Find Sum of Array in Python

What You’ll Learn

Summing an array means visiting every element once and adding it to a running total (the accumulator pattern). Example: [1, 2, 3, 4, 5] → 15. Empty lists sum to 0; negatives are allowed. This tutorial covers a manual loop, user input, a live preview, worked Python examples, edge cases, and complexity.

📝 Problem & Approach

Given a list of integers, return the sum of all elements.

# [1, 2, 3, 4, 5] -> 15
# [10, -2, 7]     -> 15
# []              -> 0

Inputs & Outputs

Minimal workflow

function array_sum(arr):
    total = 0
    for value in arr:
        total += value
    return total

Method comparison

🔮 Live Preview

Enter integers separated by commas or spaces and compute the sum instantly.

Press “Compute sum”.

Examples Gallery

Three complete Python programs — sum a fixed list, read user input, and show a running total with negatives. Click View Output to reveal sample console results.

def sum_array(arr: list[int]) -> int:
    total = 0
    for value in arr:
        total += value
    return total

array = [1, 2, 3, 4, 5]
print(f"Sum of the array elements: {sum_array(array)}")

Sum of the array elements: 15

def sum_array(arr: list[int]) -> int:
    total = 0
    for value in arr:
        total += value
    return total

n = int(input("Enter number of elements: ").strip())
if n < 0:
    raise ValueError("n must be nonnegative.")

arr = []
for i in range(n):
    arr.append(int(input(f"Enter element {i + 1}: ").strip()))

print(f"Sum of the array elements: {sum_array(arr)}")

Enter number of elements: 5
Enter element 1: 1
Enter element 2: 2
Enter element 3: 3
Enter element 4: 4
Enter element 5: 5
Sum of the array elements: 15

arr = [10, -2, 7]
total = 0
print("Running total:")
for value in arr:
    total += value
    print(f"  after {value:+d}: {total}")
print(f"Final sum: {total}")

Running total:
  after +10: 10
  after -2: 8
  after +7: 15
Final sum: 15

🔎 Worked Walkthrough — [1, 2, 3, 4, 5]

Watch the accumulator climb from 0 to 15.

Same table works for [10, -2, 7]: 0 → 10 → 8 → 15.

⚖️ Facts Worth Knowing

Handy follow-ups interviewers sometimes ask.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

You must read every element, so linear time is optimal for an unsorted list.

💡 Best Practices