A hashmap is a way of organizing data by assigning each item, or key, to a unique value. It is like an address book or mailbox, but includes a hash function to match keys to values. Hash collisions can occur when too much data is entered.
In computing, a hashmap or hash table is a form of structuring data so that each piece of data is linked to a unique identification number or symbol. This allows for easy recovery, especially from a large database containing thousands of entries. The hashmap is not a specific part of the program, but rather refers to a form of data organization that assigns each item, known as a key, in the database to a value representing that item. The hash function is the part of the program that links keys to their values.
Telephone directories are an excellent example of a hashmap. The way the address book is structured represents the hashmap structure. It focuses on linking different values and linking only one value to another matching term or phone number. The names in the directory are the keys and the telephone numbers are the values associated with them. Each person in the book is assigned a different number and no two are alike.
Mailboxes also serve as a metaphor for the hashmap. Each person has a key to a single box. The box can contain multiple pieces of information – invoices, letters, postcards – all intended for the person who holds the key to the box. When the correct key is entered or matched to the correct box, it opens to reveal its contents.
Fully functional hashmaps would link a key to a “box” of memory in a computer database. Unfortunately, a factor known as a hash collision can occur, where two keys go into the same box. This happens when a large amount of data is entered and there aren’t enough “boxes” to accommodate it all.
Unlike an address book or mailbox, hashmaps include a third element in addition to the key and the value or mailbox it corresponds to. It also includes a hash function. A person knows which mailbox is theirs, but a computer needs to be told which key goes to which mailbox. The hash function examines the key, translates it into a series of numbers or codes, and matches it to the box containing the corresponding code. The name of the key, such as “Mary”, is only there so that it’s easier for people to interpret, while the actual code is a string of numbers that needs to be read by the computer.
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