map
The array method map() takes a function, applies the function to each element of an array, and returns a new array containing the results of the function application. Let’s begin with a small example. In the script below, we iterate over each element of an array and add 1 to the element. The results are stored in a new array.
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/**
* Add 1 to each array element. Use the for...of statement.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const array = [1, 2, 3, 4, 5];
const result = [];
for (const x of array) {
result.push(x + 1);
}
ns.tprintf(`${result}`);
}
As you know, JavaScript arrays have the forEach() method. The method accepts a function and applies the function to each array element. The method does not return the result of the function application. In fact, the method forEach() returns undefined. To save the results of the function application, you must insert each result into an array. Here is a rewrite of the script add1-for.js , but using the array method forEach().
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/**
* Add 1 to each array element. Use the forEach() method.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const array = [1, 2, 3, 4, 5];
const result = [];
const addOne = (x) => result.push(x + 1);
array.forEach(addOne);
ns.tprintf(`${result}`);
}
The array method map() is like a better version of forEach() insofar as map() returns the result of applying the provided function. The script add1-forEach.js can be rewritten by using map() as follows:
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/**
* Add 1 to each array element. Use the map() method.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const array = [1, 2, 3, 4, 5];
const addOne = (x) => x + 1;
const result = array.map(addOne);
ns.tprintf(`${result}`);
}
You save 1 line of code and your intention is now more clearly expressed than when using forEach().
When should you use map() instead of forEach(), and vice versa? Your decision can be based on personal preference. Both methods essentially perform the same task: apply a function to each array element. You can base your decision on the difference in the outcome of each method. Do you want an array of the results of the function application? If yes, use the method map(). Otherwise, use the method forEach(). You can also base your decision on which method better expresses/declares your intent.
Exercises
Exercise 1. Read more about the array method map() here.
Exercise 2. Given an array of all integers between 1 and 10, inclusive, use the method map() to raise each element to the power of 2.
Exercise 3. Your pets database includes the age of each pet:
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const db = [
{ name: "Anonymouse", age: "1" },
{ name: "Charlie Chihuahua", age: 2 },
{ name: "Chirp O'Tweet", age: 1 },
{ name: "Frankie Frankfurt", age: "2" },
{ name: "Garry Longtongue", age: "1" },
{ name: "Goldie Horn", age: 1 },
{ name: "Hamsuke Hamton", age: 1 },
{ name: "Harry Speedbump", age: 2 },
{ name: "Robbie Hopster", age: 1 },
{ name: "Scratchy Meowser", age: "3" },
{ name: "Tabby Whiskers", age: 2 },
{ name: "Terry Terrier", age: 2 },
{ name: "Woofy McBark", age: "3" },
];
Some age values are given as strings, whereas all age values should be integers. Obtain a subset of the database where each pet object has its age as a string. Use the array method forEach() to correct the age values in the subset. Repeat the exercise, but use the array method map(). The typeof operator might be useful.
Exercise 4. Refer to the pets database from the script pet-db.js . Extract all pet names where the corresponding pets are 1 year old.
Exercise 5. Repeat Exercise 10 from the section Line them up, but use map().
Exercise 6. The JavaScript method Math.random() returns a random floating point number between 0 and 1. Use map() to write a function that returns an array of 10 random floating point numbers.
Exercise 7. Consider the following program.
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/**
* Print a number to the terminal.
*
* @param {NS} ns The Netscript API.
* @param {number} n Print this number.
*/
function count(ns, n) {
ns.tprintf(`${n}`);
}
/**
* Count up.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
for (let i = 1; i < 11; i++) {
count(ns, i);
}
}
Rewrite the program to use map() to achieve the same effect.
Exercise 8. Write your own function map() without using the array method map().
- Repeat the previous 2 exercises, but use your implementation of
map(). - Use your implementation of
map()to increment the age of each pet in the database from the scriptpet-db.js.