assoc

早期freebsd实现

Using associations

	Associations are special values that act like matrices, except
	that they are more general (and slower) than normal matrices.
	Unlike matrices, associations can be indexed by arbitrary values.
	For example, if 'val' was an association, you could do the following:

		val['hello'] = 11;
		val[4.5] = val['hello'];
		print val[9/2];

	and 11 would be printed.

	Associations are created by the 'assoc' function.  It takes no
	arguments, and simply returns an empty association.  You can then
	insert elements into the association by indexing the returned value
	as shown above.

	Associations are multi-dimensional.  You can index them using one to
	four dimensions as desired, and the elements with different numbers
	of dimensions will remain separated.  For example, 'val[3]' and
	'val[3,0]' can both be used in the same association and will be
	distinct elements.

	When references are made to undefined elements of an association,
	a null value is simply returned.  Therefore no bounds errors can
	occur when indexing an association.  Assignments of a null value
	to an element of an association does not delete the element, but
	a later reference to that element will return the null value as if
	the element was undefined.  Elements with null values are implicitly
	created on certain other operations which require an address to be
	taken, such as the += operator and using & in a function call.

	The elements of an association are stored in a hash table for
	quick access.  The index values are hashed to select the correct
	hash chain for a small sequential search for the element.  The hash
	table will be resized as necessary as the number of entries in
	the association becomes larger.

	The size function returns the number of elements in an association.
	This size will include elements with null values.

	Double bracket indexing can be used for associations to walk through
	the elements of the association.  The order that the elements are
	returned in as the index increases is essentially random.  Any
	change made to the association can reorder the elements, this making
	a sequential scan through the elements difficult.

	The search and rsearch functions can search for an element in an
	association which has the specified value.  They return the index
	of the found element, or a NULL value if the value was not found.

	Associations can be copied by an assignment, and can be compared
	for equality.  But no other operations on associations have meaning,
	and are illegal.