THIS IS A CALCULATOR PROGRAM LIKE FOR EXAMPLE: 2+3*4


WHY DO I GET NULL RESULT?
THE CONVERSION TO INFIX TO POSTFIX IS RIGHT BUT WHEN I EVALUATE THE RESULT IS NULL...

PLEASE PLEASE HELP...T__T

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MAX 100

typedef struct
{
      char data[MAX];
      int top;

} STACK;



void init(STACK *stack);
void get_infix(char infix[]);
STACK convertToPostfix(char infix[], char postfix[]);
int Operator(char ch);
int precedence(char op1, char op2);
int ISP_ICP(char ch);
void push(STACK *stack, char value);
char pop(STACK *stack);
char Top(STACK *stack);
int isEmpty(STACK *stack);
int isFull(STACK *stack);
void display(char infix[], char postfix[]);
char eval(STACK* S);
void main(void)
{     STACK S;
      char infix[MAX], postfix[MAX],result;
      clrscr();
      init(&S);
     S=convertToPostfix(infix, postfix);
      infix[strlen(infix)-2] = '\0';
      display(infix, postfix);
     eval(&S);
getch();

}

void init(STACK *stack)
{
      stack->top = -1;
}

void get_infix(char infix[])
{
      int i;

      printf("Enter infix expression below: \n");
      fflush(stdin);

      for (i=0;i<MAX;)
      {
            if ((infix[i] = getchar()) == '\n')
            {
          i++;
          break;
            }
            else if (!(isspace(infix[i])))
          i++;
      }
      infix[i] = '\0';
}

STACK convertToPostfix(char infix[],char postfix[])
{
      int i, length;
      int j=0;
      char top_ch;
      STACK stack;

      init(&stack);
      get_infix(infix);
      length = strlen(infix);

      if (length)
      {
            push(&stack, '(');
            strcat(infix, ")");
            length++;

            for (i=0;i<length;i++ )
            {
                if (isalpha(infix[i]))
                {  printf("Invalid postfix expression...\nYou inputted a character!");
                    break;
                }
                if (isdigit(infix[i]))
                  {
                  postfix[j++] = infix[i];
                  }
                else if (infix[i] == '(')
                  {
                  push(&stack, '(');
                  }
                    else if (Operator(infix[i]))
                  {
                  while (1)
                      {top_ch = Top(&stack);

                      if (top_ch == '\0')
                          {
                          printf("\nInvalid infix expression\n");exit(1);
                          }
                      else
                          {
                                    if (Operator(top_ch))
                              {
                                if (ISP_ICP(top_ch) >= ISP_ICP(infix[i]))
                                    postfix[j++] = pop(&stack);
                                else
                                    break;
                                }
                          else
                            break;
                          }
                        }
                        push(&stack, infix[i]);
                  }
              else if (infix[i] == ')')
                  { while (1)
                      {  top_ch = Top(&stack);
                          if (top_ch == '\0')
                              {
                                printf("\nInvalid infix expression\n");exit(1);
                              }
                              else
                              {
                                  if (top_ch != '(')
                                    {
                                        postfix[j++] = top_ch;
                                        pop(&stack);
                                    }
                                    else
                                    {
                                          pop(&stack);
                                          break;
                                    }
                              }
                      }
                  }

            }

      }


      postfix[j] = '\0';
      return stack;
}

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

int ISP_ICP(char ch)
{
      if ( ch == '+' || ch == '-' )
            return 1;
      else if ( ch == '*' || ch=='/'|| ch=='%' )
            return 2;
}


int precedence(char op1, char op2)
{
      if ( ISP_ICP(op1) > ISP_ICP(op2) )
            return 1;
      else if ( ISP_ICP(op1) < ISP_ICP(op2) )
            return -1;
      else
            return 0;
}

void push(STACK *stack, char x)
{
      if ( !(isFull(stack)) )
      {
            (stack->top)++;
            stack->data[stack->top] = x;
      }
}

char pop(STACK *stack)
{
      char ch;

      if ( !(isEmpty(stack)) )
      {
            ch = stack->data[stack->top];
            (stack->top)--;
            return ch;
      }
      else
            return '\0';
}

char Top(STACK *stack)
{
      if ( !(isEmpty(stack)) )
            return (stack->data[stack->top]);
      else
            return '\0';
}

int isEmpty(STACK *stack)
{
        if ( stack->top == -1 )
            return 1;
      else
            return 0;
}

int isFull(STACK *stack)
{
      if ( stack->top == MAX )
            return 1;
      else
            return 0;
}


void display(char infix[], char postfix[])
{
      printf("\n\n");
      printf("Infix expression: %s\n", infix);
      printf("Postfix expression: %s\n\n", postfix);
}

char eval(STACK *S)
{	char result;
	char op2,top;
	STACK k;
	init(&k);

while(!isEmpty(S))
{
	top=Top(&S);
	if(isdigit(top)!=0)
{
push(&k,pop(&S));
}
	else if(Operator(top))
{
	if(top=='+')
	{
	result=pop(&k)+pop(&k);
	push(&k,result);
	}

	else if(top=='*')
	{
	result=(pop(&k)*pop(&k));
	push(&k,result);
	}

	else if(top=='-')
	{
	op2=pop(&k);
	result=pop(&k)-op2;
	push(&k,result);
	}

	else if(top=='/')
	{
	op2=pop(&k);
	if(op2!=0)
	{
		result=pop(&k)/op2;
		push(&k,result);
	}
	else
	printf("invalid divisor!");
	}
	(S->top)--;
}
}
printf("result is %s",pop(&k));
getch();
}

Try running your code via a debugger...

Your problem is that in the convertToPostfix function your are returning a null stack. Print out the content of the top of the stack just before you return it . The content prints out to be -1 . Consequently in the eval function when you check to see if the stack is empty, this return true and your code does not enter the while loop