Checking NULL for array of structures

Question

struct student
{
   int roll;
   char *name;
};

int main()
{
  int i;
  struct student arr[2];
  arr[0].roll = 12;
  arr[1].name = "John";

  arr[1].roll = 13;
  arr[1].name = "Craig";

  struct student *ptr;

  ptr=arr;

  // This is perfect.

  for(i = 0; i<2; i++)
  {
    printf("%d %s", ptr->roll, ptr->name);
  }

  // This is also ok.
   printf("%d %s", ptr->roll, ptr->name);

   ptr++ // getting to next structure.

   printf("%d %s", ptr->roll, ptr->name);

  // But this isn't ok

  while(*ptr || ptr->name != NULL)
  {
    ptr++;
  }

  return 0;
}

How to check pointer in while loop?

ptr points to an array if you increment it, ptr start pointing to a memory out size array — Grijesh Chauhan
– Grijesh Chauhan, Commented Sep 20, 2013 at 7:01

Community · Accepted Answer · 2017-05-23 11:56:35Z

4

ptr points to an array if you increment it, ptr start pointing to a memory out size array that is not null. You can do something like:

ptr = arr;
while (ptr <  (arr +  sizeof(arr)/sizeof(arr[0])) ){
   ptr++;
}

Note: this technique will nnot work for dynamic arrays.

To learn what is this formula about read: Weird behavior when printing array in C?

edited May 23, 2017 at 11:56

CommunityBot

11 silver badge

answered Sep 20, 2013 at 7:03

Grijesh Chauhan

58.6k20 gold badges145 silver badges214 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

user1502952 Over a year ago

Yes, but why this is running an extra loop, even this was the same case when i did while(ptr->name != NULL) i.e an extra garbage value

Grijesh Chauhan Over a year ago

@user1502952 Why extra? my answer is about how to save ptr not pointing to outside array that is actually undefined behavior. Suppose if if for some arr index value name points to a NULL then add an extra check.

Grijesh Chauhan Over a year ago

@user1502952 do like: while (ptr < (arr + sizeof(arr)/sizeof(arr[0]) -1) && ptr->name != NULL ){ do your code here}

Dariusz Over a year ago

Why -1? I fail to see why this is necassary - it even seems invalid! It'll skip last element. For 1 element in an array it will not iterate at all.

Grijesh Chauhan Over a year ago

@Dariusz Math mathematically I may be uncorrected here, Should I remove ?

|

Dariusz · Accepted Answer · 2013-09-20 07:04:29Z

2

You can use ptr check for null if you traversing an array of pointers AND when you know that the end of such array is marked with NULL. You are not.

You actually are traversing an array of structures with the ptr pointing straight to the memory location of the first element.

You have to keep track of the size of the array (or amount of its filled elements) and just stop after you have gone through them all.

int count = 2;
struct student arr[count];
struct student* ptr = arr;

for (int i=0; i<count; i++) { 
  // do your stuff

  ptr++; // place it in for if you like
}

answered Sep 20, 2013 at 7:04

Dariusz

22.5k9 gold badges81 silver badges116 bronze badges

1 Comment

Grijesh Chauhan Over a year ago

I guess it easier! then my answer.

nitish712 · Accepted Answer · 2013-09-20 07:17:16Z

1

You can use this:

struct student *ptr=arr;
int max_len = sizeof(arr)/sizeof(*arr);
while(max_len--)
{
    //do something...
    ptr++;
}

One more thing I want to point out is you need to allocate memory for the char* pointer before allocating it with a string.The last loop which you have written doesn't execute at all, since all are NULL pointers.

Other way around is to put some special value in the roll number(say a negative value) of the last struct in the array. You then traverse the array until the roll number is positive. Note that this method can also be used for dynamic arrays.

struct student *arr = (struct student*)malloc(20*sizeof(struct student));
*(arr+19).roll = -1; //this acts as a sentinel to indicate the end of the array....
struct student *ptr=arr;
while(ptr->roll > 0)
{
      //do something...
      ptr++;
}

edited Sep 20, 2013 at 7:17

answered Sep 20, 2013 at 7:10

nitish712

19.8k5 gold badges28 silver badges35 bronze badges

3 Comments

user1502952 Over a year ago

Yes an extra garbage value.

nitish712 Over a year ago

I don't get why there will be a garbage value. Both the loops traverse until the end of array and stop.

user1502952 Over a year ago

that is perfect the only point is to take fixed size array, i hadn't done that done at the point of checking(arr[2]).

tab · Accepted Answer · 2013-09-20 10:28:54Z

while(*ptr || ptr->name != NULL)

First things first: the value of *ptr is a struct, which can't be converted to a boolean value and so while(*ptr) won't compile.

In order to null-terminate an array of structs, you must choose some struct member which you know will never be NULL when the struct is initialized (usually an important pointer), and use it as the flag. You do have that: the name field.

while(ptr->name != NULL)

Then the only problem is that your array isn't actually NULL terminated. You need to reserve an extra, third element at the end, just like with strings. An easy way to do so is by zero-initializing the whole array at declaration. So:

struct student arr[3] = {0};
arr[0].roll = 12;
arr[0].name = "John";
...

Which would have the same effect as manually setting ar[2].name = NULL;

Collectives™ on Stack Overflow

Checking NULL for array of structures

4 Answers 4

8 Comments

1 Comment

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

8 Comments

1 Comment

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related