JavaScript programs to find the sum of all even numbers below another number

JavaScript program to find the sum of all even numbers below another number:

In this JavaScript program, we will learn how to find the sum of all even numbers smaller than a given number. It will find the sum of all even numbers from 1 to that number. A number is called an even number if it is perfectly divisible by 2.

We will use a loop to iterate through the numbers from 1 to that number and for each number, we will check if it is even or not with a separate function. If it is even, we will add it to a separate variable that will hold the final sum. At the end of the program, it will print the value of the final sum.

Method 1: By using a for loop:

With this approach, we will use a for loop to find the sum of the even numbers smaller than another number. Below is the complete JavaScript program:

function isEven(n) {
  return n % 2 == 0;
}

function findSum(no) {
  let sum = 0;

  for (var i = 1; i <= no; i++) {
    if (isEven(i)) {
      sum += i;
    }
  }
  return sum;
}

console.log(findSum(100));

Download the program on GitHub

Here,

• The isEven method is used to check if a number is even or not. It checks if the number is perfectly divisible by 2. It uses the modulo operator, %. The modulo operator returns the remainder of a division operation. The n%2 operation will return the remainder of n/2. So, if n is even, the operation n%2 will return 0.
• The findSum method takes the number as its parameter and returns the sum of all even numbers from 1 to that number.
• It uses one for loop that runs from 1 to no and for each number, it checks if it is even or not. If it is even, it adds that value to the sum variable. The sum variable is initialized as 0. So, once the for loop ends, it will hold the total sum of all even numbers in this range.

If I execute the above program, it will give the below output:

2550

Method 2: By using a while loop:

We can also use a while loop to solve it. The below program shows how it works:

function isEven(n) {
  return n % 2 == 0;
}

function findSum(no) {
  let sum = 0;
  let i = 1;

  while (i <= no) {
    if (isEven(i)) {
      sum += i;
    }
    i++;
  }
  return sum;
}

console.log(findSum(100));

Download the program on GitHub

• It is similar to the previous program.
• The only difference is that we are using a while loop instead of a for loop. It runs from i=1 to i=no and on each iteration, it adds the value of i to the sum variable if it is even.

If you run this program, it will print similar output.

Method 3: By using a do...while loop:

The do...while loop works similar to a while loop. It executes the code in the do block and checks the condition in the while block. The following program shows how to use a do...while loop to find the sum of even numbers:

function isEven(n) {
  return n % 2 == 0;
}

function findSum(no) {
  let sum = 0;
  let i = 1;

  do {
    if (isEven(i)) {
      sum += i;
    }
    i++;
  } while (i <= no);

  return sum;
}

console.log(findSum(100));

Download the program on GitHub

Method 4: By jumping between the numbers:

We can jump between the even numbers in a loop and add all the numbers to get the required sum. The loop can start from 2 and on each step, it can move to the next even number. This approach is faster than the previous examples:

function findSum(no) {
  let sum = 0;

  for (var i = 2; i <= no; i += 2) {
    sum += i;
  }
  return sum;
}

console.log(findSum(100));

Download the program on GitHub

The for loop starts from i = 2 and on each step, it increments the value of i by 2. It adds the values of i to the sum variable in the loop. The sum variable will hold the sum of all even numbers. It will print the same output.

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