First, it usually does not matter that much in practice. Most hash functions are “good enough”. But if you really care, you should know that it is a research subject by itself. There are thousand of papers about that. You can still get a PhD today by studying & designing hashing algorithms. Your second hash … Read more
ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable. Both are thread safe, but there are obvious performance wins with ConcurrentHashMap. When you read from a ConcurrentHashMap using get(), there are no locks, contrary to the HashTable for which all operations are simply synchronized. HashTable was released in old versions of Java whereas ConcurrentHashMap is a java 5+ … Read more
I’m looking for an explanation of how a hash table works – in plain English for a simpleton like me! For example, I know it takes the key, calculates the hash (I am looking for an explanation how) and then performs some kind of modulo to work out where it lies in the array where … Read more
Bash 4 Bash 4 natively supports this feature. Make sure your script’s hashbang is #!/usr/bin/env bash or #!/bin/bash so you don’t end up using sh. Make sure you’re either executing your script directly, or execute script with bash script. (Not actually executing a Bash script with Bash does happen, and will be really confusing!) You declare an associative array by doing: You can fill it up with … Read more
First you shoud maybe read this article. When you use lists and you are looking for a special item you normally have to iterate over the complete list. This is very expensive when you have large lists.A hashtable can be a lot faster, under best circumstances you will get the item you are looking for with … Read more
Java’s Set and Map interfaces specify two very different collection types. A Set is just what it sounds like: a collection of distinct (non-equal) objects, with no other structure. A Map is, conceptually, also just what it sounds like: a mapping from a set of objects (the distinct keys) to a collection of objects (the values). Hashtable and HashMap both implement Map, HashSet implements Set, and they all use hash codes … Read more
Yes, it is a hash mapping or hash table. You can read a description of python’s dict implementation, as written by Tim Peters, here. That’s why you can’t use something ‘not hashable’ as a dict key, like a list: You can read more about hash tables or check how it has been implemented in python and why it is implemented that … Read more
There are several differences between HashMap and Hashtable in Java: Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values. One of HashMap’s subclasses is LinkedHashMap, so in the event that you’d want predictable iteration order (which is insertion order by default), you … Read more
Use JavaScript objects as associative arrays. Associative Array: In simple words associative arrays use Strings instead of Integer numbers as index. Create an object with JavaScript allows you to add properties to objects by using the following syntax: An alternate syntax for the same is: If you can, also create key-to-value object maps with the following … Read more