sum() function in Python

Introduction

The sum() function adds up all numbers in a list or other group. It helps you quickly find the total without writing a loop.

You want to find the total cost of items in a shopping list.

You need to add up scores from a game to get the final result.

You want to calculate the total distance traveled from a list of daily distances.

You have a list of numbers and want to find their sum easily.

Syntax

Python

sum(iterable, start=0)

iterable is a group of numbers like a list or tuple.

start is optional and adds a number to the total at the beginning (default is 0).

Examples

Adds all numbers in the list: 1 + 2 + 3 + 4 = 10

Python

sum([1, 2, 3, 4])

Adds numbers in a tuple: 10 + 20 + 30 = 60

Python

sum((10, 20, 30))

Adds numbers in the list and then adds 10: (5 + 5 + 5) + 10 = 25

Python

sum([5, 5, 5], 10)

Sample Program

This program adds all numbers in the list numbers using sum(). It also shows how to add a starting value.

Python

numbers = [4, 7, 1, 3]
total = sum(numbers)
print(f"The total is {total}")

# Using start value
total_with_start = sum(numbers, 5)
print(f"Total with start value 5 is {total_with_start}")

Important Notes

sum() works only with numbers inside the iterable.

If the iterable is empty, sum() returns the start value (default 0).

Summary

sum() quickly adds all numbers in a list or tuple.

You can add a starting number to the total with the optional start argument.

It saves time and makes your code simpler than writing loops.

Practice

1. What does the sum() function do in Python?

easy

A. Counts the number of elements in an iterable

B. Multiplies all numbers in an iterable

C. Adds all numbers in an iterable and returns the total

D. Finds the largest number in an iterable

Solution

1. Step 1: Understand the purpose of sum()

   The sum() function takes an iterable like a list or tuple and adds all its numbers together.

2. Step 2: Compare options with the function's behavior

   Only Adds all numbers in an iterable and returns the total correctly describes adding all numbers and returning the total.

3. Final Answer:

   Adds all numbers in an iterable and returns the total -> Option C

4. Quick Check:

   sum() adds numbers [OK]

Hint: Remember: sum() adds, not multiplies or counts [OK]

Common Mistakes:

• Confusing sum() with multiplication
• Thinking sum() counts elements
• Assuming sum() finds max value

2. Which of the following is the correct syntax to sum numbers in a list nums starting from 10?

easy

A. sum(nums, 10)

B. sum(10, nums)

C. sum(nums + 10)

D. sum(start=10, nums)

Solution

1. Step 1: Recall the sum() function signature

   The sum() function syntax is sum(iterable, start=0), where the second argument is the starting value.

2. Step 2: Identify correct argument order

   sum(nums, 10) uses sum(nums, 10), which correctly passes the iterable first and start second.

3. Final Answer:

   sum(nums, 10) -> Option A

4. Quick Check:

   sum(iterable, start) = sum(nums, 10) [OK]

Hint: Iterable first, start second in sum() [OK]

Common Mistakes:

• Swapping the order of arguments
• Using plus sign inside sum()
• Using keyword argument incorrectly

3. What is the output of the following code?

numbers = [2, 4, 6]
result = sum(numbers, 5)
print(result)

medium

A. Error

B. 12

C. 11

D. 17

Solution

1. Step 1: Calculate sum of list elements

   The list numbers contains 2, 4, and 6. Their sum is 2 + 4 + 6 = 12.

2. Step 2: Add the start value

   The start value is 5, so total is 12 + 5 = 17.

3. Final Answer:

   17 -> Option D

4. Quick Check:

   sum([2,4,6],5) = 17 [OK]

Hint: Add start value after summing list [OK]

Common Mistakes:

• Forgetting to add the start value
• Adding start value before summing
• Assuming sum returns list length

4. The code below throws an error. What is the problem?

values = [1, 2, '3', 4]
total = sum(values)

medium

A. sum() requires a start argument

B. List contains a string which cannot be added

C. sum() only works with tuples

D. Missing parentheses in sum() call

Solution

1. Step 1: Check list element types

   The list values contains integers and a string '3'. sum() cannot add strings to numbers.

2. Step 2: Understand sum() type requirements

   sum() requires all elements to be numbers. Mixing types causes a TypeError.

3. Final Answer:

   List contains a string which cannot be added -> Option B

4. Quick Check:

   sum() needs all numbers [OK]

Hint: Ensure all list items are numbers before sum() [OK]

Common Mistakes:

• Ignoring mixed data types
• Thinking sum() needs start argument
• Assuming sum() works on strings

5. You have a list of daily sales: sales = [100, 200, 0, 150, 300]. You want to calculate the total sales but ignore days with zero sales. Which code correctly uses sum() to do this?

hard

A. sum(sale for sale in sales if sale != 0)

B. sum(sales) - sales.count(0)

C. sum(sales, 0) if 0 not in sales else 0

D. sum(sales) / sales.count(0)

Solution

1. Step 1: Filter out zero sales using a generator expression

   The expression sale for sale in sales if sale != 0 creates a sequence of sales excluding zeros.

2. Step 2: Use sum() on filtered sales

   sum() adds only the non-zero sales, giving the correct total.

3. Final Answer:

   sum(sale for sale in sales if sale != 0) -> Option A

4. Quick Check:

   sum(filtered sales) = correct total [OK]

Hint: Use sum() with condition inside generator [OK]

Common Mistakes:

• Subtracting the count of zeros instead of filtering
• Using sum() with wrong condition
• Dividing sum by zero count