!");
		getch();
	}
		return 0;
}