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#algorithms #Arrays #data structures #Deep Dive #Hashtables #Heaps #Java #linked lists #Queues #Search algorithms #Sort algorithms #Stacks #Trees

Taking Hash Tables Off The Shelf // dev.to/vaidehijoshi/taking-hash-tables-off-the-shelf // #hashtables

#hashtables

I also like how the whole project was MAKE A HASHTABLE and in the main file for the user interface I use C++’s built in hashtable <map>

#hashtables

W2D5* - Hashing and Hashes

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.

Our project today was to build our own HashMap, with the assistance of some specs. I understand the concept of it and reason why hashes and sets are preferred for their efficiency with search. But building one was something I struggled with. I’m going to have to revisit this later tonight.

#appacademy #sf #hashtables

Hashtable Operations

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

...And the funs just beginning find more operations and methods at official Hashtable documentation.

#C #Hashtables

Hashtable Generics

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} };

This creates a Dictionary in which the keys are all String and the values are all int.

#C #Hashtables

Hashtables Explained

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

-------------------------------------------------------------------------------------------

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

-------------------------------------------------------------------------------------------

Changing a Hashtable:

We can write into a hashtable using the following format:

variable[key] = value;

eg: person["age"] = 100;

We can use the same method to write new keys and values into the hashtable.

#C #Hashtables

#php #arrays #php arrays #hashtables

Hashtables Explained

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

-------------------------------------------------------------------------------------------

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

-------------------------------------------------------------------------------------------

Changing a Hashtable:

We can write into a hashtable using the following format:

variable[key] = value;

eg: person["age"] = 100;

We can use the same method to write new keys and values into the hashtable.

#C #Hashtables

#hashtables

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