Thread: Evaluating an Expression

This compiles and runs correctly, but it says "The value is 0" Am I not returning what I should be?

Thanks for any help

Code:

#include <stdio.h>        
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#define FAILED -1;

typedef struct node stack;
typedef stack * stack_ptr;
struct node
{                             
   int key;             
   stack_ptr next; 
}; 

int evaluate(char *);
int isempty(stack_ptr);
void push(stack_ptr *, int);
int pop(stack_ptr *);
void cleanup(stack_ptr *);
void print(stack_ptr);

int main()
{
 char str[80];
 int res;
 stack_ptr top = NULL;

   printf("Enter the postfix form expression:\n");
   gets(str);

   res = evaluate(str);
   printf("the result is %d: \n", res);

   return (0);
}

int evaluate(char *str)
{
 int num=0;
 char ch=0;
 stack_ptr top = NULL;


   while((ch == *str))
   {
      num;
      if (ch == ' ') 
      {
         str++;
         continue;
      }

      if (ch == '+')
         num = pop(&top) + pop(&top);
      else
         if (ch == '*')
            num = pop(&top) * pop(&top);
         else
            while (1) 
            {
               if (isdigit(ch))
               {
                  num = 10*num + (ch -'0');
                  ch = *++str;
               } 
               else
                  if (ch == ' ') 
                     break;
                  else
                  {
                     printf("wrong input!\n");
                     exit (0);
                  }
            }
            push(&top, num);
            str++;
   }
   return num;
}


int isempty(stack_ptr top)
{
   return (top == NULL);
}

void push(stack_ptr *top, int v)
{
 stack_ptr tmp;

   tmp = (stack_ptr) malloc(sizeof(stack));
   if (tmp == NULL)
   {
      printf("Out of space!\n");
      exit (0);
   }

   tmp->key = v;
   tmp->next = *top;
   *top = tmp;
}

int pop(stack_ptr *top)
{
 stack_ptr tmp;
 int x;

   if (isempty(*top)) 
   {
      printf("wrong input!\n");
      exit (1);
   }

   tmp = *top;
   x = tmp->key;
   *top = tmp->next;

   free(tmp);
   return x;
}


void cleanup(stack_ptr *top)
{
 stack_ptr tmp;

   while(*top != NULL )
   {
      tmp = (*top)->next;
      free(*top);
      *top = tmp;  
   }
}

void print(stack_ptr top)
{
 stack_ptr tmp =top;

   if (top != NULL)
      while (tmp != NULL) 
      {
         printf("\tvalue is %d\n",tmp->key);
         tmp = tmp->next;
      } 
   else
      printf("stack is empty\n");
}

In the function evaluate() you initialize top = NULL. You then try to reference the address of this pointer before you give it an actual address.

Code:

 stack_ptr top = NULL;


   while((ch == *str))
   {
      num;
      if (ch == ' ') 
      {
         str++;
         continue;
      }

      if (ch == '+')
         num = pop(&top) + pop(&top);
      else
         if (ch == '*')
            num = pop(&top) * pop(&top);

What should top be set to?

And if there are any other issues, (that was the first i noticed), please put some comments in the code.

I dont know, lol, thats what you should know, it points to nothing right now (NULL), it needs to point to the beginning of a struct. You declared it that way, to point to struct data type.

I have modified your code. You have some errors in your code. Some of the changed that I have made, I've put a commnet.

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#define FAILED -1;

typedef struct node stack;
typedef stack * stack_ptr;
struct node
{
   int key;
   stack_ptr next;
};

int evaluate(char *);
int isempty(stack_ptr);
void push(stack_ptr *, int);
int pop(stack_ptr *);
void cleanup(stack_ptr *);
void print(stack_ptr);

int main()
{
 char str[80];
 int res;
 stack_ptr top = NULL;   // not required 

   printf("Enter the postfix form expression:\n");
   gets(str);

   res = evaluate(str);
   printf("the result is %d: \n", res);

   return (0);
}

int evaluate(char *str)
{
 int num=0;
 char ch=0;
 stack_ptr top = NULL;


   while((ch = *str))
   {
      num=0;       //CHANGED
      if (ch == ' ')
      {
	 str++;
	 continue;
      }

      if (ch == '+')
	 num = pop(&top) + pop(&top);
      else
	 if (ch == '*')
	    num = pop(&top) * pop(&top);
	 else
	    while (1)
	    {
	       if (isdigit(ch))
	       {
		  num = 10*num + (ch -'0');
		  ch = *(++str);     //changed 
	       }
	       else
		  if (ch == ' ')
		     break;
		  else
		  {
		     printf("wrong input!\n");
		     exit (0);
		  }
	    }
	    push(&top, num);
	    str++;
   }
   return num;
}


int isempty(stack_ptr top)
{
   return (top == NULL);
}

void push(stack_ptr *top, int v)
{
 stack_ptr tmp;

   tmp = (stack_ptr) malloc(sizeof(stack));
   if (tmp == NULL)
   {
      printf("Out of space!\n");
      exit (0);
   }

   tmp->key = v;
   tmp->next = *top;
   *top = tmp;
}

int pop(stack_ptr* top)
{
 stack_ptr tmp;
 int x;

   if (isempty(*top))
   {
      printf("wrong input!\n");
      exit (1);
   }

   tmp = *top;
   x = tmp->key;
   *top = tmp->next;

   free(tmp);
   return x;
}


void cleanup(stack_ptr *top)
{
 stack_ptr tmp;

   while(*top != NULL )
   {
      tmp = (*top)->next;
      free(*top);
      *top = tmp;
   }
}

void print(stack_ptr top)
{
 stack_ptr tmp =top;

   if (top != NULL)
      while (tmp != NULL)
      {
	 printf("\tvalue is %d\n",tmp->key);
	 tmp = tmp->next;
      }
   else
      printf("stack is empty\n");
}

The major error in your code was,

Code:

 while((ch == *str))

It sould be corrected as,

Code:

 while((ch = *str))

I've tried that in the past, and it says "Wrong input!"

I've also heard from a few people that I should change it to:

Code:

  while((ch == *str))

I've also corrected the beginning of my evaluate function so it drops into my while loop:

Code:

int evaluate(char *str)
{
 int num=0;
 char ch = *str;
 stack_ptr top = NULL;

I think the way you enter the expression is wrong. According to your program, (not the best way mind you ....) an expression is like this

Code:

 NUMBER<space>NUMBER<space>OPERATOR

for example,

Code:

3 4 +

You could imporve on this.

Aoubt the following while condition ,

Code:

while((ch == *str))

This is wrong. It won't serve your purpose. What it will do is just compare "ch" with "*str". What you want to do is assign the value of "*str" to "ch" and continue the loop till NULL Is reached.
so your code should be like this.

Code:

while((ch = *str))

I am sorry, but let me reiterate what i said in the first reply. When the heck are you going to assign top to something, you use the address here, but nevre initialize it but to NULL!

Code:

int evaluate(char *str)
{
 int num=0;
 char ch=0;
 stack_ptr top = NULL;


   while((ch = *str))
   {
      num=0;       //CHANGED
      if (ch == ' ')
      {
	 str++;
	 continue;
      }

      if (ch == '+')
	 num = pop(&top) + pop(&top); //you need to initialize to something other than NULL before you reference the address.
      else
	 if (ch == '*')
	    num = pop(&top) * pop(&top); //you need to initialize to something other than NULL before you reference the address.
	 else
	    while (1)
	    {
	       if (isdigit(ch))
	       {
		  num = 10*num + (ch -'0');
		  ch = *(++str);     //changed 
	       }
	       else
		  if (ch == ' ')
		     break;
		  else
		  {
		     printf("wrong input!\n");
		     exit (0);
		  }
	    }
	    push(&top, num);
	    str++;
   }
   return num;
}

Maybe if you try what he was suggesting with the = instead of ==, and assign top to what ever it needs to be, you wont get "Wrong input" becuase pop() will actually be able to use the address!!!

Here the statment

Code:

stack_ptr top = NULL;

is to say that the stack is empty. This is not a problem. It has to be NULL initially because the stack is empty.

It gives the message "WRONG INPUT" because it's wrong input. There is nothing to argue there. If you check the code, first it will check to see if CH is "+" or "*" if so , it will pop the first two items from the stack. IF the correct POSTFIX was given, by the time an OPERATOR is reached, there should be operands in the stack. Say if you give an expression like ,

Code:

+34 45

This will give a wrong input , as it should be, since first it encounters a "+" sign , tried to pop the stack, the stack is empty, so "WRONG INPUT".

The reason he is getting this message is because it is a wroing input. The statment,

Code:

stack_ptr top = NULL;

is not wrong.