How to find the missing number in an int array of 1 to 100 in JS

Overview

Consider an array of numbers from 1 to 100. In this array, duplicate elements are not allowed. Also, there is one number missing in this array. We will find that by following the steps provided below:

Step 1: Calculate the sum of numbers from 1 to 100 using the sum of natural numbers formula given below:

function findMissedNum(arrayOfNumbers, n = 100) {
    if(arrayOfNumbers.length === n) {
      console.log("no number is missed");
    }
    if(arrayOfNumbers.length < (n - 1) ) {
      console.log("more than one number is missed")
    }
    let totalSum = (n * (n+1)) / 2;
    let sumOfArray = 0;
    
    for(let i of arrayOfNumbers){
        sumOfArray += i;
    }
    
    return totalSum - sumOfArray;
}
// create an array with values 0 to 100
let arrayOfNumbers = Array.from( Array(101).keys())
// remove the 0th element 
arrayOfNumbers.shift();
// now the array will 1 to 100
// remove the value at index 50.(means remove 51 from the array)
arrayOfNumbers.splice(50, 1)
// now the array is 1 to 100 but without the number 51
let missedNum = findMissedNum(arrayOfNumbers, 100);
console.log("The missing number is :", missedNum)

Explanation

Lines 1–16: We create a findMissedNum function that will find the missing number of an array using the above algorithm.

Line 18: We create a new array, arrayOfNumbers.

The Array(101) array returns an array with 101 empty elements.

On the created array, we call the keys method, which returns an array iterator object that contains the key of each index. In our case, iterator from is from 0 to 100.
We use the Array.from method with the array iterator as an argument. This method returns a new array with the values of the array iterator. We get an array with values 0 to 100.

Line 21: We use the shift method to remove the first element of the array. Now the array will be 1 to 100.

Line 26: We use the slice method to remove the element at index 50. Element 51 will be removed from the array.

Line 30: We call the findMissedNum method. This method returns the missing number 51 as result.

How to find the missing number in an int array of 1 to 100 in JS

Overview

Example

Explanation