Is there a way of calculating the sum of even numbers in an array?

Question

I have been trying to find a way to calculate the sum of even or odd numbers in an array. I figured out how to actually output only even or odd numbers in in the console, but I'm not sure how I would go about calculating the sum of even or odd numbers. The only solution I have found is to calculate the the whole array

My code most likely doesn't follow the best of code practices so be easy on me, but feel free to point out better ways to accomplish the result. I'm all ears.

The code under gives the current even numbers in array and outputs them to the browsers console

This is where I'm at so far

    <!DOCTYPE html>
    
    <html>
    <head>
      <meta charset="UTF-8">
      <title>For - loop </title>
      <script>
     window.onload = startFunction;

    function startFunction()
    {
        document.getElementById("buttonOne").onclick = buttonFunction;
    }

    function buttonFunction ()
    {
        var list=[22,45,63,223,12,56,89];
        var number = 0;
        var rest = 0;
        var counter;
        
        for(counter=0;counter<20;counter+=1)
        {
        
            number = list[counter];
            
            rest = number%2;
            
            if(rest == 0)  //type (rest==0) to find even numbers in the array
            {
                console.log(number);
            }
        }
    }
  </script>
</head>
<body>
    <button id="buttonOne">Enter</button>
</body>
</html>

Why are you doing `counter<20` in your `for` loop? Your `list` has less than 20 elements? — gen_Eric, Dec 02 '20 at 19:21

Unmitigated · Accepted Answer · 2020-12-02T19:32:02.747

1

You can do this quite simply with Array#reduce.

const list=[22,45,63,223,12,56,89];
let evenSum = list.reduce((acc,curr)=>acc + (curr % 2 == 0 ? curr : 0), 0);
console.log('Even sum:', evenSum);
let oddSum = list.reduce((acc,curr)=>acc + (curr % 2 == 1 ? curr : 0), 0);
console.log('Odd sum:', oddSum);

If performance is particularly important, you can calculate both sums with a single reduce operation.

const list=[22,45,63,223,12,56,89];
let [evenSum, oddSum] = list.reduce((acc,curr)=>(acc[curr % 2] += curr, acc), [0, 0]);
console.log('Even sum:', evenSum);
console.log('Odd sum:', oddSum);

edited Dec 02 '20 at 19:32

answered Dec 02 '20 at 19:26

Unmitigated

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I'm curious, is there performance improvements on two `reduce` vs one `foreach`/`for` loop with an if statement? – imvain2 Dec 02 '20 at 19:27
@imvain2 curious myself. I find that the code above is alot easier on the eyes and brain though – im_bad_at_this Dec 02 '20 at 19:30
O(2n) vs O(n) you know the result ;) – kishore Rajendran Dec 02 '20 at 19:31
@im_bad_at_this I've updated the answer to find both sums together. – Unmitigated Dec 02 '20 at 19:32
@iota perfection. – im_bad_at_this Dec 02 '20 at 19:37
efficient and simple – kishore Rajendran Dec 02 '20 at 19:39
1

@iota Completed! – im_bad_at_this Dec 02 '20 at 19:44

sercan · Answer 2 · 2020-12-02T19:22:43.007

0

let list = [22,45,63,223,12,56,89];
let number = 0, rest = 0, result = 0;

for(let counter = 0; counter < 20; counter += 1) {
  number = list[counter];
  rest = number%2;
  if(rest == 0) {
    result += number; //here
  }
}

console.log(result); //printing result

edited Dec 02 '20 at 19:22

answered Dec 02 '20 at 19:20

sercan

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19

Is there a benefit to user "let" istead of "var"? – im_bad_at_this Dec 02 '20 at 19:21
@im_bad_at_this [Is there a performance difference between 'let' and 'var' in JavaScript](https://stackoverflow.com/questions/21467642/is-there-a-performance-difference-between-let-and-var-in-javascript) – sercan Dec 02 '20 at 19:24

score 0 · Answer 3 · answered Dec 02 '20 at 19:22

Technically you can just loop through the array and if even, add that to a variable else add that to a different variable.

list = [22, 45, 63, 223, 12, 56, 89]
evensum = 0;
oddsum = 0;

list.forEach(function(e) {
  if (e % 2 == 0) {
    evensum += e;
  } else{
    oddsum += e;
  }
});

console.log(evensum, oddsum)

Is there a way of calculating the sum of even numbers in an array?

3 Answers3