Trouble using Math.max.apply over array in JavaScript

Question

I am trying to find the maximum number within each of four subarrays, push each max to a new array that includes all four maxes and return that array. When I run this code it returns an array with four values, but they are all null. What am I doing wrong?

largest = [];

function largestOfFour(arr) {
  for (i = 0; arr.length > i; i++) {
    var tempSub = arr.slice(i , i + 1);
    largest.push(Math.max.apply(Math, tempSub));
  } 
  return largest;
}

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

@RocketHazmat The second parameter is for a test I believe on freecodecamp. I tried running without it but nothing changed. — slurrr
– slurrr, Commented Sep 17, 2015 at 17:25
That's because largestOfFour(arr) only lists one parameter in its signature. — gen_Eric
– gen_Eric, Commented Sep 17, 2015 at 17:27

Guffa · Accepted Answer · 2015-09-17 17:31:04Z

3

When you are using slice to get part of the array, you are getting an array with the item inside it. The variable tempSub doesn't contain an array of values like [4, 5, 1, 3], it contains an array of arrays of values, like [[4, 5, 1, 3]]. When you send that into the max method it will not be able to get the maximum value because you don't send the values into the method, you send a single value that is an array.

Use arr[i] to get an item in the array instead of slice.

Also, you should declare the variable largest inside the function, so that the function can be used more than once without retaining the result from previous calls. The variable i should also be local.

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

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

// show result in snippet
document.write(JSON.stringify(result));

edited Sep 17, 2015 at 17:31

answered Sep 17, 2015 at 17:25

Guffa

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1 Comment

slurrr Over a year ago

I noticed that and tried something similar but I still had the tempSub code in there. That is exactly what I am trying to do and thanks for explaining it so clearly.

gen_Eric · Accepted Answer · 2015-09-17 17:26:45Z

3

You could use Array.map to run a function on each array that gets its largest value:

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

Or using ES6 arrow functions (which not many browsers actually support):

function largestOfFour(arr) {
    return arr.map(v => Math.max.apply(Math, v));
}

edited Sep 17, 2015 at 17:26

answered Sep 17, 2015 at 17:21

gen_Eric

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4 Comments

slurrr Over a year ago

That is a very nice alternative way to accomplish what I am trying to do, thanks for sharing. Much more eloquent than my rudimentary code above.

Guffa Over a year ago

Note that the map method is not supported in all browsers either. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

gen_Eric Over a year ago

@Guffa: True, but I think you can safely assume no one's using IE < 9, no? Also there's a polyfill if you really need to support IE 8.

Guffa Over a year ago

@RocketHazmat: You can definitely not safely assume that. This for example says that about 11% is using it: netmarketshare.com/… Other sources have other statistics for IE 8, but they are never zero.

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