Javascript array while loop. Comparing results: arr[len] vs arr[len-1]

Question

Good day everyone!

I've found this great code from this site:

var points = [30,100];
document.getElementById("demo").innerHTML = myArrayMax(points);

function myArrayMax(arr) {
    var len = arr.length;
    var max = -Infinity;
    while (len--) {
      if (arr[len] > max) {
        max = arr[len];
      }
    }
    return max;
}

source: Find the min/max element of an Array in JavaScript

It really works really well and very fast. I've got a question (out of curiosity). When I tried to add (or manipulate) something, the result didn't match based from the first code. Here is the code:

var points = [30,100];
document.getElementById("demo").innerHTML = myArrayMax(points);

function myArrayMax(arr) {
    var len = arr.length;
    var max = -Infinity;
    while (len--) {
      if (arr[len-1] > max) {
        max = arr[len-1];
      }
    }
    return max;
}

I've assumed that the array[0] is the first array element, also I used post-decrement function, and yet I didn't get the expected result.

What will happen on the last round (len === 0) in your modified example? — Andreas
– Andreas, Commented Nov 2, 2018 at 16:00
Take note that you also can use the builtin Math.max like this: Math.max.apply(null, [30, 100]) — Simon Zyx
– Simon Zyx, Commented Nov 2, 2018 at 16:20
Using Infinitie is a generally bad practice. let's assume that first iteratebale value match our condition best var max = arr[arr.length-1] — qiAlex
– qiAlex, Commented Nov 2, 2018 at 16:25

Niche · Accepted Answer · 2018-11-02 16:15:44Z

1

First of all, using arr[len-1] adds a new problem to the mix: Index out of bounds. This happens when the loop iterates the last element in the loop (len == 0 after len--) and you end up looking for arr[-1] which doesn't exist. Also, you're not checking anymore for the last index in the array (arr[arr.length - 1]), since len-- already substracts from the index. Like so:

var points = [30,100];
document.getElementById("demo").innerHTML = myArrayMax(points);

function myArrayMax(arr) {
    var len = arr.length; // len == 2
    var max = -Infinity;
    while (len--) { // len == 1
      // checking for arr[len - 1] the first time
      // results in checking for arr[0]. arr[1] will be skipped
      if (arr[len-1] > max) { 
        max = arr[len-1];
      }
    }
    return max;
}

The function works well in the first place, so... Why fix what isn't broken?

Check out arithmetic operators for more info on len--.

edited Nov 2, 2018 at 16:15

answered Nov 2, 2018 at 16:04

Niche

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10 Comments

Niche Over a year ago

Yes, but the while condition also substracts 1 from len's value. Check out arithmetic operators

ExCode Over a year ago

Based on my observation: When it enters in while loop, the initial value of len is 2. The condition is accepted and proceeds in if statement. When I used len-1, it should direct in arr[1]=100 not 30. When the value finished storing the value in var max, the len will be deducted and repeats the while loop. May I know why it bypasses the if (arr[1] > max) or was my understanding in using the while loop is wrong ? Thanks for answering my question.

Niche Over a year ago

The initial value of len is 2. Then the while condition comes and evaluates (and operates) upon len--. Now len's value is 1. Now, on the first iteration, checking for arr[len-1] will go to the 0 position of your array.

Niche Over a year ago

Saying len-- is the same as saying len -= 1

ExCode Over a year ago

I see. I thought that it is a post-decrement, not a standard arithmetic operator. Thank you very much.

|

Nattamai Jawaharlal Manikandan · Accepted Answer · 2018-11-02 16:03:35Z

0

function myArrayMax(arr) {
var len = arr.length;
var max = -Infinity;
while (len--) { // here itself you are decrementing the value of len
  if (arr[len-1] > max) { // here also you are checking the previous one. Then you won't get chance to check the last array element.
    max = arr[len-1];
  }
}
return max;
}

// moreover you will get index out of bound exception while going in last iteration.

answered Nov 2, 2018 at 16:03

Nattamai Jawaharlal Manikandan

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1 Comment

ExCode Over a year ago

Thank you very much

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