JavaScript - Return value from array with map function

Question

I want to extract the largest number of every single array. I believe it should be possible with the .map function.

This is what I have so far, but unfortunately this returns " [ undefined, undefined, undefined, undefined ]"

Code:

function largestOfFour(largestOfFour) {
    largestOfFour.forEach(function(number){
        number.sort(function(a, b){
            return b - a;
        });
        highestValue = number.map(function(number){
            return number[0];
        });
    });
};

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

Your problem is that you have `number` as the parameter to your `forEach` and then `number` again in your `map` function. So inside your `map` function, what do you think `number` refers to (hint, it isn't an array). — Matt Burland, Nov 25 '15 at 15:45
Use `arr.map(Math.max.apply.bind(Math.max, null));` Check [this answer](http://stackoverflow.com/a/32679435/2025923) for explanation and demo — Tushar, Nov 25 '15 at 15:45

score 2 · Answer 1 · answered Nov 25 '15 at 15:41

2

There's a shorter way of finding the largest number in each array using Math.max:

function largestOfFour(arr) {
  return arr.map(function (el) {
      return Math.max.apply(null, el);
  });
}; // [ 5, 27, 39, 1001 ]

DEMO

answered Nov 25 '15 at 15:41

Andy

61,948
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There's _even_ shorter way. Use `arr.map(Math.max.apply.bind(Math.max, null));` Check [this answer](http://stackoverflow.com/a/32679435/2025923) for explanation and demo – Tushar Nov 25 '15 at 15:46
1

@Tushar, [short, clever code is not better than more verbose simple code](http://zzzzbov.com/blag/debugging-clever-code). – zzzzBov Nov 25 '15 at 15:56
@zzzzBov True. But once you understand the syntax(or with comments), it's good and concise code. – Tushar Nov 25 '15 at 16:01
@Andy I know about the Math method, but I don't understand what " apply.bind" does. Could you elaborate on that? – Cake Nov 25 '15 at 16:04
You should ask @Tushar – Andy Nov 25 '15 at 16:10
@Cake Check my first comment and read the description – Tushar Nov 25 '15 at 16:12

zzzzBov · Accepted Answer · 2015-11-25T15:57:27.760

You're iterating over the outer array,

function largestOfFour(largestOfFour) {
    largestOfFour.forEach(function(number){

and then sorting the inner arrays,

number.sort(function(a, b){
    return b - a;
});

and then of those inner arrays you're trying to acquire the [0] property from each value, which is undefined

highestValue = number.map(function(number){
// you redefine `number` here to be the value within the inner array
    return number[0];
});

What you probably want to do is map the outer array:

function largestOfFour(largestOfFour) {
    largestOfFour.map(function(numbers){

sort the inner arrays:

numbers.sort(function(a, b){
    return b - a;
});

and then return the first value:

return numbers[0];

All together it probably should be:

function largestOfFour(largestOfFour) {
    return largestOfFour.map(function(numbers){
        numbers.sort(function(a, b){
            return b - a;
        });
        return numbers[0];
    });
}

Although there are simpler ways to find the maximum values of an array of arrays.

One such example is to use Array.prototype.reduce:

function largestValues(outer) {
    return outer.map(function (inner) {
        return inner.reduce(function (prev, current) {
            return Math.max(prev, current);
        }, Number.NEGATIVE_INFINITY);
    });
}

Perfect, very good explanation on why it wasn't working too! — Cake, Nov 25 '15 at 15:57

score 0 · Answer 3 · answered Nov 25 '15 at 15:42

http://jsfiddle.net/y48qz6hx/ Just fixed your code

function largestOfFour(largestOfFour) {
    largestOfFour.forEach(function(number, index){
        number.sort(function(a, b){
            return b - a;
        });
        largestOfFour[index] = number[0];
    });

    alert(largestOfFour);
};

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

maioman · Answer 4 · 2015-11-25T15:49:29.160

0

try this:

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

   console.log( ar.map(x=> Math.max.apply(null, x)) )

edited Nov 25 '15 at 15:49

answered Nov 25 '15 at 15:42

maioman

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I've got `Uncaught SyntaxError: Unexpected token =>` – Alexander Elgin Nov 25 '15 at 15:43
The function name is "largestOfFour". Not the first element of the array. – Andy Nov 25 '15 at 15:44
@AlexanderElgin, that's because this answer is in ES6 and probably no help to you. – Andy Nov 25 '15 at 15:44
@Andy the OP asks: 'I want to extract the first number of every single array...' (not the largest like in function name ) – maioman Nov 25 '15 at 15:48
Then why is the function name "largestOfFour" and the code sorting each array? The OP probably meant to write the first element of a sorted array. – Andy Nov 25 '15 at 15:51
@Andy I corrected it to find the largest – maioman Nov 25 '15 at 15:51
@AlexanderElgin arrow function should be supported on all updated browsers, what are you using? – maioman Nov 25 '15 at 15:53
maybe not http://caniuse.com/#feat=arrow-functions – maioman Nov 25 '15 at 15:56

JavaScript - Return value from array with map function

4 Answers4