Python Program to Find Sum of Even Numbers

You can find the sum of even numbers in a list using sum(num for num in numbers if num % 2 == 0) where numbers is your list of integers.

Examples

Input[1, 2, 3, 4, 5]

Output6

Input[10, 15, 20, 25, 30]

Output60

Input[]

Output0

How to Think About It

To find the sum of even numbers, look at each number one by one and check if it divides evenly by 2 using %. If yes, add it to a total sum. At the end, the total sum will be the sum of all even numbers.

Algorithm

1

Get the list of numbers.

2

Create a variable to hold the sum, starting at 0.

3

For each number in the list, check if it is even using <code>number % 2 == 0</code>.

4

If it is even, add it to the sum variable.

5

After checking all numbers, return or print the sum.

Code

python

numbers = [1, 2, 3, 4, 5]
sum_even = sum(num for num in numbers if num % 2 == 0)
print(sum_even)

Output

6

Dry Run

Let's trace the list [1, 2, 3, 4, 5] through the code

1

Start with sum_even = 0

sum_even = 0

2

Check 1: 1 % 2 != 0, skip

sum_even = 0

3

Check 2: 2 % 2 == 0, add 2

sum_even = 2

4

Check 3: 3 % 2 != 0, skip

sum_even = 2

5

Check 4: 4 % 2 == 0, add 4

sum_even = 6

6

Check 5: 5 % 2 != 0, skip

sum_even = 6

7

Return sum_even

6

Number	Is Even?	Sum So Far
1	No	0
2	Yes	2
3	No	2
4	Yes	6
5	No	6

Why This Works

Step 1: Check each number

We use num % 2 == 0 to find if a number is even because even numbers have no remainder when divided by 2.

Step 2: Add even numbers

Only numbers passing the even check are added to the total sum using sum().

Step 3: Return the total

After checking all numbers, the sum contains the total of all even numbers.

Alternative Approaches

Using a for loop and accumulator

python

numbers = [1, 2, 3, 4, 5]
sum_even = 0
for num in numbers:
    if num % 2 == 0:
        sum_even += num
print(sum_even)

This approach is more explicit and easier to understand for beginners but longer than using sum with a generator.

Using filter and sum

python

numbers = [1, 2, 3, 4, 5]
sum_even = sum(filter(lambda x: x % 2 == 0, numbers))
print(sum_even)

This uses functional programming style with filter and lambda but can be less readable for new learners.

Complexity: O(n) time, O(1) space

Time Complexity

The program checks each number once, so time grows linearly with the number of elements, making it O(n).

Space Complexity

Only a few variables are used regardless of input size, so space complexity is O(1).

Which Approach is Fastest?

All approaches run in O(n) time; using sum with a generator is concise and efficient, while loops and filter add readability or style differences.

Approach	Time	Space	Best For
sum with generator	O(n)	O(1)	Concise and efficient
for loop with accumulator	O(n)	O(1)	Clear and beginner-friendly
filter and sum	O(n)	O(1)	Functional programming style