Permutation using Recursion in Javascript

Question

Through youtube, I've learned to obtain a multi-dimensional output of permutations using recursion, i.e anagrams with passing in an array of letters as an argument (i.e ['a', 'b', 'c']) but is there a way to modify the code to pass in a string instead 'abc' to obtain the output below??

Into [ 'abc', 'bac', bca', 'acb', 'cab', 'cba' ]

This is my code. Is there a way to modify to obtain the desired result, using a string as the argument(i.e 'abc')? Thank you, much appreciated

function anagrams(inputString){
  if(inputString.length ===0) return [[]];
  const stringArr = inputString;
  const firstEl = stringArr[0];
  const restArr = stringArr.slice(1);
  const anagramWithoutFirst = anagrams(restArr);
  const masterArr = [];

  anagramWithoutFirst.forEach(anagram => {
    for (let i=0; i<=anagram.length; i++){
      const anagramWithFirst = [...anagram.slice(0, i), firstEl, ...anagram.slice(i)]
      masterArr.push(anagramWithFirst)
    }
  })
  return masterArr
}

console.log(anagrams(['a', 'b', 'c']))

Is there a problem with mapping the result on join, e.g. anagrams(['a', 'b', 'c']).map(e => e.join(""))? — ggorlen
– ggorlen, Commented Mar 22, 2022 at 13:59

Alex · Accepted Answer · 2022-03-22 08:21:16Z

1

This can be achieved using a higher order function which takes in the original function and returns a new function which splits the text up, calls the original function then joins the result back together

function anagrams(inputString){
  if(inputString.length ===0) return [[]];
  const stringArr = inputString;
  const firstEl = stringArr[0];
  const restArr = stringArr.slice(1);
  const anagramWithoutFirst = anagrams(restArr);
  const masterArr = [];

  anagramWithoutFirst.forEach(anagram => {
    for (let i=0; i<=anagram.length; i++){
      const anagramWithFirst = [...anagram.slice(0, i), firstEl, ...anagram.slice(i)]
      masterArr.push(anagramWithFirst)
    }
  })
  return masterArr
}

function splitAndUnsplit(f) {
  return (str) => (f(str.split('')).map(xs => xs.join('')))
}

console.log(splitAndUnsplit(anagrams)('abc'))
console.log(anagrams(['a', 'b', 'c']))

Edit:

To expand on @Scott Sauyet's great comment, we need two sets of parens as we are calling two functions.


function splitAndUnsplit(f) {
  return function(str){
    const inputArr  = str.split('') 
    const outputArr = f(inputArr)
    const outputStr = outputArr.map(xs => xs.join('')))
    return outputStr
  }
}

The first set of parens is used to create a function which will accept and return a string, and the second set is required to call that function with a supplied value

edited Mar 22, 2022 at 8:21

answered Mar 21, 2022 at 22:09

Alex

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2 Comments

Enochy Over a year ago

Heya Alex! Thank you for your response. when you are console.logging the splitAndUnsplit, why are there two sets of parens? and not like anagrams('abc')

Scott Sauyet Over a year ago

That's how the higher-order function works. splitAndUnsplit(anagrams) returns a function which itself takes your initial string. You could name it and then call it, like const stranagram = splitAndUnsplit (anagram) and then call it with stranagram ('abc'). But you can also call it directly. An alternative would be const stranagram = (s) => anagram (s.split ('')) .map (xs => xs.join('')). But Alex extracted that pattern into a reusable function.

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