JavaScript Data Structures: Hash Table: Get values

2020-04-13

Intro 🌐

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

Today, we'll learn how to get the values of our Hash Table.

Requirements 💭

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

a method to get the values (values)

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 values
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 (= value) to the values array
return the values array

As you can see, the steps are nearly the same as in our last part, where we got our keys.

Example

We want to get all the values.

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

// desired data:
[33, "miku86", 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 (= value) to the values array
[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 (= value) to the values array
[33, "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 (= value) to the values array
[33, "miku86", false];

// desired data:
[33, "miku86", 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++;
  }

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

    // 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 (= value) to the values array
          values.push(item[1]);
        }
      }
    }

    // return the values array
    return values;
  }
}

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 values
console.log(newHashtable.values());
// [ 33, 'miku86', false ] ✅

✅

Next Part ➡️

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

Next time, we'll learn how to get all key-value pairs (= entries) from our Hash Table.

Need some mentoring? Click here!

Questions ❔

How would you implement the values-function?
How would you write this code in a functional style?
Should we remove duplicate values?