Arrays

Module Data.Arrays

ixfx has a host of functions for working with arrays. See the link above for a complete overview. On this page we’ll give a quick overview of working with arrays in Javascript interleaved with some ixfx functions.

In Javascript, arrays are essentially stacks, with items stored ‘left-to-right’ in the order added. Arrays can hold whatever you want, sometimes we even use arrays to hold other arrays, a way of making a multi-dimensional data structure like a table.

Arrays in Javascript are mutable meaning their contents can change even though the object reference stays the same. In this document, we’ll only use them in an immutable way.

Basics

Creating

Arrays are simple to create

Run
// Create an empty array
const a1 = [];

// Create with some starting elements
const a2 = ['starting'];
const a3 = ['one', 'two'];

// Create using the contents of other arrays
const a4 = [...a2, ...a3]; // ['starting', 'one', 'two']

Create an array from an iterable or generator

Array.from(iterable);

Adding

When working with immutable arrays, we don’t actually add anything to an array, but rather create a new array with previous elements and any new elements.

Use destructuring to add element(s) to an array. Where you put the destructured values will determine their position. Let’s say ‘a’ is the initial array.

Run
const a = ['some','array']
const b = [...a, 'with', 'new']; // ['some','array','with','new']
const c = ['with', 'new', ...a]; // ['with', 'new', 'some', 'array']
const d = ['with', ...a, 'new']; // ['with', 'some', 'array', 'new']

Note in these cases, a never changes.

Filter

To filter is to go through every item in an array and see if it matches a condition, called a predicate. If the predicate returns true, the item is returned, forming a new array. The original array is not changed.

Run
const a = [`apple`,`banana`,`carrot`,`bean`];

// Check if item starts with letter b
const withB = a.filter( v => v.startsWith(`b`)); // [`banana`, `bean`]

In the above example we’re using a succint way to express the predicate. But it could be expanded as below, just make sure it returns true or false.

Run
const a = [`apple`,`banana`,`carrot`,`bean`];

// Get items at 50% chance
const r = a.filter( v => {
  const r = Math.random();
  if (r > 0.5) return false;
  return true; // Item will be included
});
// This could be expressed in more compressed form as: a.filter(v => Math.random() > 0.5)

filter will return an empty array if no matching items were found.

Removing items

When working with immutable arrays, we don’t really remove items, but rather create a new array without certain items. filter comes in handy for this.

For example, we want an array without carrot:

Run
const a = [`apple`,`banana`,`carrot`,`bean`];
const withoutCarrot = a.filter( v => v === `carrot`); // [`apple`,`banana`,`bean`]

If it seems ugly to always create new variables when using immutable arrays, use let rather than const:

let a = [`apple`,`banana`,`carrot`,`bean`];
a = a.filter( v => v === `carrot`); // [`apple`,`banana`,`bean`]

Accessing contents

You can ‘loop over’ or ‘enumerate’ the contents of an array.

// for-of
for (const v of myArray) {
  // 'v' will be an item from array
}

// forEach
myArray.forEach(v => {
  // 'v' will be an item from array
})

Items in arrays have an integer index, which starts at 0. Meaning the first item is at position in 0. Use at to get an item by index. If you use negative numbers it will count from the last element.

Run
const ar = ['a','b','c','d'];
// First
ar.at(0); // 'a'
// Second, etc
ar.at(1); // 'b'
// Last
ar.at(-1); // 'd'

Includes

filter can be used to find items that match a predicate and return a new array. However there are times you just want to check if something exists, without getting it back.

some allows you to use a filtering predicate:

Run
const ar = ['apple','banana','carrot'];
if (ar.some( v => v.startsWith(`b`))) {
  // do something if we have an item that starts with 'b'...
}

find allows you to use a filtering predicate, returning the first item that matches.

Run
const ar = ['apple','banana','carrot','bean'];
const anythingB = ar.find( v => v.startsWith(`b`)); // `banana`
if (anythingB) {
  // Do something if we find any item starting with 'b'...
}

find will return undefined if no matching item was found.

Random

ixfx has functions for getting a random element or index:

Run
import { Arrays } from 'https://unpkg.com/ixfx/dist/data.js';
const v = [`blue`, `red`, `orange`, `pink`, `purple`];
Arrays.randomElement(v);
Arrays.randomIndex(v);

// Randomise order
const shuffled = Arrays.shuffle(v);

weightedSource can be used for skewing the distributing of random elements, eg. to favour picking elements at the end of the array over elements at the beginning.

Run
import { weightedSource } from 'https://unpkg.com/ixfx/dist/random.js';
import { Arrays } from 'https://unpkg.com/ixfx/dist/data.js';
const v = [`blue`, `red`, `orange`, `pink`, `purple`];

// Favour items in the middle
Arrays.randomElement(v, weightedSource(`bell`));

Remove a random element from an array with randomPluck . It doesn’t modify the array, but returns the randomly selected item and a new array without it.

Run
import {randomPluck} from 'https://unpkg.com/ixfx/dist/arrays.js';

const r = randomPluck([`apples`, `oranges`, `pears`]);
r.value; // A random value
r.array; // A copy of the array without the plucked item

Sorting

ixfx has sorting functions for common needs:

Run
import { Arrays } from 'https://unpkg.com/ixfx/dist/data.js';
const v = [
  { colour:`blue`, size: 10 },
  { colour:`red`,  size: 5 },
  { colour:`orange`, size: 7 },
  { colour:`pink`, size: 1 }
];

// Sort objects by their 'colour' property
const byColour = Arrays.sortByProperty(v, `colour`);
// Sort objects by their 'size' property
const bySize = Arrays.sortByNumericProperty(v, `size`);

Grouping

groupBy allows you to group an array by some generated key.

Run
import { groupBy } from "https://unpkg.com/ixfx/dist/arrays.js"
const data = [
 { age: 39, city: `London` },
 { age: 14, city: `Copenhagen` },
 { age: 23, city: `Stockholm` },
 { age: 56, city: `London` }
];
const map = groupBy(data, item => item.city); // Get back a Map
console.log(map);

This will yield the Map:

{
  London: [
    { age: 39, city: `London` },
    { age: 56, city: `London` }
  ],
  Stockhom: [
    { age: 23, city: `Stockholm` }
  ],
  Copenhagen: [
    { age: 14, city: `Copenhagen` }
  ]
}

See more things in the Arrays module