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JavaScript Data Structures: Hash Table: Get all entries

April, 2020

Intro 🌐

Last time, we learned how to get the values from our hash table.

Today, we’ll learn how to get the whole entries (= all key-value pairs) of our Hash Table.


Requirements 💭

We need the following parts to get the values from our Hash Table:

  • a method to get the entries (entries)

Starter Code ▶️

We start with the code with the set method, so that we can use the set method to add some data.

class Hashtable {
  constructor() {
    this.data = [];
    this.size = 0;
  }

  hash(key) {
    const chars = key.split("");
    const charCodes = chars.map((char) => char.charCodeAt());
    const charCodeSum = charCodes.reduce((acc, cur) => acc + cur);
    return charCodeSum;
  }

  set(key, value) {
    const hash = this.hash(key);

    if (!this.data[hash]) {
      this.data[hash] = [];
    }

    this.data[hash].push([key, value]);

    this.size++;
  }
}

If you are not familiar with the hash function, re-read this post.


Thoughts 💭

First, we should think about the constraints and possibilities:

  • first, we declare an empty array for the entries
  • then we iterate over the data array
  • if there is data (= array of key-value pairs) at this specific index, iterate over this data (= the single key-value pairs)
  • add the data (= key-value pair) to the entries array
  • return the entries array

As you can see, the steps are nearly the same as in our keys function and values function


Example

We want to get all the entries.

// current hash table data:
[
  [["age", 33]],
  [
    ["name", "miku86"],
    ["mean", false],
  ],
];

// desired data:
[
  ["age", 33],
  ["name", "miku86"],
  ["mean", false],
];

Steps

// current hash table data:
[
  [["age", 33]],
  [
    ["name", "miku86"],
    ["mean", false],
  ],
];

// then we iterate over the data array
[["age", 33]];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["age", 33];

// add the data (= key-value pair) to the entries array
[["age", 33]];

// then we iterate over the data array
[
  ["name", "miku86"],
  ["mean", false],
];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["name", "miku86"];

// add the data (= key-value pair) to the entries array
[
  ["age", 33],
  ["name", "miku86"],
];

// if there is data (= array of key-value pairs) at this specific index
// then iterate over this data (= the single key-value pairs)
["mean", false];

// add the data (= key-value pair) to the entries array
[
  ["age", 33],
  ["name", "miku86"],
  ["mean", false],
];

// desired data:
[
  ["age", 33],
  ["name", "miku86"],
  ["mean", false],
];


Implementation ⛑

// a Hash Table class
class Hashtable {
  constructor() {
    this.data = [];
    this.size = 0;
  }

  hash(key) {
    const chars = key.split("");
    const charCodes = chars.map((char) => char.charCodeAt());
    const charCodeSum = charCodes.reduce((acc, cur) => acc + cur);
    return charCodeSum;
  }

  set(key, value) {
    const hash = this.hash(key);

    if (!this.data[hash]) {
      this.data[hash] = [];
    }

    this.data[hash].push([key, value]);

    this.size++;
  }

  entries() {
    // declare an empty array for the entries
    const entries = [];

    // iterate over the data array (I call a single array a "bucket")
    for (let bucket of this.data) {
      // if there is data (= array of key-value pairs) at this specific index
      if (bucket) {
        // iterate over this data (= the single key-value pairs)
        for (let item of bucket) {
          // add the data (= key-value pair) to the entries array
          entries.push(item);
        }
      }
    }

    // return the entries array
    return entries;
  }
}

Note: I’m using a for ... of-loop. If you don’t know how this works, you can read about it on MDN. You can use whatever you want to use, a default for-loop, a for ... in-loop, a functional approach etc.


Result

// create a new hash table
const newHashtable = new Hashtable();

// add three new key-value pairs
newHashtable.set("name", "miku86");
newHashtable.set("mean", false);
newHashtable.set("age", 33);

// show the hash table data
console.log(newHashtable.data);
// [ <301 empty items>, [ [ 'age', 33 ] ], <115 empty items>, [ [ 'name', 'miku86' ], [ 'mean', false ] ] ]

// show the entries
console.log(newHashtable.entries());
// [['age', 33], ['name', 'miku86'], ['mean', false]] ✅


Next Part ➡️

We managed to write a function to get all entries, great work!

As you can see, most of the logic was nearly the same as in our keys function and values function.

Next time, we’ll recap the Hash Table!

Need some mentoring? Click here!


Further Reading 📖


Questions ❔

  • How would you implement the entries-function?
  • Is there an alternative to our current approach, e.g. using already existing methods like get?
  • How would you write this code in a functional style?

miku86

Hi! I'm Michael 👋 I'm a problem solver and mentor. My goal is to build great products and help you to become a better developer.