Sum only required array elements

Question

I need to Sum all elements of array, up to the selected value of input (first four elements in this example):

HTML:

<input id="VAL" value="4" /> <button onclick="SUM()">BUTTON</button> SUM: <span id="bubu"></span>

JS:

var numbers = [1, 2, 3, 4, 5]; function getSum(x,y) {return x+y;} function SUM() { document.getElementById("bubu").innerHTML = numbers.reduce(getSum); }

Now the sum of all elements work correctly. But how can I add a loop, to get only required sum?

Fiddle DEMO

you need to be more clear, what is required sum? is it the number for elements from the array you want to sum, or soething else. — Vaibhav Vishal
– Vaibhav Vishal, Commented Sep 25, 2018 at 16:39
i finally understood the question, he wants to sum only the first n values of the array, and n = input number. — zebnat
– zebnat, Commented Sep 25, 2018 at 16:40
add first n elements of an array is what you need to search on google. — Vaibhav Vishal
– Vaibhav Vishal, Commented Sep 25, 2018 at 16:45

T.J. Crowder · Accepted Answer · 2018-09-25 16:45:15Z

There are at least two ways:

~~filter~~ slice first, removing the values you don't want to sum up (edited because of the OP's comment "I mean the numer of elements"), or
In your reduce callback, don't add y if it's not one of the values you want to include (by checking the index you receive as the third argument to the callback)

This answer encourages improving fishing skills. Good answer!

Emeeus · Accepted Answer · 2018-09-25 17:22:07Z

Using a for loop, it's pretty simple.

var numbers = [1, 2, 3, 4, 5]; let upTo = 4; var res = 0; for(let i = 0; i<upTo; i++){ res += numbers[i]; } console.log(res)

Using reduce()

var numbers = [1, 2, 3, 4, 5]; let upTo = 4; var res = numbers.reduce((a,e,i)=>(i<upTo)?a+e:a+0) console.log(res)

Example: https://jsfiddle.net/fsbhoe45/

zebnat · Accepted Answer · 2018-09-25 17:00:55Z

Use the reduce index parameter: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

var numbers = [1, 2, 3, 4, 5]; window.SUM = function() { var numberOfElements = document.getElementById("VAL").value; document.getElementById("bubu").innerHTML = numbers.reduce(function(x, y, i){ if(i <= numberOfElements-1) { return x+y; } else { return x; } }); }

https://jsfiddle.net/k9vrLsnm/3/

trk · Accepted Answer · 2018-09-25 17:07:54Z

You could also do this using reduce alone if you supplied a limit to handler that indicates the index up to which you need the sum for. For e.g. if you need sum up to 4 elements then limit is 3.

function onClickHandler(limit) { return numbers.reduce(function (acc, val, index) { return index < limit ? acc : acc + val }, 0) }

Hope this helps.

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