javascript : how to calculate an average in a nested array inside an array

Question

How do I calcualte the overall average of an array like this: example array:

[[80, 90, 70], [70,80,60],[90,100,80]]

what I am trying right now

for (let i = 0; i < grades.length; i++) {
  for (let y = 0; y < grades[i].length; y++) {
    lastTotalScore += grades[i[y]];
    lastAverageScore = lastTotalScore / grades[i].length;
    overallTotalScore += lastAverageScore;
    overallAverageScore = overallTotalScore / grades.length;
  }
}

Thanks

Andy · Accepted Answer · 2022-02-20 08:40:57Z

1

flatten the array, and then reduce over the elements adding them up, and then dividing by the length of the flattened array.

const arr = [
  [80, 90, 70],
  [70, 80, 60],
  [90, 100, 80]
];

// Flatten the nested arrays
const flat = arr.flat();

// `reduce` over the numbers, and then divide
// by the number of elements in the array
const avg = flat.reduce((acc, c) => {
  return acc + c;
}, 0) / flat.length;

console.log(avg);

answered Feb 20, 2022 at 8:40

Andy

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Comments

Nitheesh · Accepted Answer · 2022-02-20 11:05:41Z

Calculate the total from all numbers. Divide it with length of total numbers.

Working Sample

const grades = [[80, 90, 70], [70, 80, 60], [90, 100, 80]];
let lastTotalScore = 0;
let length = 0;
for (let i = 0; i < grades.length; i++) {
    length += grades[i].length;
    for (let y = 0; y < grades[i].length; y++) {
        lastTotalScore += grades[i][y];
    }
}
console.log(`Average = ${lastTotalScore / length}`);

OR

Convert the two dimentional array to a linear structure using Array.flat and calculate its sum by looping with Array.reduce and divide by its length.

Working Sample

const grades = [[80, 90, 70], [70, 80, 60], [90, 100, 80]];
const flatGrade = grades.flat();
const sum = flatGrade.reduce((acc, curr) => acc + curr, 0);
console.log(`Average = ${sum / flatGrade.length}`);

Amila Senadheera · Accepted Answer · 2022-02-20 08:41:02Z

0

This would also work. Using flat and reduce.

const input = [
  [80, 90, 70],
  [70, 80, 60],
  [90, 100, 80],
];

const { sum, count } = input.flat().reduce(
  (prev, curr) => {
    prev.sum += curr;
    prev.count += 1;
    return prev;
  },
  { sum: 0, count: 0 }
);

console.log(sum / count);

answered Feb 20, 2022 at 8:41

Amila Senadheera

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Comments

Atif · Accepted Answer · 2022-02-20 10:16:01Z

0

As some of the others suggested, flattening the array does the trick nicely. The only additional thing I would consider is to wrap the code into a function, which can then be reused, to keep things DRY.

const someArray = [[80, 90, 70], [70, 80, 60], [90, 100, 80]];

function getAverage(arr) {
    const oneArr = arr.flat();
    const avg = oneArr.reduce((sum, value) => sum += value) / oneArr.length; 
    return avg;  
}

console.log(`The average grade is: ${getAverage(someArray)}.`);

answered Feb 20, 2022 at 10:16

Atif

313 bronze badges

Comments

Parvesh Kumar · Accepted Answer · 2022-02-20 16:06:06Z

0

Here's one that uses only reduce.

const arr = [
  [80, 90, 70],
  [70, 80, 60],
  [90, 100, 80],
];
const overallAverage = arr.reduce((acc, curr) => acc + curr.reduce((a, c) => a + c) / curr.length, 0) / arr.length;
console.log(overallAverage);

answered Feb 20, 2022 at 16:06

Parvesh Kumar

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