Modified version of summing an array with recursion in Javascript

Question

I'm struggeling a bit with recursion and just solved an exercise where you have to sum up all the elements of an array.

function sumArr (para) {

  if (para.length === 0) {
    return 0
  }

  else {
    return para.pop() + sumArr(para)
    
  }
}

When invoked sumArr ( [2,3,4] ) it returns 7, as expected. However, now I changed the function a bit


function sumArr (para) {

  if (para.length === 0) {
    return 0
  }

  else {
    return para.pop() - sumArr(para)
    
  }
}

and when invoked sumArr ( [2,3,4] ) it returns 3, which I can't wrap my head around.

Thanks for reading or even helping me understand recursion a bit better!

Do you have to use a function to do that calculation or could you use anything in javascripts tool box? like reduce() for example — fedesc
– fedesc, Commented Jul 21, 2020 at 8:36
The pop() method removes the last element from an array and returns that element 4-(3-(2-(0))) = 3 — Nikita Madeev
– Nikita Madeev, Commented Jul 21, 2020 at 8:38
It works as expected for addition because it's commutative and associative. Subtraction is not associative: (a - b) - c != a - (b - c) — Barmar
– Barmar, Commented Jul 21, 2020 at 8:41

Nina Scholz · Accepted Answer · 2020-07-21 08:45:33Z

1

You could return a string instead of a calculated value and have alook to the brackets.

function sumArr(para) {
    if (para.length === 0) return 0;      

    let pop = para.pop(),
        temp = `(${pop} - ${sumArr(para)})`;

    console.log(pop, temp);
    return temp;
}

console.log(sumArr([2, 3, 4]));

answered Jul 21, 2020 at 8:45

Nina Scholz

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Comments

Mark · Accepted Answer · 2020-07-21 14:07:41Z

1

JavaScript arrays are passed by reference and value. Using Array.pop mutates the collection, which is leading to side-effects like what you see. Instead, Array.slice as given below will return a copy of the array without the first element:

function sum(arr) {
  if (arr.length) {
    return arr[0] + sum(arr.slice(1));
  } else {
    return 0;
  }
}

answered Jul 21, 2020 at 14:07

Mark

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