Build a new array from two other arrays in Javascript

Question

I have two arrays:

alphabet is composed of letters (e.g. the English alphabet). dictionary is a list of objects with words and definitions, 'word' being the key for the word value.

I want to build a new array called frequency which would contain the number of words that start with each letter in the alphabet array, with indices corresponding to the alphabet array.

I've managed to come up with the following:

alphabet.forEach(function(letter) {
        frequency = dictionary.filter(item => item['word'].toLowerCase().startsWith(letter)).length;
    });

This gets me individual values for frequency. What is the best syntax in Javascript for building an array of those values? Is it another filter? Or should the code use the current forEach manually with a incremental index?

Convert frequency variable to a key-value pair such as object, map or set — User863
– User863, Commented Sep 13, 2021 at 12:09
Can you provide sample data for alphabet variable and how you want the output result to be — Yves Kipondo
– Yves Kipondo, Commented Sep 13, 2021 at 12:10
Please add the values of alphabet, dictionary also the expected value for frequency to make it more clear. — Nitheesh
– Nitheesh, Commented Sep 13, 2021 at 12:18

The Otterlord · Accepted Answer · 2021-09-13 12:29:29Z

First, let's get some example data. We'll ignore anything that isn't related to the question.

const alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('')
var dictionary = [{'word': 'alpha'}, {'word': 'beta'}, {'word': 'balloon'}]

Then we'll create a results variable, which will hold an array of all results in alphabetical order.

var results = []

Now we loop over the same way you did before, but we'll push each result to our list.

alphabet.forEach(function(letter) {
  frequency = dictionary.filter(item => item['word'].toLowerCase().startsWith(letter)).length;
  results.push(frequency);
});

As an alternative solution that allows us to more easily look through our results, we'll make results an object, and use letters as keys.

var results = {};
alphabet.forEach(function(letter) {
  frequency = dictionary.filter(item => item['word'].toLowerCase().startsWith(letter)).length;
  results[letter] = frequency;
});

Invizi · Accepted Answer · 2021-09-13 12:53:54Z

1

Using reduce, you can easily make a new object with the letter as the key and an array of matches as the value.

const words = ["abba", "ant", "apple", "bannana", "barn", "car", "cat"];

const alphabet = "abcdefghijklmnopqrstuvwxyz".split("");

const result = alphabet.reduce((a, letter) => {
  a[letter] = words.filter(word => word.toLowerCase().startsWith(letter));
  return a;
}, {});

console.log(result);

answered Sep 13, 2021 at 12:53

Invizi

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