Javascript looping through arrays of arrays

Question

So I've been fighting with this a few hours now- the goal is to create a new array of the highest numbers in each array of 4. However, I can't seem to get it to loop more than once. How am I screwing up this for loop?

function largestOfFour(arr) {
 for (var i = 0; i < arr.length; i++) {
 var allTop = ""; 
 var top = arr[i].sort(function(a, b) {
 return b - a; 
 });
 i++;
 allTop.push(top[0]);
 }
}
largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

• takendarkk: Your snippet does not work at all allTop.push is not a function.

Answer 1

The variable allTop should be defined before the loop as an array, and returned after the loop ends:

function largestOfFour(arr) {
 var allTop = [];
 
 for (var i = 0; i < arr.length; i++) {
 var top = arr[i].sort(function(a, b) {
 return b - a;
 });
 allTop.push(top[0]);
 }
 
 return allTop;
}
console.log(largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]));

• chauxvive: Ahhhhhh a scope problem. Mostly. Thank you!

Answer 2

A better approach is using the function map along with the function Math.max

function largestOfFour(arr) {
 return arr.map(function(a) {
 return Math.max.apply(null, a);
 });
}
var result = largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);
console.log(result);

Full ES6:

var largestOfFour = (arr) => arr.map(a => Math.max(...a));
var result = largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);
console.log(result);

• chauxvive: This is very similar to the other approach I was trying- thank you for demonstrating how Math.max and map can work together

Answer 3

Try this:

function largestOfFour(arr) {
 let allTop = [];
 arr.forEach(a => {
 allTop.push(Math.max.apply(Math, a));
 });
 return allTop;
}
console.log(
 largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]])
);

Answer 4

Other solution would be to use function reduce along with the function Math.max

function largestOfFour(arr) {
 return arr.reduce((a, x) => {
 a.push(Math.max.apply(null,x));
 return a;
 }, []);
}
console.log(largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]));