hashtable | 易学教程

Good way to hash a float vector?

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I am well aware of all the problems involved in comparing floats. This is exactly the reason for this question. I'm looking to create a fast hash table for values that are 3D vectors (3 floats - x,y,z). It can be assumed that the length of the vector is always 1.0 ( sqrt(x*x+y*y+z*z) is 1.0) Essentially this means that I'm looking for a hash function that takes values that are almost equal to the same unsigned int value and a corresponding equality operator that is true if the hash values are equal (not not necessarily only if they are equal) Edit - False positives (i.e. vectors that are

Why are hash maps better than trie maps?

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问题 By trie map I mean an associative array, where the payloads are stored in a trie instead of a hash table. When I'm using a hash map/table, the keys I use are typically strings. What are the advantages of a hash map over some trie based map? I have read that a hash map is faster - but it seems to me that a consistent hash functions would have to check each element of the (char) array for the final hash - iterating over the array once. In a trie you would similarly have to iterate over the

Difference between Hashtable and Collections.synchronizedMap(HashMap)

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As far as I know, java.util.Hashtable synchronizes each and every method in the java.util.Map interface, while Collections.synchronizedMap(hash_map) returns a wrapper object containing synchronized methods delegating calls to the actual hash_map (correct me if I am wrong). I have two questions : What difference does it make to synchronize each and every method and to have a wrapper class? What are the scenarios to choose one over the other? What happens when we do Collections.synchronizedMap(hash_table) ? Will this be equal to simply using a normal java.util.Hashtable ? Nadir Muzaffar Here are

Hash tables VS associative arrays

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Recently I have read about hash-tables in a very famous book " Introduction to Algorithms ". I haven't used them in any real applications yet, but want to. But I don't know how to start. Can anyone give me some samples of using it, for example, how to realize a dictionary application (like ABBYY Lingvo) using hash-tables? And finally I would like to know what is the difference between hash-tables and associative arrays in PHP, I mean which technology should I use and in which situations? If I am wrong (I beg pardon) please correct me, because actually I am starting with hash-tables and I have

Building a sorted dictionary using ToDictionary

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问题 I'm not an expert in C# and LINQ. I have a Dictionary , which I understand a hash table, that is, keys are not sorted. dataBase = new Dictionary<string, Record>() Record is a user-defined class that holds a number of data for a given key string. I found an interesting example that converts this Dictionary into a sorted dictionary by LINQ: var sortedDict = (from entry in dataBase orderby entry.Key ascending select entry) .ToDictionary(pair => pair.Key, pair => pair.Value); This code works

The fundamentals of Hash tables?

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I'm quite confused about the basic concepts of a Hash table. If I were to code a hash how would I even begin? What is the difference between a Hash table and just a normal array? Basically if someone answered this question I think all my questions would be answered: If I had 100 randomly generated numbers (as keys), how would I implement a hash table and why would that be advantageous over an array? Psuedo-code or Java would be appreciated as a learning tool... The answers so far have helped to define hash tables and explain some theory, but I think an example may help you get a better feeling

What is the use of a Hash range in a dynamodb table?

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问题 I am new to dynamodb (ddb). I was going through its documentation and it says to add Hash Key and a Hash Range key. In the documentation it says that ddb will create an usorted index on the hash key and a sorted index on the hash range. What is the purpose of having these 2 keys rather than just one key. Is it because the first key is used like : A HashTable which contains : key - range of keys for each value in the hash range 2nd HashTable hash range key - Actual data value. This would help

C# - parsing json formatted data into nested hashtables

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问题 I’m trying to work with some json formatted data in C#, but, I’m having some problems determining the proper way to approach the problem. My issue is that the json formatted data will be in an unknown format (I know that sounds odd … please read on). Basically, the json formatted data will be some collection of name/value pairs where the values may or may not be arrays of nested name/value pairs. To make things more fun, the nesting of the name/value pair arrays can continue on ad infinitum.

How to keep the order of elements in hashtable

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I have a hashtable . values() method returns values in some order different from the order in which i am inserted.How can i get the values in the same order as i inserted?Using LinkedHashmap is an alternative but it is not synchronized. cletus Use a LinkedHashMap . Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map

Multi-valued hashtable in Java

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Is it possible to have multiple values for the same key in a hash table? If not, can you suggest any such class or interface which could be used? No. That's kind of the idea of hash tables. However, you could either roll your own with a Map<YourKeyObject, List<YourValueObject>> and some utility methods for creating the list if it's not present, or use something like the Multimap from Google Collections . Example: String key = "hello"; Multimap<String, Integer> myMap = HashMultimap.create(); myMap.put(key, 1); myMap.put(key, 5000); System.out.println(myMap.get(key)); // prints either "[1, 5000]