Project 4 
Explanation of Adding People and Friendships

This is a quick walk-through of how memory changes as we add new people
and friendships to Facebook.  Consider a people.txt file that only contains the
following:

person alice rhodes
person bob rhodes
person carla sewanee
friend alice bob
friend alice carla

(This is a simplified version of the sample people.txt file.)

The steps in the walk-through below are illustrated in the accompanying diagram.
The parts of the diagram in red show what is new about each step.

Step (1): Start of the program.  We haven't read anything from people.txt yet.
all_people is empty.  There is nothing on the stack.

Step (2): We read from people.txt to add a person named alice who goes to rhodes.
We allocate a person on the heap (with new) and set that person's name to alice 
and school to rhodes.  Alice starts out with no friends (everyone begins with no 
friends). We push_back a pointer to this dynamically-allocated person into all_people.  
(Note that because alice was allocated on the heap, we *only have* a pointer to alice.
We don't have an actual variable that stores alice's information, only a pointer
that points to the information.  So pushing_back a pointer to alice is all we can do
anyway.)

Step (3): We read from people.txt to add a person named bob who goes to rhodes.
We allocate a person on the stack and set that person's name and school, then
push_back a pointer to Bob into all_people.

Step (4): We add carla to the database.

Step (5): We read from people.txt to make alice and bob friends.  In other words,
we need to update alice's list of friends to add bob, and update bob's list of friends
to add alice.  We do this with a linear search over all_people to find the appropriate
pointers to alice and bob.  Once we find the pointer to alice, we get alice's friends
and push_back a pointer to bob.  Then with the pointer to bob, we get bob's friends
and push_back a pointer to alice.

Step (6): We repeat the same idea to make alice friends with carla.  Note that at the
end of all of this, alice has two friends (bob and carla), but bob and carla each have
only one friend (alice).