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# JavaScript program for Mean of range in an array

Given an array of numbers and a range, the goal is to calculate the mean of all the numbers within that range. This problem can be approached in various ways, but in this tutorial, we will explore a simple and efficient solution using JavaScript. We will begin by defining the problem in more detail, including examples that illustrate how the function should work. Then, we will break down the solution step by step, providing clear explanations and code snippets along the way. By the end of this tutorial, you will have a solid understanding of how to write a JavaScript program that calculates the mean of a range in an array. So let’s get started!

Before we begin, let's define what we mean by "mean". In mathematics, the mean (also known as the average) is calculated by adding up all the values in a set of numbers and then dividing the sum by the number of values. In the context of an array, the mean of a range of values is the average of all the values in that range.

## Problem Statement

Given an array of numbers and a range consisting of two numbers, the goal is to calculate the mean of all the numbers within that range. The range is inclusive, meaning it should include both the first and last numbers in the range. The function should take in the array and range as input and output the mean of the numbers within the range. If the input array is empty, the function should return null. If the range is invalid (i.e., the first number is greater than the second number), the function should return null as well. The output should be a float with two decimal places.

Let’s understand this with some examples −

### Example 1

Input: Array: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Range: [3, 7] Output: Mean of range [3, 4, 5, 6, 7] is 5.00

In Example 1, we have an array of numbers [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and the range [3, 7]. The mean of the range [3, 4, 5, 6, 7] is 5.00, so the output should be 5.00.

### Example 2

Input: Array: [2, 4, 6, 8, 10, 12] Range: [1, 5] Output: Mean of range [2, 4, 6, 8, 10] is 6.00

In Example 2, we have an array of numbers [2, 4, 6, 8, 10, 12] and the range [1, 5]. The mean of the range [2, 4, 6, 8, 10] is 6.00, so the output should be 6.00.

### Example 3

Input: Array: [3, 5, 7, 9, 11, 13] Range: [7, 3] Output: null

In Example 3, the range [7, 3] is invalid because the first number is greater than the second number, so the output should be null.

### Example 4

Input: Array: [] Range: [1, 5] Output: null

In Example 4, the input array is empty, so the output should also be null.

Now let’s understand the algorithm for the above problem statement.

## Algorithm

**STEP 1** − Define the function 'meanInRange' that takes two parameters, an array 'arr' and a range 'range'.

**STEP 2** − Check if the length of the input array 'arr' is zero. If it is, return null.

**STEP 3** − Destructure the 'range' array range into two variables 'start' and 'end'.

**STEP 4** − Check if the value of 'start' is greater than 'end'. If it is, return null.

**STEP 5** − Use the 'slice' method to extract a subarray from the input array 'arr' that includes only the elements between the start and end indices of the given range.

**STEP 6** − Use the 'reduce' method to calculate the sum of all the elements in the subarray.

**STEP 7** − Divide the sum by the length of the subarray to obtain the mean.

**STEP 8** − Return the mean as a string formatted to two decimal places, along with a message that indicates the range of the values used to compute the mean.

**STEP 9** − End the function.

Now let’s understand this algorithm with an example where we implement this algorithm using JavaScript.

## Example

In this implementation, we define a function 'meanInRange' that takes in an array 'arr' and a range 'range' as input. The function first checks if the input array is empty or if the range is invalid and returns null if either condition is true. Otherwise, it slices the input array to get the range, calculates the sum of the numbers in the range using 'reduce', and calculates the mean. Finally, the function logs the mean rounded to two decimal places to the console using 'console.log'.

We then call the 'meanInRange' function with different input arrays and ranges to test its functionality and print the output to the console.

function meanInRange(arr, range) { if (arr.length === 0) { return null; // return null if the input array is empty } const [start, end] = range; if (start > end) { return null; // return null if the range is invalid } const rangeArr = arr.slice(start - 1, end); // slice the array to get the range const sum = rangeArr.reduce((acc, num) => acc + num, 0); // calculate the sum const mean = sum / rangeArr.length; // calculate the mean // return the mean rounded to two decimal places return `Mean of range [${rangeArr}] is ${mean.toFixed(2)}`; } // Example usage: console.log(meanInRange([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [3, 7])); // should output "Mean of range [3,4,5,6,7] is 5.00" console.log(meanInRange([2, 4, 6, 8, 10, 12], [1, 5])); // should output "Mean of range [2,4,6,8,10] is 6.00" console.log(meanInRange([3, 5, 7, 9, 11, 13], [7, 3])); // should output "null" console.log(meanInRange([], [1, 5])); // should output "null"

## Conclusion

In this tutorial, we explored how to find the mean of a range of values within an array using JavaScript. We discussed the problem statement and provided an algorithm and a JavaScript program that solves the problem. We also learned several usage examples and how to handle edge cases like an empty array or an invalid range. By following this approach, you can easily compute the mean of a range of values within an array in your JavaScript programs.