hashing, hash tables. hashCodes, and all types of maps

continuity

"A hash function that is used to search for similar (as opposed to equivalent) data must be as continuous as possible; two inputs that differ by a little should be mapped to equal or nearly equal hash values."[

how many references must you change to insert a link at the end of a singly linked list

1

necessary properties of a hash function

1) . Determinism 2) . Uniformity 3) Defined range 4) Data normalization 5) continuity 6) Non-invertible

how many references must you change to insert a link in the middle of a singly linked list

2

Uniformity

A good hash function should map the expected inputs as evenly as possible over its output range - That is, every hash value in the output range should be generated with roughly the same probability - The reason for this last requirement is that the cost of hashing-based methods goes up sharply as the number of collisions—pairs of inputs that are mapped to the same hash value—increases. If some hash values are more likely to occur than others, a larger fraction of the lookup operations will have to search through a larger set of colliding table entries. When testing a hash function, the uniformity of the distribution of hash values can be evaluated by the chi-squared test.

hash function

A hash function is any function that can be used to map data of arbitrary size onto data of a fixed size. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes

set

A set is like a hash map except it only stores keys, without values. in java we use hash maps to implement this

queue

FIFO

three types of maps in java

HashMap, TreeMap, and LinkedHashMap

Non-invertible

In cryptographic applications, hash functions are typically expected to be practically non-invertible, meaning that it is not realistic to reconstruct the input datum x from its hash value h(x) alone without spending great amounts of computing time (see also One-way function)

example of data normalization

In some applications, the input data may contain features that are irrelevant for comparison purposes. For example, when looking up a personal name, it may be desirable to ignore the distinction between upper and lower case letters

stack

LIFO

Search algorithms that use hashing consist of two separate parts

The first step is to compute a hash function that transforms the search key into an array index. Ideally, different keys would map to different indices. This ideal is generally beyond our reach, so we have to face the possibility that two or more different keys may hash to the same array index. Thus, the second part of a hashing search is a collision-resolution process that deals with this situation

purpose of hash table

The purpose of a hash table is to have O(c) constant time complexity in adding and getting the elements With a hash table if you want to retrieve an element you just pass the key and the hash function will return you the desired element

hash collisions

Typically, the domain of a hash function (the set of possible keys) is larger than its range (the number of different table indices), and so it will map several different keys to the same index which could result in collisions. So then, each slot of a hash table is associated with (implicitly or explicitly) a set of records, rather than a single record. For this reason, each slot of a hash table is often called a bucket, and hash values are also called bucket listing[citation needed] or a bucket index.

dynamic hash table

When the hash function is used to store values in a hash table that outlives the run of the program, and the hash table needs to be expanded or shrunk, the hash table is referred to as a dynamic hash table.

HashMap

a collection class that is designed to store elements as key-value pairs

dequeue

a double-ended queue is an abstract data type that generalizes a queue, for which elements can be added to or removed from either the front or back. use a double-ended linked list to implement this

hash function (in terms of tables)

a function which when given a key, generates an address in the table. - allows for fast access into an array

which of the following is not true, a reference object to a class object: a) can be used to access public methods in the object b) has a size dependent on its class c) has the data type the class d) does not hold the object itself

b

which of the following is not true, iterators would be useful if you wanted to: a) do an insertion sort on a linked list b) insert a new link at the beginning of a list c) swap two links at arbitrary locations d) delete all links with a certain key valur

b

when you create a reference to a link in a linked list, it ___________

can refer to any link you want

assuming current points to the next-to-last link in a singly linked list, what statement will delete the last link from the list

current.next = null

should we use a single or double-ended linked list when implementing a queue

double, its more efficient

of the list discussed in this chapter, which one would be best for implementing a queue

double-ended list

access to the links in a linked list is usually through the __________ link

first

HashSet

is a regular set - all objects in a set are distinct. HashSet stores and retrieves elements by their content, which is internally converted into an integer by applying a hash function. Elements from a HashSet are retrieved using an Iterator. The order in which elements are returned depends on their hash codes

when all references to a link are changed to refer to something else, what happens to the link

java's garbage collection process destroys it

assuming a copy takes longer than a comparison, is it faster to delete an item with a certain key from a linked list or from an unsorted array

linked list

would better to use a linked list or array to implement a stack

linked list because it uses memory more efficiently

what is java's answer to dictionaries

maps

Determinism

meaning that for a given input value it must always generate the same hash value - This requirement excludes hash functions that depend on external variable parameters, such as pseudo-random number generators or the time of day.

can a map in java contain duplicate keys

no

is the map interface a subtype of the collection interface

no

how many times would you need to traverse a singly linked list to delete the item with the largest key

once if the link contains a previous reference

maps

provide a way of looking up one thing based on the value of another.

hashCode() in java

provides a numeric representation of an object

separate chaining collision resolution

putting the keys that collide in a linked list. A hash table then is an array of lists

in the insertFirst() method in the linkedList.java program the statement newLink.next = first; means that:

the next field of the new link will refer to the old first link

collision

when two objects have the same hash value