使用JavaScript的哈希表实现

分类：编程问答

如何解决使用JavaScript的哈希表实现

我正在尝试使用javascript实现哈希表。目前，一切正常，但是我在给定特定键的情况下无法使用get方法检索哈希表中的值。我正在使用线性探测以避免碰撞。当我对键“ Alejandro”进行哈希处理时，我会将键映射到0索引。然后，将其添加到哈希表中。然后我尝试“ Rosalby”，它也映射到0索引。我使用线性探测来找到下一个可用的插槽，在本例中，空索引为1，然后将Rosalby的值放在该插槽中。到目前为止，看来我的碰撞管理得很好。然而;当我尝试在get方法中获取值时，无法获得正确的值，这就是哈希表的样子。

此外，我想提到的是，我使哈希表的大小变大了，并且给定了特定的键，我得到了正确的值，这仅仅是因为我没有碰撞。预先谢谢你。

// Hash table implementation
class HashTable {
  // constructor functio
  constructor(size) {
    this.size = size;
    this.buckets = this.initArray(size);
    this.limit = 0;
  }

  // init array function
  initArray(size) {
    // init an array
    const array = [];

    // populate the array base on the size
    for (let i = 0; i < size; i++) {
      // push null to the array
      array.push(null);
    }
    // return the array
    return array;
  }

  // mapping key to index
  hash(key) {
    let total = 0;

    // get unique code of character in the string
    for (let i = 0; i < key.length; i++) {
      let keyCode = key.charCodeAt(i)
      //  console.log("Key code:",keyCode)

      // sum up the unique code
      total += keyCode;
      // console.log(total); 
    }
    // mod the total to the size of the hash table
    const hashIndex = total % this.size;
    // return that index
    return hashIndex;
  }

  // put method
  put(key,value) {
    // throw an erro if hashTable is full
    if (this.limit >= this.size) throw "Hash Table is full";

    // hash the key
    let hashIndex = this.hash(key);
    // console.log(hashIndex);

    // linear probing
    while (this.buckets[hashIndex] != null) {
      hashIndex++;

      hashIndex = hashIndex % this.size;
    }

    // add the value at that key to the buckets array
    this.buckets[hashIndex] = value;

    // increase the limit
    this.limit++;
  }

  // get method
  get(key) {
    // hash the key
    let hashIndex = this.hash(key);
    let value = this.buckets[hashIndex];

    // return the value at that index
    return value;
  }
} // end of hash table

// sanity check
const ht = new HashTable(3);
console.log(ht);
// ht.hash("Rosalby");
console.log();
ht.put("Alejandro","555-5555");
ht.put("Rosalby","123-1231");
ht.put("Lalaland","000-0000");
// ht.put("Lalaland","919-1919");
console.log();
console.log(ht);
console.log("Alejandro:",ht.get("Alejandro"),"Rosalby:",ht.get("Rosalby"),"Lalaland:",ht.get("Lalaland"));

Hash Table console logs

解决方法

这是哈希表的快速实现：

class HashTable
{
    constructor(getHash)
    {
        if(!getHash) throw "Argument is null: getHash";
        
        this.getHash = getHash;
        this.buckets = {};
        this._count = 0;
    }
    
    //  throws an exception if the key already exists
    add(key,value)
    {
        const hashCode = this.getHash(key);
        const bucket = this.buckets[hashCode];
        if(!bucket)
        {
            this.buckets[hashCode] = [{key,value}];
            ++this._count;
            return;
        }
        for(let length = bucket.length,i = 0; i < length; ++i)
        {
            const item = bucket[i];
            if(item.key == key) throw "Duplicate key.";
        }
        bucket.push({key,value});
        ++this._count;
    }
    
    //  replaces the value if the key already exists
    set(key,i = 0; i < length; ++i)
        {
            const item = bucket[i];
            if(item.key == key) 
            {
                item.value = value;
                return;
            }
        }
        bucket.push({key,value});
        ++this._count;
    }

    get(key,defaultValue)
    {
        const hashCode = this.getHash(key);
        const bucket = this.buckets[hashCode];
        if(!bucket) return defaultValue;
        for(let length = bucket.length,i = 0; i < length; ++i)
        {
            const item = bucket[i];
            if(item.key == key) return item.value;
        }
        return defaultValue;
    }

    has(key)
    {
        const hashCode = this.getHash(key);
        const bucket = this.buckets[hashCode];
        if(!bucket) return false;
        for(let length = bucket.length,i = 0; i < length; ++i)
        {
            const item = bucket[i];
            if(item.key == key) return true;
        }
        return false;
    }
    
    get count()
    {
        return this._count;
    }
}

HashTable.unitTests = function()
{
    function getHash(value)
    {
        const stringValue = String(value);
        let result = 0;
        for(let length = stringValue.length,i = 0; i < length; ++i) result ^= stringValue.charCodeAt(i);
        return result;
    }
    
    let ht;
    
    //  empty
    ht = new HashTable(getHash);
    if (ht.count != 0) throw "failed";
    if (ht.has(null)) throw "failed";
    if (ht.has("")) throw "failed";
    if (ht.get(null) != void (0)) throw "failed";
    if (ht.get("") != void (0)) throw "failed";
    if (ht.get(null,1) != 1) throw "failed";

    //  add new
    ht = new HashTable(getHash);
    ht.add("a",1);
    if (ht.count != 1) throw "failed";
    if (!ht.has("a")) throw "failed";
    if (ht.get("a") != 1) throw "failed";

    //  set new
    ht = new HashTable(getHash);
    ht.set("a",1);
    if (ht.count != 1) throw "failed";
    if (!ht.has("a")) throw "failed";
    if (ht.get("a") != 1) throw "failed";

    //  add duplicate
    ht = new HashTable(getHash);
    ht.add("a",1);
    try { ht.add("a",1); throw "failed"; } catch (ex) { if (ex != "Duplicate key.") throw "failed"; }

    //  add+set duplicate
    ht = new HashTable(getHash);
    ht.add("a",1);
    ht.set("a",1);
    if (ht.count != 1) throw "failed";
    if (!ht.has("a")) throw "failed";
    if (ht.get("a") != 1) throw "failed";

    //  set+add duplicate
    ht = new HashTable(getHash);
    ht.set("a",1); throw "failed"; } catch (ex) { if (ex != "Duplicate key.") throw "failed"; }

    //  set duplicate
    ht = new HashTable(getHash);
    ht.set("a",1);
    if (ht.count != 1) throw "failed";
    if (!ht.has("a")) throw "failed";
    if (ht.get("a") != 1) throw "failed";

    //  add multiple
    ht = new HashTable(getHash);
    ht.add("a",1);
    ht.add("b",2);
    if (ht.count != 2) throw "failed";
    if (!ht.has("a")) throw "failed";
    if (!ht.has("b")) throw "failed";
    if (ht.get("a") != 1) throw "failed";
    if (ht.get("b") != 2) throw "failed";

    //  add with collision
    ht = new HashTable(getHash);
    ht.add("ab",1);    //  hash code 3
    ht.add("ba",2);    //  hash code 3
    if (ht.count != 2) throw "failed";
    if (!ht.has("ab")) throw "failed";
    if (!ht.has("ba")) throw "failed";
    if (ht.get("ab") != 1) throw "failed";
    if (ht.get("ba") != 2) throw "failed";

    //  set with collision
    ht = new HashTable(getHash);
    ht.set("ab",1);    //  hash code 3
    ht.set("ba",2);    //  hash code 3
    if (ht.count != 2) throw "failed";
    if (!ht.has("ab")) throw "failed";
    if (!ht.has("ba")) throw "failed";
    if (ht.get("ab") != 1) throw "failed";
    if (ht.get("ba") != 2) throw "failed";
}

HashTable.unitTests();
console.log("OK");
<html>
<head>
<script src="HashTable.js"></script>
</head>
<body>
</body>
</html>

一些说明：

  • 哈希函数是作为哈希表的配置而有意提供的；使班级更可重用；
  • 使用this._count是实施容量限制的更好方法（虽然未实现，但易于添加）；
  • 单元测试可以扩展更多。但是，我们在这里所拥有的足以表明HashTable类是可操作的；
  • ^运算符（按位XOR）通常可以更好地分布所生成的哈希码。参见Why are XOR often used in java hashCode() but another bitwise operators are used rarely?。

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