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

Updated on:  | 26 Comments

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.

Table of contents

Sum and average of first n natural numbers

Sum and average of n numbers in Python

  1. Accept the number n from a user

    Use input() function to accept integer number from a user.

  2. Run a loop till the entered number

    Next, run a for loop till the entered number using the range() function. In each iteration, we will get the next number till the loop reaches the last number, i.e., n.

  3. Calculate the sum

    In each iteration, keep adding the current number into the sum variable to calculate the addition. Use a formula sum = sum + current number.

  4. Calculate the average

    At last, after the loop ends, calculate the average using a formula average = sum / n. Here, The n is a number entered by the user.

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)Code language: Python (python)
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:  55Code language: Python (python)

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:  31.2Code language: Python (python)

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
Code language: Python (python)

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))
Code language: Python (python)

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
Code language: Python (python)

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]]Code language: Python (python)

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)Code language: Python (python)

Next steps

Let me know your comments and feedback in the section below.

Solve:

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About Vishal

I’m Vishal Hule, the Founder of PYnative.com. As a Python developer, I enjoy assisting students, developers, and learners. Follow me on Twitter.

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Comments

  1. Princewill says

    Hello Vishal,
    Please can you help me with this question below.

    Given the final percentage a student has gotten at the end of a semester, you need to write a program that decides if the student has passed or failed the semester.

    If the percentage is higher than or equal to 60, the student has passed the semester. If the percentage is lower than 60, the student has failed the semester.

    However, the percentage is not the only thing that determines if a student has passed or failed. A student does not pass if their score is 5 points below the class average.

    For instance, if the average class score is 70, the student must have a minimum score of 65 to pass.

    If the average class score is 50, the student still needs a score of 60 to pass based on our first condition.

    Reply

    • Damascene says 

      # Calculation of pass marks

passMarks=60

classAverage=float(input('Please enter your class average marks: '))
studentMarks=float(input('please enter your score: '))

if (studentMarks>=(classAverage-5))and (studentMarks>=passMarks):
    print('Congz you have passed the semester')


else:
    print('unfortunately you failed the semester')
      Reply

  2. Bill says

    Hi, can you help me with this code as I am not getting the right total sum and confused about where have I gone wrong?

    
SMALL_MAH = 3000
MEDIUM_MAH = 10000
LARGE_MAH = 20000

# Paste the `recommend_power_bank` function from your Part A solution here
def recommend_power_bank(x):
     if x <= SMALL_MAH:
      return str('small')
     elif x  MEDIUM_MAH:
      return str('large')
# Write the rest of your solution here
battery = int(input('Device battery capacity (mAH): '))
more = input('More? (y/n): ')
battery = 0
capacity = battery + battery   
while more == 'y':
    battery = int(input('Device battery capacity (mAH): '))   
    capacity = battery + battery      
    more = input('More? (y/n): ').lower()   
    if more == 'n':
       break 
    elif more == 'y':
         continue
print('Total required battery capacity: ' + str(capacity)+ ' mAH')
print('Power bank recommendation: ' +str(recommend_power_bank(int(capacity))))
    Reply

    • Damascene says 

      SMALL_MAH = 3000
MEDIUM_MAH = 10000
LARGE_MAH = 20000

# Paste the `recommend_power_bank` function from your Part A solution here
def recommend_power_bank(x):
     if x <= SMALL_MAH:
      return str('small')
     elif x <= MEDIUM_MAH:
      return str('medium')
     else:
          return str('large')
# Write the rest of your solution here


while True:
         batteryCapacity = int(input('Device battery capacity (mAH): '))  
         newCapacity=recommend_power_bank(batteryCapacity)
         print('Total required battery capacity:',batteryCapacity,' mAH')
         print('Power bank recommendation: ',newCapacity)
         
         more = input("press 'q' to quit or enter to continue: ")   
         if more == 'q':
            break

print('Ok Bye!')
      Reply

    • Usman Zafar says

      sum = 0

      SMALL_MAH = 3000
      MEDIUM_MAH = 10000
      LARGE_MAH = 20000

      # Paste the `recommend_power_bank` function from your Part A solution here
      def recommend_power_bank(x):
      if x <= SMALL_MAH:
      return str('small')
      elif x < MEDIUM_MAH:
      return str('large')
      # Write the rest of your solution here
      battery = int(input('Device battery capacity (mAH): '))
      more = input('More? (y/n): ')
      battery = 0
      capacity = battery + battery
      while more == 'y':
      battery = int(input('Device battery capacity (mAH): '))
      capacity = battery + battery
      more = input('More? (y/n): ').lower()
      if more == 'n':
      break
      elif more == 'y':
      continue
      print('Total required battery capacity: ' + str(capacity)+ ' mAH')
      print('Power bank recommendation: ' +str(recommend_power_bank(int(capacity))))

      Reply

  3. Ramya says

    Such a profound Python website with excellent information and hands on coding exercises. Very happy and glad I came across your website today. Thank you !

    Reply

  4. Shadi says

    Hi, thank you for the tutorial.
    Could you please list then prons and cons of the summation using loop compare to using the math formula directly, regarding the computation space and time complexity.
    Many thanks in advance!

    Reply

  5. Anna says

    Can you help with this please
    write a python program that helps users calculate how much they should pay for the items they have put in their cart at the supermarket. Your program should ask the person how many items they put in their cart. Your program should then ask the user to input the price of each item independently.Your program should finally print the total price of all total items

    Reply

    • Chikki says

      Hey! As it’s my very first program and I tried to solve your question, hope it will help you

      n = int(input("Enter the total item no. In cart "))
pay=0
I=1
while (i <= n):
    Print("Enter the price of item no. ",i )
    item = float(input())
    pay = pay+item
    i = i+1
print("your total payable amount is: ")
      Reply

      • Damascene says 

        #Cost of item

#Ask the user to enter the number of items that he wants to put in the cart.

nItems=input('Please enter the number of items that you want to put in the cart:')
nItems=int(nItems)
cost=0
i=1
while (i<=nItems):

    
#Ask the user to enter the cost of each item.
    costItem=int(input('please enter the cost of one item: '))

    cost=cost+costItem
    i=i+1

print(' the cost of item is',cost)
        Reply

  7. Daniel says

    please give me these answer formula. I need this question formula very badly.
    Q:1 write a function sum that calculates the sum of begin number to end number by step.
    for example: sum(10,30,3)=> 10+13+16+19+22+25+28
    Q:2 read a file(input_text.txt)
    a. write a program that counts each alphabetic character occurrence. “Yesterday all my troubles seemed so far away.”

    'a': 5
'b': 1
'c': 0
'd': 2
'e': 6

    b. write a program that sorts in descending order by frequency of character and write the sorted result to a file(sorted_result.txt)

    'e': 6
'a': 5
'd': 2
'b': 1

    Q.3 write a mini calculator function that has three parameters(first two are operands and the third parameter is the operator) and returns the result.
    for example

    calculator(35,5, '+' : 35+5=>40
calculator(10, 15, '-'): 10-15=> -5

    Q.4 A prime number is a number that has only 1 and itself as a divisor (for example, 1,3,5,7,11,13….)
    a. write a function that parameter n is whether a prime number or not.
    b. write a program that prints prime numbers between 1 to 100
    Thank you advance for your replay.

    Reply

    • Damascene says 

      #Ask the user to enter the starting number.
startingNumber=int(input('please enter the starting number: '))

#Ask the user to enter the ending number.
endingNumber=int(input('Enter the ending number: '))
#Ask the user to enter interval between two numbers
interval=int(input('Enter interval between two numbers: '))
sum=0

for i in range(startingNumber,endingNumber,interval):
    sum=sum+i

print('The sum is:',sum)
      Reply

  8. Thabiso says

    ● Create a new file called while.py
    ● Write a program that always asks the user to enter a number.
    ● When the user enters -1, the program should stop requesting the user to
    enter a number,
    ● The program must then calculate the average of the numbers entered
    excluding the -1.
    ● Make use of the while loop repetition structure to implement the
    program.
    ● Compile, save and run your file

    can you please help me with this exercise

    Reply

    • Vishal says

      Hey Thabiso, Please try this

      sum=0 
count=0;
while True: 
  n = input("Enter Number ") 
  n = int (n)
  if n==-1:
    break
  else:
    count += 1 
    sum += n
print ("sum using while loop ", sum) 
average = sum / count 
print("Average using a while loop ", average)
      Reply

  9. Hoàng says

    Write a Python program to calculate the average of a given list.
    Example : list=[22,25,26,32,41,35,13,16,21]
    average = 20, len =3

    Reply

    • Hoàng says

      list=[22,25,26,32,41,35,13,16,21]

      import random
new_list=random.sample(list[0:],3)
import statistics
statistics.mean(new_list)
j=statistics.mean(new_list)
if round(j)==20:
	print(new_list)

      # program run slowly
      # Do you have a better way? Thanks

      Reply

  10. chand says

    Python code

    Need help in writing code in finding average using while loop.

    Can you help with the code

    Question
    Write a program that first prompts the user to enter the number of numbers to be entered and then uses a while loop to repeatedly prompt the user for those numbers and adds the numbers to a running total. When the correct number of numbers have been entered, the program should print the average.
    You can assume the number of numbers entered is an integer greater than zero and that each subsequent number is a float.

    Reply

  11. VED PRAKASH says

    While calculating average using while loop why it’s necessary to divide the “sum” by “total_numbers” instead of “n” even if we take n >=1.

    Please explain.

    Reply

    • Vishal says

      Hey Ved Prakash,
      we decrementing the value of n in each iteration i.e. n-=1. so and the end of the last iteration of while loop n will become ZERO.
      that’s why before the start of the loop iteration we are taking the value of n into another variable which is nothing but total_number.

      Reply

      • Chitra Tanasap says

        Hi Vishal,
        I have a question when we calculated sum and want to define as str, how to do it. After enter all the marksList and print ( “” +str(sumOfMarks)) and print(“” + str(averageOfMarks)).
        I always get the error int and str

        #calculate the sum and average
        #sumOfMarks = sum(marksList)
        #averageOfMarks = sum(marksList)/5

        Reply

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