Intro 🌐
Last time, we learned how to add data to our hash table .
Today, we'll learn how to get data out of our Hash Table.
Requirements 💭
We need the following parts to get data from our Hash Table:
- a method to get data(
get) - a key we want to search for (
key) - a hash function to hash our
key(hash)
Starter Code ▶️
We start with the code from the last lesson, 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 have to hash the key with our hash function
- if there is data at this specific array index (= the hashed key), iterate over this data, search for the key and return the key-value pair
- if there is data at this specific array index (= the hashed key), but not with our desired key, return null
- if there is no data at the array index of the hashed key, return null
Example
We want to get the key-value pair with the key name.
// current hash table data:
hashTableData = [
[
["name", "miku86"],
["mean", false],
],
[["age", 33]],
];
// desired data:
["name", "miku86"];
Steps:
// current hash table data:
hashTableData = [
[
["name", "miku86"],
["mean", false],
],
[["age", 33]],
];
// we hash the key (= `name`) with our imaginary (!) hash function
0;
// we go to the specific array index (= 0)
[
["name", "miku86"],
["mean", false],
];
// we go to the first key-value pair in this array
["name", "miku86"];
// this is the key (= "name") we search for, therefore we return the key-value pair
["name", "miku86"];
// desired data:
["name", "miku86"];
✅
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++;
}
get(key) {
// hash the key
const hash = this.hash(key);
// look if there is any data at this specific array index
if (this.data[hash]) {
// iterate over this data
for (const item of this.data[hash]) {
// look if the first value of the array is the desired key, e.g. name
if (item[0] === key) {
// if so, then return the key-value pair
return item;
}
}
}
// if there is no data at this specific array index
// OR if there is data, but not with the correct key
// then return null
return null;
}
}
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);
console.log(newHashtable.data);
// [ <301 empty items>, [ [ 'age', 33 ] ], <115 empty items>, [ [ 'name', 'miku86' ], [ 'mean', false ] ] ]
console.log(newHashtable.get("name"));
// [ 'name', 'miku86' ] ✅
console.log(newHashtable.get("mean"));
// [ 'mean', false ] ✅
console.log(newHashtable.get("age"));
// [ 'age', 33 ] ✅
console.log(newHashtable.get("nothing to see"));
// null, because wrong array index ✅
console.log(newHashtable.get("naem"));
// null, because correct array index, but wrong key ✅
✅
Next Part ➡️
We managed to write a simple function that gets us our data, great work!
Next time, we'll learn how to get all keys from our Hash Table.
Need some mentoring? Click here!
Further Reading 📖
Questions ❔
- How would you implement the
get-function? - How would you write this code in a functional style?
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