how to create tree using # define

I want to create a tree. I saw the following code

#define NODEALLOC(struct treenode*)malloc(sizeof(struct treenode))

    struct treenode{

          int data;
         struct treenode * left;
         strruct treenode *right;

   }
typedef struct treenode *treeptr;
treeptr root;

treeptr create(int d)开发者_JS百科
 {

treeptr root;
root =NODEALLOC;
root->data=d;
root->left=null;
root->right=null;
 return root;
}

I am not understanding the #define statement.can anyone help me?

First, it's wrong, it needs a space between the NODEALLOC and the (

Second, it's evil. There are good uses for macros; this is not one of them.

Here's how it works:

It's a simple string substitution, so when you see root=NODEALLOC; it becomes root=(struct treenode*)malloc(sizeof(struct treenode))

malloc(n) allocates n bytes of memory, sizeof tells malloc how big n needs to be, and (struct treenode*) casts malloc's returned void* into the correct type.

Don't use that. Use a function if you don't wan't to write the malloc more than once. Using macros like this is something you should better avoid (it's legal, but not considered a good practice).

If you don't know what macros are in the first place, read.

It's simple. Replace NODEALLOC in code with its definition. In your case:

treeptr create(int d)
{
  treeptr root;
  root = (struct treenode*)malloc(sizeof(struct treenode)); // was NODEALLOC;
  root->data=d;
  root->left=null;
  root->right=null;
  return root;
}

Thus, this is a simple allocation.

However, using a macro only for this is not really helpful, it can even confuse readers of your code. Usually, a macro should do something useful.