Average value between each element of the array

I need help with my React Native app. I need to calculate the average value between each element of the array. The logic is as follows, if there are 4 elements with values 3,5,6,9, then you need to calculate the average value between 3 and 5 and divide by 1, between 3, 5, 6 and divide by 2, etc. The first value is counted as the initial value, so it is not taken into account.

I would appreciate any help.

	return (
		<View style={Styles.sectionContainer}>
			{readings.map(
				({
					id,
					date,
					volume,
				}) => {
					return (
						<View key={id} style={Styles.card}>
							<Text
								style={[
									Styles.sectionHeading,
									Styles.darkAppText,
									{ fontWeight: "bold" },
								]}
							>
								average: {average}
							</Text>
						</View>
					);
				}
			)}
		</View>
	);
}

I was trying to do this inside map function, but it gives average between 2 last elements:

if (id == 1) {
  average = 0;
} else if (id > 1) {
  for (i = 1; i < readings.length; i++) {
    average = (readings[i].volume readings[i - 1].volume) / i;
  }
}

Although you write that the first value is not to be taken into account, you seem to still want to output something for it. Maybe that is the role of the if (id == 0) statement, although there is no (explained) reason why the entry with id 0 should be the first one. Yet you seem to want to map that first entry to 0.

You can use this function for that:

<!-- begin snippet: js hide: false console: true babel: null -->

<!-- language: lang-js -->

function runningVolumeAverage(arr) {
    let sum = 0;
    return arr.map(({volume}, i) => {
        sum += volume;
        return i && sum / i;
    });
}

const readings = [{ volume: 3 }, { volume: 5 }, { volume: 6 }, { volume: 9 }];
console.log(runningVolumeAverage(readings));

<!-- end snippet -->

If you want to exclude that zero, and want to have one fewer value in your output than you have in your input, then apply .slice(1) to the returned array:

console.log(runningVolumeAverage(readings).slice(1));

In your actual code you could integrate it as follows (assuming all the rest of your code is fine, and only the average was the missing piece of info):

const averages = runningVolumeAverage(readings);
return (
        <View style={Styles.sectionContainer}>
            {readings.map(
                ({
                    id,
                    date,
                    volume,
                }, i) => {  // <-- get the index
                    return (
                        <View key={id} style={Styles.card}>
                            <Text
                                style={[
                                    Styles.sectionHeading,
                                    Styles.darkAppText,
                                    { fontWeight: "bold" },
                                ]}
                            >
                                average: {averages[i]} // <-- get the average
                            </Text>
                        </View>
                    );
                }
            )}
        </View>
    );

The below functions compute the average values according to what you've described and returns them as an Array:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const data = [3,5,6,9];

function avg(first, ...rest) {
  const v = rest.reduce((a, b) => a + b, first);
  return v / rest.length;
}

function getAvgAll(values) {
  const results = [];
  for (let i = 1; i < values.length; i += 1) {
    results.push(avg(...values.slice(0, i + 1)));
  }
  return results;
}

console.log(getAvgAll(data));

<!-- end snippet -->

EDIT :Your problem has nothing to do with React Native rather its simply a JS job.here is the code

function calculateAverageValues(arr) {
  if (arr.length < 2) {
    return []; // If there are less than 2 items,return empty
  }

  const outputArray = [];
  let sum = arr[0]; // Initialize the sum with the first item 

  for (let i = 1; i < arr.length; i++) {
    sum += arr[i]; // Add the current element to the sum.
    const average = sum / i; // Calculate the average value.
    outputArray.push(average); // Push the average value to output 
 
  }

  return outputArray;
}

Example usage:

const inputArray = [3, 5, 6, 9];
const result = calculateAverageValues(inputArray);
console.log(result); // Output:[ 8, 7, 7.666666666666667 ]

For the first element and the last element in the array I choose to return a value of 0 you could change it for whatever value you want.

const array = [3,5,6,9]
const arrayValuePerIndex = array.map((item, index, origArray) => {
    return index === 0 ? 0 : index< origArray.length - 1? (item + origArray[index + 1]) / (index + 1) : 0; 
});

console.log(arrayValuePerIndex) // [ 0, 5.5, 5, 0 ]