Thread: assign null value to a struct and check if it is null

Hi everyone,

I want to initialize my struct with a null value, and at some point check whether the struct is null.

Code:

struct mystruct s = {0};

and then when i need to check it:

Code:

if(s == {0}) then do something

but it doesn't seem to be the right way of doing it.

Any help?

Thanks in advance.

Hi qny,

When I tried to do that, it gave me "invalid operand to binary ==" error..

Originally Posted by ymc1g11

Hi qny,

When I tried to do that, it gave me "invalid operand to binary ==" error..

Oops, my bad. Sorry.
You need to compare member by member (for those members that are comparable)

Code:

if ((s.member1 == snull.member1) &&
    (s.member2 == snull.member2) &&
/*  ...  */
    (s.memberN == snull.memberN)) { /* do something */ }

Originally Posted by laserlight

Just compare the members of the struct with 0. The use of memcmp may be an appropriate alternative. As I said, if you really do want to talk about a "null" struct, then you need to define precisely what that means. As yet, you have not even shown us your struct definition.

laserlight,

here's the struct i created:

Code:

struct tnode {
	char *word;
	int count;
	struct tnode *left;
	struct tnode *right;
};

I am trying to implement it as a tree. So, to do that, I initialized the root of the tree as NULL as follows:

Code:

struct tnode root = {0};

So every time when I add an element(which is a char*), it will check if the current node is null, add the element to root if yes, else traverse the tree recursively.

Much thanks in advance

Originally Posted by laserlight

The thing is, the root is not "null". It is zero initialised.

It might make more sense to have root be a pointer instead. If you do want it to be a struct tnode object, then you can either compare its members with 0, or just the word member.

sorry i just cant get my head round it as a beginner

so i should declare it in my main as this:

Code:

struct tnode *root;

and in my method sort,
-how do i check if the 'root' had been assigned anything?
-do i need to first declare the 'root' is empty?

this is my method which sort words into the tree (made of struct tnode):

Code:

void sort(struct tnode **r, char *w){    
    if(r == null){
        r = (struct tnode *) malloc(sizeof(struct tnode));
        r->word = w;
        r->count = 1;
        r->*left = null;
        r->*right = null;
        (printf("%s\n", r->word));
    }else{
    
        int cmp = strcmp(w, r->word);
        if(cmp<0){
            sort(r->left, w);
        }else if(cmp>0){
            sort(r->right, w);
        }else r->count++;
        
    }
}

Please help~ and much thanks~!

sorted! cheers!