Question

In the following code for a binary search tree:

template <class TKey>
class bst<TKey>::node *bst<TKey>::insert(node *T, TKey &key)
{
  if (T == NULL) {
    T = new node;
    T->key = key;
  } else if (T->key == key) {
    cout << "key " << key << " already in tree" << endl;
  } else {
    
    int dir = T->key < key;
    T->link[dir] = insert(T->link[dir], key);   
  }
  return T;
}

I’m confused what the line

int dir = T->key < key;

is doing. I could understand int dir = T->key, although of course that wouldn’t make sense, but I’ve not seen the < operator used in that way before. Any clues?

Answer 1

T->key < key is a condition. It will evaluate to either true or false.
If it evaluates to true, dir will get value 1, otherwise it will get value 0.

int dir = T->key < key;

is short form for writing

int dir;
if(T->key < key)
    dir = 1;
else
    dir = 0;

When a boolean is assigned to an int, it gets the value 0 or 1 corresponding to false or true.