Python program to calculate sum and average of first n natural numbers

In this lesson, you will learn how to calculate the sum and average of the first n natural numbers in Python.

Also, you will get to know how to calculate the addition and average of user-entered numbers, list of numbers. And the use of built-in function sum().

This tutorials is part of Python Basics.

Sum and average of first n natural numbers

Program:

n = int(input("Enter number"))
sum = 0
# loop from 1 to n
for num in range(1, n + 1, 1):
    sum = sum + num
print("Sum of first ", n, "numbers is: ", sum)
average = sum / n
print("Average of ", n, "numbers is: ", average)

Output

Enter number 10
Sum of first  10 numbers is:  55
Average of  10 numbers is:  5.5

Use built-in function sum()

You can also take the advantage of built-in function sum() to calculate the sum of an iterable like range and list.

n = 10
res = sum(range(1, n + 1))
print("Sum of first ", n, "numbers is: ", res)

# Output Sum of first  10 numbers is:  55

Sum and average of a list

Use the below steps to calculate the sum and average of numbers present in the given list.

• Iterate a Python list using a for loop and add each number to a sum variable.
• To calculate the average, divide the sum by the length of a given list (total numbers in a list)

# list with int and floats
num_list = [10, 20.5, 30, 45.5, 50]

# Approach 1 using built-in function sum
res = sum(num_list)
avg = res / len(num_list)
print("sum is: ", res, "Average is: ", avg)
# Output sum is:  156.0 Average is:  31.2

# Approach 2 using a for loop
res1 = 0
for num in num_list:
    res1 += num
avg1 = res1 / len(num_list)
print("sum is: ", res1, "Average is: ", avg1)
# Output sum is:  156.0 Average is:  31.2

Sum and average using a mathematical formula

In the above programs, we calculated the sum and average using the looping technique. Now, let’s see how to calculate the sum and average directly using a mathematical formula.

Assume n is a number

• The sum of the first n natural number = n * (n+1) / 2
• the average of first n natural number = (n * (n+1) / 2) / n

Example

n = 20
# formula to calculate sum
res = n * (n + 1) / 2
print('sum of first', n, 'numbers is:', res)
# Output sum of first 20 numbers is: 210.0

# formula to calculate average
average = (n * (n + 1) / 2) / n
print('Average of first', n, 'numbers is:', average)
# Output Average of 20 numbers is: 10.5

Sum and average of multiple user-entered numbers

If you want to calculate the sum and percentage of multiple user-entered numbers, please refer to the following program.

Refer to how to accept list of numbers as a input in Python.

input_string = input('Enter numbers separated by space ')
print("\n")
# Take input numbers into list
numbers = input_string.split()

# convert each item to int type
for i in range(len(numbers)):
    # convert each item to int type
    numbers[i] = int(numbers[i])

# Calculating the sum and average
print("Sum = ", sum(numbers))
print("Average = ", sum(numbers) / len(numbers))

Output

Enter numbers separated by space 10 20 30 40 50

Sum =  150
Average =  30.0

While loop to calculate sum and average

You can also use the Python while loop to calculate the sum and average of n numbers. Follow these steps:

• Decide the value of n.
• Run a while loop till n is greater than zero.
• In each iteration, add the current value of n to the sum variable and decrement n by 1.
• Calculates the average by dividing the sum by n (total numbers).

n = 20
total_numbers = n
sum = 0
while n >= 0:
    sum += n
    n -= 1
print("sum =", sum)
# Output sum = 210

average = sum / total_numbers
print("Average = ", average)
# Output Average =  10.5

Practice Problem: Add two matrices in Python

matrixOne = [[6,9,11],
    [2 ,3,8]]

matrixTwo = [[15,18,11],
    [26,16,19]]

# Result shoud be
result = [[0,0,0],
         [0,0,0]]

Solution

matrixOne = [[6,9,11],
    [2 ,3,8]]

matrixTwo = [[15,18,11],
    [26,16,19]]

result = [[0,0,0],
         [0,0,0]]

# First iterate rows
for i in range(len(matrixOne)):
   # Second iterate columns
   for j in range(len(matrixOne[0])):
       result[i][j] = matrixOne[i][j] + matrixTwo[i][j]
print("Addition of two Matrix In Python")
for res in result:
   print(res)

Next steps

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Solve:

• Python exercise for beginners
• Python Quiz for beginners