#hashtables

DATA STRUCTURES AND ALGORITHMS: DEEP DIVE USING JAVA 2018 Free Download Now

DATA STRUCTURES AND ALGORITHMS: DEEP DIVE USING JAVA 2018 Free Download Now

DATA STRUCTURES AND ALGORITHMS: DEEP DIVE USING JAVA 2018 Free Download Now Learn about Arrays, Linked Lists, Trees, Hashtables, Stacks, Queues, Heaps, Sort algorithms and Search algorithms Useful Links ULTIMATE WEB DESIGNER & DEVELOPER COURSE: BUILD 23 PROJECTS! 2018 free Download Now ADVANCED C PROGRAMMING: POINTERS…

Today was our first lecture that was done solely on a whiteboard. It’s nice to see that we’re changing directions and starting to become more focused on computer science and shifting away from Ruby.

Every Hashtable supports the same set of methods, regardless of its keys and values.

Like ArrayList it can be thought of as sending questions or commands to the Hashtable.

Some common methods

hashtable.Keys returns a list of the keys used in a hashtable as an ICollection. eg:

String[] animal_array = {"cats", "dogs", "ponies"}; String[] plant_array = {"tulips", "peonies", "ponga"}; var organisms = new Hashtable() { {"animals", animal_array}, {"plants", plant_array} }; var keys = organisms.Keys; // => "animals" and "plants"

hashtable.Values returns the list of values in the hash as an ICollection

var keys = organisms.Values; // => animal_array and plant_array

hashtable.ContainsKey() returns true if the hashtable contains the specified key.

Console.WriteLine(organisms.ContainsKey("animals")); // yep, it's there

hashtable.ContainsValue()returns true if the hashtable contains the specified value.

Console.WriteLine(organisms.ContainsValue("plant_array")); // yep

A Hashtable is not generic, which means if you want to constrain the type of elements that go into the collection, you use the Dictionary<T> class instead.

It works the same way, as below:

var lettersToNumbers = new Dictionary<string, int> { {"a", 23}, {"b", 45}, {"c", 67}, {"d", 89} };

Hashtables are another kind of collection, they differ from arrays by recording a key with each value. Whereas an array uses a sequential number system to identify the elements.

So a Hashtable entry has a {key, value}, each can be any type of operator: "String, int, bool...etc.

Writing a Hashtable:

var person = new Hashtable() {     {"firstName", "Frank"}, {"lastName", "Hardy"}, {"age", 25} };

Creates a hashtable with the following contents

the "firstName" key has value "Frank" the "lastName" key has value "Hardy" the "age" key has value 25.

Reading from Hashtables:

You "key into" a hash using the same bracket [...] notation as an array.

The key (any object) goes between the brackets.

Console.WriteLine(person["firstName"]);

result: Frank

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Keys can be anything! They can be a declared variable of an array or different hashtable.

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Changing a Hashtable:

We can write into a hashtable using the following format:

variable[key] = value;

eg: person["age"] = 100;