A Prefix Array, also known as a Prefix Sum Array or Cumulative Sum Array, is an Array with the calculation of cumulative sums of elements in a source array. It stores the cumulative sum of elements up to a certain index in the array. This can also be done in-place, so that the target rewrites values of the source.

Prefix and Suffix arrays are useful for range computations. For example, LeetCode 1608. Special Array With X Elements Greater Than or Equal X

Here’s how it works:

Given an array of numbers, the prefix array would be another array where each element prefix[i] stores the sum of elements from index 0 to index i in the array.

For example, given nums = [1, 2, 3, 4, 5], the prefix array would be [1, 3, 6, 10, 15], because:

prefix[0] = 1                      // sum from nums[0] to [0]
prefix[1] = 1 + 2 = 3              // sum from nums[0] to [1]
prefix[2] = 1 + 2 + 3 = 6          // sum from nums[0] to [2]
prefix[3] = 1 + 2 + 3 + 4 = 10     // sum from nums[0] to [3]
prefix[4] = 1 + 2 + 3 + 4 + 5 = 15 // sum from nums[0] to [4]
Code language: JavaScript (javascript)

Here is an example of creating a prefix array in-place, in JavaScript:—

const nums = [1, 2, 3, 4, 5];
nums.forEach((num, i) => nums[i] += nums[i-1] ?? 0);
// [1, 3, 6, 10, 15]
Code language: JavaScript (javascript)

Suffix Array:

A suffix array is similar to a prefix array, but it stores cumulative sums in reverse order. Instead of storing the sum of elements up to a certain index from left to right, a suffix array stores the sum of elements from right to left.

Given the array nums = [1, 2, 3, 4, 5], the suffix array would be:—

suffix[4] = 5
suffix[3] = 5 + 4 = 9
suffix[2] = 5 + 4 + 3 = 12
suffix[1] = 5 + 4 + 3 + 2 = 14
suffix[0] = 5 + 4 + 3 + 2 + 1 = 15

//  [15, 14, 12, 9, 5]
Code language: JavaScript (javascript)

Suffix Array Code

Creating a suffix array function is a little trickier than writing a prefix array. It can be done with a new array or in-place, replacing values of an existing array. Here are both.

function createSuffixArray(nums) {
    const suffixArray = new Uint32Array(nums);
    for (let i = suffixArray.length - 2; i >= 0; i--) {
        suffixArray[i] += suffixArray[i+1];
    }
    return suffixArray;
}

const nums = [1, 2, 3, 4, 5];
createSuffixArray(nums);
// [15, 14, 12, 9, 5]
Code language: JavaScript (javascript)

By pre-allocating a Uint32Array of the appropriate length and iterating over the input array from right to left, I efficiently compute the cumulative sums and store them directly in the suffix array without needing to reverse the array or create intermediate copies. This approach is concise and efficient.

This can also be done in-place.

function createSuffixArrayInPlace(nums) {
    for (let i = nums.length - 2; i >= 0; i--) {
        nums[i] += nums[i+1];
    }
    return nums; // Optional: Return the modified nums array
}

const nums = [1, 2, 3, 4, 5];
const suffixArray = createSuffixArrayInPlace(nums);
// [15, 14, 12, 9, 5]
Code language: JavaScript (javascript)

Top-level Array generics didn’t make it into EcmaScript. Here’s how to hand-roll them.

const myFilter = Function.prototype.call.bind(Array.prototype.filter);
Code language: JavaScript (javascript)

And that gives is a function that can be called as:—

const isNotUpper =  e=>e>"Z"; // lexicographic comparison
myFilter("asdAxE", isNotUpper).join(""); // "asdx"
Code language: JavaScript (javascript)

How does this filter “magic” work? Let’s look step-by-step with another example, Array.prototype.forEach. But first, let’s get some “basics” out of the way.

Function.prototype.call

Function.prototype.call calls the function it’s called on, passing its first argument as the this value to that function.

In the following example, window.prompt is called three times, passing the value of each character, followed by the index in which it occurs, followed by the this value (a String object).

[].forEach.call("asd", prompt);
Code language: JavaScript (javascript)

The interpreter will execute the above as the following:

thisArg = new String("asd");

prompt("a", 0, thisArg);
prompt("s", 1, thisArg);
prompt("d", 2, thisArg);
Code language: JavaScript (javascript)

(Function window.prompt ignores the last argument.)

We can do likewise with console.log, which prints any number of arguments.

[].forEach.call("asd", console.log);
Code language: JavaScript (javascript)

In each call, the arguments passed are (1) the value at each index, (2) the index, and (3) the String object (promoted from a string value), used as the thisArg.

Function.prototype.call.call

We can further abstract the forEach call with:—

Function.prototype.call.call (func, thisArg, ...args )
Code language: JavaScript (javascript)

— as:—

Function.prototype.call.
    call([].forEach, "asd", console.log)
Code language: JavaScript (javascript)

— resulting:—

In the above code, the this value to call is [].forEach, the this value to forEach is "asd", promoted to a String object, and the callback function, the first argument to forEach, is console.log.

Function.prototype.call.bind

If the second call method is replaced with call to Function.prototype.bind, forEach will be bound as the this value to a function from call as:—

let arrayForEach = Function.prototype.call.bind([].forEach);
Code language: PHP (php)

The steps by which this new function is created are a bit tricky, but it essentially creates a new function with a `[[BoundThis]] value assigned to the first argument (promoted to an object) (See: 10.4.1.3 BoundFunctionCreate ( targetFunction, boundThis, boundArgs )).

The forEach method can now be called generically, without call.

arrayForEach("asd", console.log)
Code language: JavaScript (javascript)

The bound function arrayForEach is passed with "asd", which is promoted to a string object with length=3, and used as the this arg for [].forEach. Function [].forEach is called with, console.log three times, such as:

console.log(thisArg[i], i, thisArg)
Code language: JavaScript (javascript)

We can reuse this top-level Array.prototype.forEach:

const arrayForEach = 
Function.prototype.call.bind([].forEach);
arrayForEach("asd", console.log)
arrayForEach("qwe", console.log)
Code language: JavaScript (javascript)

Function.prototype.call Shortcut

Just as Array.prototype.forEach is found on every array instance such as [].forEach, so too is Function.prototype.call found on every function instance, such as (function(){}).call.

Base Object to Call and Arrow Functions

Arrow functions get their this value from the lexical environment and Bound functions have a bound thisArg (more on this later). This:—

(e=>e).call("foo")
Code language: JavaScript (javascript)

— results undefined

For our intent, this doesn’t matter. We can still use call a layer of abstraction out, as call.call. We don’t access the Base object that far back. We can also use other functions or the built-in function constructor function, Function.

(function(){return this}).call({})
Code language: JavaScript (javascript)

— which returns the object argument, {}.

Calling of forEach is stored by binding forEach to the call method:

let boundForEach = Function.prototype.call.bind([].forEach);
Code language: JavaScript (javascript)

This can be later called with any thisArg and any callback.

boundForEach("asd", prompt);
Code language: JavaScript (javascript)

Bound Method for Array.prototype.filter

Back to the problem at hand, we want to invoke Array.prototype.filter when it’s called and with the arguments passed in:

const myFilter = Function.prototype.call.bind(Array.prototype.filter);
Code language: JavaScript (javascript)

And that gives is a function that can be called as:—

const isNotUpper =  e=>e>"Z"; // lexicographic comparison
myFilter("asXd", isNotUpper);
Code language: JavaScript (javascript)

— results:—

['a', 's', 'd']
Code language: JavaScript (javascript)

Function myFilter is called with array-like object to act as the this value for the actual function call to Array.prototype.filter.Array.prototype.filter promotes its thisArg to a String object and calls the second parameter, isNotUpper (with that String as the thisArg).

Just as we get Array.prototype.forEach with [].forEach, so too can we get Function.prototype.call from any function (except arrow or bound functions).

Function.call is the same Function.prototype.call, but gets it this arg a Function, the base object, if called as Function.call(). As mentioned, we’re not calling it like that, rather, we’re using the value of that function as the Base object for bind.

From: https://leetcode.com/problems/filter-elements-from-array/discuss/5024144/Function.prototype.call.bind

NaN is a global property that represents an IEEE 754 “not a number”. (There is also a static NaN property of the built-in Number object, Number.NaN for pointless duplication (mdn)).

In older versions of ECMAScript NaN and other global properties like undefined were writable. You shouldn’t be doing that and Brendan, et al made it illegal; even throwin errors in strict mode..

The value type of NaN is “number”, as can be seen by the literal NaN property or by NaN values.

typeof NaN; // "number"
typeof (1 / "foo"); // "number"

Any value compared to NaN using the comparison operators == or === results false.

NaN == NaN; // false
undefined == NaN; // false

Method isNaN almost seems to work:—

isNaN(NaN); // true

— until it does type conversion, even attempting to run the algorithm when no argument is supplied.

isNaN(); // true
isNaN(undefined); // true
isNaN("-."); // true
isNaN(""); // false
isNaN("-2."); // false

The answer to that is to use Number.isNaN, which only returns true for actual NaN values and does not do type conversion.

Number.isNaN(); // false
Number.isNaN("1"); // false
Number.isNaN(""); // false
Number.isNaN("-."); // false
Number.isNaN("0xf"); // false
Number.isNaN("-2."); // false

Object.is can also reliably check NaN values:

 Object.is(NaN, NaN); // true
 Object.is(NaN, undefined); // false

Methods isFinite and the newer non-type-converting version Number.isFinite can work in certain situations:—

 Number.isFinite(NaN); // false

— but do not check exclusively for NaN, as they also return true for Infinity and -Infinity:—

 Number.isFinite(-Infinity); // false

But these methods are useful for numeric validation. Especially Number.isFinite, which, unlike global isFinite, does not do type conversion:

Number.isFinite("2"); // false
Number.isFinite(new Date); // false

isFinite("2"); // true
isFinite(new Date); // true

Problem

LeetCode 15. 3Sum — JavaScript.

Solution

https://leetcode.com/problems/3sum/solutions/2895033/javascript-sorted-typed-array-sliding-window/?orderBy=most_votes

const threeSum = numsArray => {
    "use strict";
    const nums = Int32Array.from(numsArray).sort();
    const results = [];
    const LAST  = nums.length - 1;

    let i = 0;
    do {
        const n = nums[i];
        if (n === nums[i-1]) continue;
        let l = i + 1;
        let r = LAST;
        const isPossible = (n + nums[l] + nums[l+1] <= 0);
        if (!isPossible) break;
        do {
            const ln = nums[l];
            const rn = nums[r];
            const testSum = n + ln + rn;

            if (testSum === 0) {
                results.push([n, ln, rn]);
                // Increment l; decrement r.
                // To avoid duplicate triplets,
                // repeat until unique values appear.
                //  E.g. [-10, 2, 3, 3, 7, 8]
                // [[-10, 2, 8] [-10, 3, 7]] NOT 2nd [-10, 3, 7]
                while (ln === nums[++l]);
                while (rn === nums[--r]);
            } else if (testSum < 0) {
                l++;
            } else {
                r--;
            }
        } while (l < r);
    } while (++i < LAST)
    return results;
};Code language: JavaScript (javascript)

Interactive Web Demo in JavaScript

https://mybanned.com//wp-content/uploads/2023/01/3sum.html

Video