Python program to find Sum of a sublist

In this article, we will learn different methods to find the sum of a sublist in Python. A sublist is a contiguous portion of a list between specified start and end indices.

Methods Used

The following are the various methods to accomplish this task ?

• Using For Loop (Brute Force)

• Using Cumulative Sum Method

• Using sum() Function

• Using math.fsum() Function

Using For Loop (Brute Force)

This method iterates through the sublist elements and adds them one by one ?

# Input list
numbers = [3, 5, 10, 5, 2, 3, 1, 20]
print("The Given List is:", numbers)

# Starting and ending indices
start_index = 1
end_index = 5

# Initialize sum variable
result_sum = 0

# Traverse from start index to end index
for k in range(start_index, end_index + 1):
    result_sum += numbers[k]

print("The resultant sum of sublist is:", result_sum)

The Given List is: [3, 5, 10, 5, 2, 3, 1, 20]
The resultant sum of sublist is: 25

Using Cumulative Sum Method

This method creates a cumulative sum array where each element contains the sum of all previous elements. Then we use the difference to get the sublist sum ?

# Input list
numbers = [3, 5, 10, 5, 2, 3, 1, 20]
print("The Given List is:", numbers)

# Starting and ending indices
start_index = 1
end_index = 5

# Create cumulative sum
cumulative = numbers.copy()  # Don't modify original list
for k in range(1, len(cumulative)):
    cumulative[k] = cumulative[k] + cumulative[k-1]

# Calculate sublist sum using cumulative array
if start_index == 0:
    result_sum = cumulative[end_index]
else:
    result_sum = cumulative[end_index] - cumulative[start_index-1]

print("The resultant sum of sublist is:", result_sum)

The Given List is: [3, 5, 10, 5, 2, 3, 1, 20]
The resultant sum of sublist is: 25

Using sum() Function

The most Pythonic approach uses list slicing with the built-in sum() function ?

# Input list
numbers = [3, 5, 10, 5, 2, 3, 1, 20]
print("The Given List is:", numbers)

start_index = 1
end_index = 5

# Get sublist using slicing and calculate sum
sublist = numbers[start_index:end_index + 1]
result_sum = sum(sublist)

print("The resultant sum of sublist is:", result_sum)

The Given List is: [3, 5, 10, 5, 2, 3, 1, 20]
The resultant sum of sublist is: 25

Using math.fsum() Function

The math.fsum() function provides more accurate floating-point summation compared to the built-in sum() function ?

import math

# Input list
numbers = [3, 5, 10, 5, 2, 3, 1, 20]
print("The Given List is:", numbers)

start_index = 1
end_index = 5

# Get sublist and calculate sum using math.fsum()
sublist = numbers[start_index:end_index + 1]
result_sum = math.fsum(sublist)

print("The resultant sum of sublist is:", result_sum)

The Given List is: [3, 5, 10, 5, 2, 3, 1, 20]
The resultant sum of sublist is: 25.0

Comparison

Conclusion

Use sum() with list slicing for most cases as it's the most readable. Use math.fsum() when working with floating-point numbers that require high precision. The cumulative sum method is efficient when you need to calculate multiple sublist sums from the same array.