DEV Community

stuxnat
stuxnat

Posted on • Updated on

String Compression Algorithm: JavaScript

I recently completed a coding challenge. One of the tasks was to write a string compression algorithm. This is a very popular algorithm, and I will be writing about how to solve it here.

The task:
Given a string of letters, if there are consecutive repeating characters, append the character followed by the number of times it is seen. If the character appears only once, do not include a number.
Ex: compress a string 'aabbcddd', to 'a2b2cd3'

The first steps we'd have to take are:

  1. Split the string into an array to be able to iterate through it
  2. Create a new string, which would hold the compressed version
  3. Create a for loop to iterate through the first string
function compress(string){
let compressed = ""
let stringArray = string.split("")
for (let i = 0; i < stringArray.length; i++){

     }
}
Enter fullscreen mode Exit fullscreen mode

Next, we would need to keep track of several things within the for loop:

  1. The current count of the consecutive letters with each iteration
  2. The letter that is being counted.

The function will now look like this:

function compress(string){
let compressed = ""
let stringArray = string.split("")
for (let i = 0; i < stringArray.length; i++){
     let count = 1 
     let letter = stringArray[i]
     }
}

Enter fullscreen mode Exit fullscreen mode

We will need to create a while loop to compare letters. The loop will have two conditions: 1) if i is less than the length of stringArray -1, 2) if the current letter in the iteration is the same as the neighboring letter

When both conditions are met, both the count and index positions will be incremented

function compress(string){
let compressed = ""
let stringArray = string.split("")
for (let i = 0; i < stringArray.length; i++){
     let count = 1 
     let letter = stringArray[i]
    while (i < stringArray.length - 1 && stringArray[i] === stringArray[i + 1] {
   count++
   i++
  }
 }
}
Enter fullscreen mode Exit fullscreen mode

For the final part, we want to add the letter and the number of times it appears to the compressed string. Then we want to return the string.

function compress(string){
let compressed = ""
let stringArray = string.split("")
for (let i = 0; i < stringArray.length; i++){
     let count = 1 
     let letter = stringArray[i]
    while (i < stringArray.length - 1 && stringArray[i] === stringArray[i + 1] {
   count++
   i++
  }
if (count === 1){
 compressed += letter 
  } else { 
  compressed =+ letter + count}
 }
}

return compressed
}
Enter fullscreen mode Exit fullscreen mode

aaaand done!

Top comments (0)