Any way to avoid a for loop? (ES6 / JavaScript)

Question

Improving my algorithm knowledge using ES6 (I am fairly new to ES6) and wondering if there is any way (if at all performant) of avoiding a for loop in this largest of each array function I wrote?

function largestEach(arr) {
   for(const [i,v] of arr.entries())
      arr[i] = v.sort((a,b) => b - a).filter((e,i) => i === 0);
   return arr.reduce((a,b) => a.concat(b));
}
largestEach([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

Console logs: [5, 27, 39, 1001] which is correct.

It is conceptual so there is no real use case I am using it for. I am not against for loops just curious what my better options were in ES6 (or JS in general). Purely curious!

I had come up with the same as @4castle. Note that if any of the inner arrays were empty you'd get a result of -Infinity for them, which is better than what the function in the question does in that case (where the output simply skips over any empty arrays, returning an array of different length to the input). — nnnnnn
– nnnnnn, Commented Oct 11, 2017 at 2:44
@4castle so the ... spread syntax essentially expands that sub array out and returns only the maximum value... Intriguing! — simplexity
– simplexity, Commented Oct 11, 2017 at 2:45
It looks like Lux came up with it also. You can vote on his :) — castletheperson
– castletheperson, Commented Oct 11, 2017 at 2:55

Lux · Accepted Answer · 2017-10-11 02:44:01Z

4

You could simply use .map(). Basically your for loop is equivalent to this:

arr = arr.map(elem => elem.sort((a, b) => b - a).filter(e,i) => i === 0)

However the next thing thats interesting is that you don't have to specify the sort function in this case. Also I wouldn't use .filter(e,i) => i === 0) but rather .pop() or [0].

So you could rewrite:

arr = arr.map(elem => elen.sort()[0])

Next you could use Math.max, so you can rewrite your entire function:

function largestEach(arr) {
  return arr.map(e => Math.max(...e))
}

answered Oct 11, 2017 at 2:44

Lux

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3 Comments

RobG Over a year ago

@some—there is no spread operator. ;-)

Felix Kling Over a year ago

You do need a custom sort callback because the default is storing the elements lexicographical: console.log([10, 9, 20].sort()).

RobG Over a year ago

@some—Mozilla does not author the content of MDN, contributors do. Anyone can become a contributor (yes, I'm one but not very active), just create an account and away you go. ... is a punctuator that's used in spread and rest syntax in the same way ; is used to define a statement, but it's not called a "statement operator". ;-)

dhilt · Accepted Answer · 2017-10-11 02:43:08Z

2

function largestEach(arr) {
   return arr.map(a => a.reduce((a, b) => Math.max(a, b)));
}

answered Oct 11, 2017 at 2:43

dhilt

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Comments

P Ackerman · Accepted Answer · 2017-10-11 02:43:28Z

2

function largestEach(arr) {
   return arr.map((a)=> Math.max.apply(null, a))
}

answered Oct 11, 2017 at 2:43

P Ackerman

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4 Comments

Panos Kalatzantonakis Over a year ago

This is the fastest solution

some Over a year ago

@PanosK.It might be the fastest, but it will also crash with a stack overflow if the array is large.

P Ackerman Over a year ago

@some How large?

some Over a year ago

It depends on the javascript engine, and it can change while the script is running. My limited testing has shown that it is a problem if there are more than 100k items (Chrome 125k, Firefox 500k, Edge 650k). You can easily test it with Math.max.apply(null,Array(900000)). (and change the number)

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