Having trouble finding the largest numbers in a subarray

Question

Here is my current code:

var largestNumbers = new Array(4);

function largestOfFour(arr) {
    var biggest = Math.max.apply(null, largestOfFour) 
    for (var index in arr) {
        for(var i=0; i < arr[index].length; i++){
            if (arr[index] > biggest){
                arr[index].push(largestNumbers);
            }
            else if (arr[index] < biggest) {
                arr[index].splice(0, 1);
            }
        }
    }
    return largestNumbers;
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

My objective is to find the largest number in each of the subarrays and then put those values into another array and then return that value. I have been searching around for things like "How to extract a subarray from array" and that has led to me using the splice function. I'm not sure if I'm using it right, but I'm trying everything I've got because I've been stuck on this problem for 2 days now. If anyone could just lead me in the path to solve this problem, I would be more than grateful. Thank you all so much.

Furthermore, I was using this site to try and guide me along. There is a snippet of code that attempts to resolve a problem similar to mine in the latter half of the page. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply

score 4 · Accepted Answer · answered Jul 24 '15 at 13:57

4

var largest = function largest(arr) {
    return arr.map(function(arr) {
        return Math.max.apply(null, arr);
    });
};

Working example

answered Jul 24 '15 at 13:57

Jeff Shaver

3,315
18
19

plus one, nice and compact ;) – musefan Jul 24 '15 at 14:00

epascarello · Answer 2 · 2015-07-24T14:03:36.587

1

Use map to create a new Array by looping over the first level.
Use Math.max to get the maximum value from a list of numbers
Use apply to convert the array to arguments in Math.max

function largestOfFour(arr) {
  return arr.map(function(array) {
    return Math.max.apply(Math, array)
  });
}

var myArray = [
  [4, 5, 1, 3],
  [13, 27, 18, 26],
  [32, 35, 37, 39],
  [1000, 1001, 857, 1]
];
console.log(largestOfFour(myArray));

edited Jul 24 '15 at 14:03

answered Jul 24 '15 at 13:58

epascarello

204,599
20
195
236

Same answer posted below – Jeff Shaver Jul 24 '15 at 13:59
And it was not there when I started to answer it. Congrats on being first. – epascarello Jul 24 '15 at 14:00
@epascarello: OI, I was first! You watch where you throw around that Congrats of yours – musefan Jul 24 '15 at 14:01
@musefan you were not the first with this type of solution. ;) – epascarello Jul 24 '15 at 14:02
@epascarello: pfft.. well mine's bigger than yours! – musefan Jul 24 '15 at 14:03
I can make it bigger by adding more line breaks. ;) – epascarello Jul 24 '15 at 14:04
Does that work? I'll have to give it a try when I get home – musefan Jul 24 '15 at 14:05
Plus one... so it's even more like Jeff's – musefan Jul 24 '15 at 14:05
@epascarello Can you please explain the reason for putting 'Math' in the first parameter for your apply function? I'm having trouble understanding what needs to go there. – Chasen Bettinger Jul 24 '15 at 14:08
@ChasenBettinger In this case anything that is valid can go in the first parameter of the apply since `Math.max` uses no context. Some people use `null` and others use `Math`. [This answer](http://stackoverflow.com/questions/21255138/how-does-the-math-max-apply-work) has an explanation that might help you understand. – epascarello Jul 24 '15 at 14:12
@epascarello Yes, that does help me! Thank you so much. I hope you have a great day! – Chasen Bettinger Jul 24 '15 at 14:15

ntnbst · Answer 3 · 2017-07-22T11:23:22.840

Applying simple nested loop's logic.

function largestOfFour(arr) {

var maxArray = [];

for(var i = 0; i < arr.length; i++) {

var maxE = 0;

   for(var j = 0; j < arr.length; j++) {

      if(maxE < arr[i][j]) {

      maxE = arr[i][j];

   }

 }

   maxArray.push(maxE);

 }
  arr = maxArray;
  return arr;

 }

 largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

OR Try this

function largestOfFour(arr) {

var maxArray = [];


for(var i = 0; i < arr.length; i++) {

 var maxE = Math.max(...arr[i]);          // this spread is only available in ES6 so can be  replaced by --> Math.max.apply(null, arr[i])

  maxArray.push(maxE);
}

arr = maxArray;

return arr;

}


 largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

musefan · Answer 4 · 2015-07-24T14:13:12.017

Just do it with a simple nested loop. Find the biggest number for each inner array and add it to the result:

function largestOfFour(arr) {
    var result = [];

    for(var i = 0; i < arr.length; i++){
        var max = 0;
        var inner = arr[i];
        for(var j = 0; j < inner.length; j++){
            if(inner[j] > max)
                max = inner[j];
        }
        result.push(max);
    }

    return result;
}

var output = largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);
// [5, 27, 39, 1001]

Here is a working example

score 0 · Answer 5 · edited Sep 02 '15 at 22:18

0

function largestOfFour(arr) {
    return arr.reduce(function(p, c, index, arr){
        p.push(Math.max.apply(null, c));
        return p;
    }, []);
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

edited Sep 02 '15 at 22:18

Jones

1,480
19
34

answered Sep 02 '15 at 21:36

Alik

5
2

score 0 · Answer 6 · answered Dec 27 '15 at 08:42

Even you can code like this also , creating a new array showing in them also

function largestOfFour(arr) {
  // You can do this!
  var newArray=[];
  for(i=0;i<arr.length;i++){
    newArray.push(Math.max.apply(Math,arr[i]));

  }
  return newArray;
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

score -1 · Answer 7 · answered Jul 24 '15 at 13:57

-1

var largestNumbers = new Array();

function largestOfFour(arr) {
    for (var i = 0 ; i< arr.length; i ++) {
        largestNumbers.push(Math.max.apply(Math, arr[i]));   
    }
}

largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

answered Jul 24 '15 at 13:57

Shree Ram Neupane

407
5
16

I wouldn't recommend a function that updates a global variable. Would be better for it to return the result, or modify an argument passed in. Still, it _does_ work so can't downvote it – musefan Jul 24 '15 at 14:09

score -1 · Answer 8 · edited May 23 '17 at 10:28

Using the approach with Math.max.apply() you can do that with one loop that iterates through the array as argument;

function largestOfFour(arr){
   var result = [];

  for(var i = 0 ; i < arr.length ; i++){
    result[i] = Math.max.apply(null, arr[i]);
  }

  return result; 
}

EDIT: As @Jeff Shaver posted a simpler solution, you should use the map function to return the result array.

Having trouble finding the largest numbers in a subarray

8 Answers8