Javascript - Averaging values from array incorrectly?

Question

So I'm attempting a coding exercise, and I can't seem to find the problem. I need to pull the median from an array. If the array is an even length, I need the average of two values.

const N = parseInt(readline());
var inputs = readline().split(' ');
for (let i = 0; i < N; i++) {
    const X = parseInt(inputs[i]);
}

var arr = inputs.sort((a, b) => a - b);
if (arr.length % 2 === 0)
{
    console.error(arr);
    console.error(arr[arr.length / 2 - 1]);
    console.error(arr[arr.length / 2]);
    var a = arr[arr.length / 2 - 1];
    var b = arr[arr.length / 2];
    var ans = (a + b) / 2;
    console.log(ans);
}
else console.log(arr[Math.floor(arr.length/2)]);

CONSOLE OUTPUT: [ '1', '2', '3', '4', '5', '6', '7', '8', '9', '10' ] 5 6 28

So from the console output, my array is sorted properly. Next, I'm pulling out the two medians correctly. My a is indeed 5, and my b is indeed 6. But adding them with (a + b) / 2 suddenly turns into 28... Why?

element in your arr is String. So "5"+"6" = "56" -> 56/2 = 28 — Thuc Nguyen
– Thuc Nguyen, Commented Mar 11, 2019 at 4:53

Amardeep Bhowmick · Accepted Answer · 2019-03-11 04:58:17Z

3

Variables a and b are being treated as strings and concatenated, then the division happens: '5' + '6' becomes 56 and dividing them by 2 results in 28. So you need to coerce them into numbers before doing the addition:

 var ans = (+a + +b) / 2;

Appending a arithmetic operator before the variables will coerce them into numbers and then you can add them.

edited Mar 11, 2019 at 4:58

answered Mar 11, 2019 at 4:49

Amardeep Bhowmick

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1 Comment

Arcaster Over a year ago

Oh man, what an eye-opener for JavaScript (and coding practice in general). Thanks. I'll be more careful of this in the future.

Noman Hassan · Accepted Answer · 2019-03-11 04:49:12Z

2

dude u are adding two strings i.e. '5' + '6' which makes it 56 divide by 2 = 28 (Yes JS is crazy). Just use parseInt,

var ans = (parseInt(a, 10) + parseInt(b, 10)) / 2;

answered Mar 11, 2019 at 4:49

Noman Hassan

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Comments

codedawi · Accepted Answer · 2019-03-11 05:09:09Z

0

It is seems you are adding Strings together. It is essentially taking the charCode rather than the Number value ex: '1'.charCodeAt(0)

I solved this by using .map on the Array of inputs:

const N = parseInt(readline());
var inputs = readline().split(' ').map(function(num) { return parseInt(num); });

var arr = inputs.sort((a, b) => a - b);
if (arr.length % 2 === 0)
{
    console.error(arr);
    console.error(arr[arr.length / 2 - 1]);
    console.error(arr[arr.length / 2]);
    var a = arr[arr.length / 2 - 1];
    var b = arr[arr.length / 2];
    var ans = (a + b) / 2;
    console.log(ans);
}
else console.log(arr[Math.floor(arr.length/2)]);

answered Mar 11, 2019 at 5:09

codedawi

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