How to calculate the difference between of the elements in an array starting from the last up to the first in JavaScript?

user_14124549 0
Viewed 87

This is an array given:

arrayNum = [1, 2, 4, 5, 8, 9];
arrayS = [];
for(var i=1, len = array1.length; i<len; i++){
    arrayS.push(arrayNum[i]-arrayNum[i-1]);
  }
console.log(arrayS);

This code calculates the difference between each two consecutive elements! However I need to calculate the difference between elements starting from the last up to the first element what would be in this particular case 9-8-5-4-2-1 = -11?!

s1=0;
for(var j=array1[array1.length-1]; j>0; j--){
    s1 = s1 - array1[j];
  }
console.log(s1);

However this is not working!

for-loop arrays javascript
4 Answers

You can use Array.reduceRight() to calculate the difference from the end of the array.

Note: that reduce/reduceRight would throw an error when reducing an empty array without an initial value. I use a ternary to check the length, and if it's empty return NaN.

const fn = arr =>
  arr.length ?
    arrayNum.reduceRight((s, n) => s - n) // if array is not empty
    :
    NaN // if array is empty

const arrayNum = [1, 2, 4, 5, 8, 9];

const result = arrayNum.reduceRight((s, n) => s - n)

console.log(result);

For the for loop to work, you need to initialize s1 without setting a value, and j with the last index. When calculating s1 check if it's undefined, and initialize it with the current number. If it's not, subtract the current number:

const array1 = [1, 2, 4, 5, 8, 9];

let s1;

for (let j = array1.length - 1; j >= 0; j--) {
  s1 = s1 === undefined ? array1[j] : s1 - array1[j];
}

console.log(s1);

user_51574540

In your original solution, you should iterate the index, rather than the element

const arrayNum = [1, 2, 4, 5, 8, 9];

s1 = arrayNum[arrayNum.length - 1];
for (var j = arrayNum.length - 2; j >= 0; j--) {
  s1 = s1 - arrayNum[j];
}
console.log(s1);

Or you could use reduce

const arrayNum = [1, 2, 4, 5, 8, 9];

const res = arrayNum.reduce(
  (acc, el, index) => acc + (index !== arrayNum.length - 1 ? -1 : 1) * el,
  0
);

console.log(res);

user_66551600

Expression 9-8-5-4-2-1 is equal to -(-9+8+5+4+2+1).

-9+8+5+4+2+1 is equal to (-(9*2) + (9+8+5+4+2+1)).

const arrayNum = [1, 2, 4, 5, 8, 9];

const res = -arrayNum.reduce((acc, num) => acc + num
, -arrayNum[arrayNum.length - 1] * 2)

console.log(res)

user_142687800

Issue is with var j=array1[array1.length-1];, not correct index to start with in for-loop.

Try the while loop, should simplify for this case.

array1 = [1, 2, 4, 5, 8, 9];

s1 = array1[array1.length-1];
j = array1.length-1;

while (--j >= 0) s1 -= array1[j];

console.log(s1);

user_78994770
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