I am fairly stupid new to programming. I want to implement a simple hashtable in javascript (for educational purposes). Everything works correctly, except when I try to overwrite some value it holds the previous value. For example when I try hashTable.set(cherry, 100), then hashTable.set(cherry, 4) and then hashTable.get(cherry) gives me 100 instead of 4. I've tried going over it with debugger, but turns out debugger doesn't help if you are fairly stupid new to programming. Heres the code:

    class HashTable {
  constructor(size) {
    this.data = new Array(size);
  }

  _hash(key) {
    let hash = 0;
    for (let i = 0; i < key.length; i  ) {
      hash = (hash   key.charCodeAt(i) * i) % this.data.length;
    }
    return hash;
  }

  set(key, value) {
    const address = this._hash(key);
    if (!this.data[address]) {
      this.data[address] = [];
    }
    if (Array.isArray(this.data[address])) {
      for (let arr of this.data[address].values()) {
        if (arr[0] === key) {
          const arr1 = arr[1];
          arr[1] === value;
          return;
        }
      }
    }
    this.data[address].push([key, value]);
  }
  get(key) {
    const address = this._hash(key);
    const currentNode = this.data[address];
    if (currentNode) {
      for (let arr of currentNode) {
        if (arr[0] === key) {
          return arr[1];
        }
      }
    }
    return undefined;
  }
}

const myHashTable = new HashTable(2);
myHashTable.set("cherry", 100);
myHashTable.set("cherry", 4);
console.log(myHashTable.get("cherry")); // returns 100 instead of 4
myHashTable.set("peach", 9);
console.log(myHashTable.get("peach"));
myHashTable.set("apple", 2);
console.log(myHashTable.get("apple"));

CodePudding user response：

class HashTable {
  constructor(size) {
    this.data = new Array(size);
  }

  _hash(key) {
    let hash = 0;
    for (let i = 0; i < key.length; i  ) {
      hash = (hash   key.charCodeAt(i) * i) % this.data.length;
    }
    return hash;
  }

  set(key, value) {
    const address = this._hash(key);
    if (!this.data[address]) {
      this.data[address] = [];
    }

    for (let el of this.data[address]) {
      if (el[0] === key) {
        el[1] = value;
        return;
      }
    }
    this.data[address].push([key, value]);
  }
  get(key) {
    const address = this._hash(key);
    const currentNode = this.data[address];
    if (currentNode) {
      for (let arr of currentNode) {
        if (arr[0] === key) {
          return arr[1];
        }
      }
    }
    return undefined;
  }
}

const myHashTable = new HashTable(2);
myHashTable.set("cherry", 100);
myHashTable.set("cherry", 4);
console.log(myHashTable.get("cherry")); // returns 100 instead of 4
myHashTable.set("peach", 9);
console.log(myHashTable.get("peach"));
myHashTable.set("apple", 2);
console.log(myHashTable.get("apple"));

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You were doing some funky stuff in the set function. That's what I changed. Now, it looks through the elements of this.data at address and checks the first element to make sure the key is not the same. If so, it just updates the value and returns early. I think that was what you had in mind.

To make this a better solution, I suggest you add a capacity to the underlying array and when the density reaches some percentage, you grow the underlaying array to reduce hash collisions.