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  1. PO
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    <p>Firstly we should say about the problem to be solved with Hashing algorithm. </p> <p>Suppose you have some data (maybe an array, or tree, or database entries). You want to find concrete element in this datastore (for example in array) as much as faster. How to do it?</p> <p>When you are built this datastore, you can calculate for every item you put special value (it named HashValue). The way to calculate this value may be different. But all methods should satisfy special condition: calculated value should be unique for every item.</p> <p>So, now you have an array of items and for every item you have this HashValue. How to use it? Consider you have an array of N elements. Let's put your items to this array according to their HashHalues.</p> <p>Suppose, you are to answer for this question: Is the item "it1" exists in this array? To answer to it you can simply find the HashValue for "it1" (let's call it f("it1")) and look to the Array at the f("it1") position. If the element at this position is not null (and equals to our "it1" item), our answer is true. Otherwise answer is false.</p> <p>Also there exist collisions problem: how to find such coolest function, which will give unique HashValues for all different elements. Actually, such function doesn't exist. There are a lot of good functions, which can give you good values.</p> <p>Some example for better understanding:</p> <p>Suppose, you have an array of Strings: A = {"aaa","bgb","eccc","dddsp",...}. And you are to answer for the question: does this array contain String S?</p> <p>Firstle, we are to choose function for calculating HashValues. Let's take the function f, which has this meaning - for a given string it returns the length of this string (actually, it's very bad function. But I took it for easy understanding).</p> <p>So, f("aaa") = 3, f("qwerty") = 6, and so on...</p> <p>So now we are to calculate HashValues for every element in array A: f("aaa")=3, f("eccc")=4,...</p> <p>Let's take an array for holding this items (it also named HashTable) - let's call it H (an array of strings). So, now we put our elements to this array according to their HashValues:</p> <p>H[3] = "aaa", H[4] = "eccc",... </p> <p>And finally, how to find given String in this array?</p> <p>Suppose, you are given a String s = "eccc". f("eccc") = 4. So, if H[4] == "eccc", our answer will be true, otherwise it fill be false.</p> <p>But how to avoid situations, when to elements has same HashValues? There are a lot of ways to it. One of this: each element in HashTable will contain a list of items. So, H[4] will contain all items, which HashValue equals to 4. And How to find concrete element? It's very easy: calculate fo this item HashValue and look to the list of items in HashTable[HashValue]. If one of this items equals to our searching element, answer is true, owherwise answer is false.</p>
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    1. POHashing - What Does It Do?
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    1. USIlnur
    1. USIlnur
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    1. POHashing - What Does It Do?
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    1. COHashing is not necessarily related to array indexing. Consider MD5 hashes.
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      1. USMike Samuel
    2. CO@Mike Samuel: Yes. Of course not only to arrays. Arrays indexes is a simpliest implementation of using hash codes.
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      1. USIlnur
 

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