Sum in python with lists

How to sum a list in Python

In Python, we look at a list as a data structure that is a mutable (or changeable) ordered sequence of elements. A list’s items are the elements or values that make up the list. Lists are characterized by values between square brackets [ ], just as characters between quotes define strings.

If you need to function with a large number of similar values, lists are ideal. They allow you to hold data that belongs together, condense your code, and apply the same methods and operations to multiple values simultaneously.

Consider all of the various collections you have on your computer while speaking about Python lists and other data structures that are forms of collections: your collection of files, your music playlists, your browser bookmarks, your emails, the collection of videos you can access on a streaming service, and more.

Summing a list in Python

This article seeks to illustrate how to find the sum of the elements in a list provided these elements are numbers. However, you may also wish to merge a list of strings or non-numerical values where sum() will not be helpful but we can engage other approaches as will be illustrated in the examples. There are various ways to sum a lists’ elements in Python. We hope that by the end of this article, you would have mastered at least one approach that resonates with you more.

Using sum() function

Python has an inbuilt function called sum() which aids in adding up the numbers in a list. Summing up lists is required virtually in every sphere that we interact with lists.

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How to Find the Sum of Elements in a List in Python

In Python, programmers work with a lot of lists. Sometimes, it is necessary to find out the sum of the elements of the lists for other operations within the program.

In this article, we will take a look at the following ways to calculate sum of all elements in a Python list:

1) Using sum() Method

Python provides an inbuilt function called sum() which sums up the numbers in a list.

Syntax

• Iterable – It can be a list, a tuple or a dictionary. Items of the iterable have to be numbers.
• Start – This number is added to the resultant sum of items. The default value is 0.

The method adds the start and the iterable elements from left to right.

Example:

Code Example:

# Python code to explain working on sum() method # Declare list of numbers numlist = [2,4,2,5,7,9,23,4,5] numsum = sum(numlist) print('Sum of List: ',numsum) # Example with start numsum = sum(numlist, 5) print('Sum of List: ',numsum)

Output:

Sum of List: 61 Sum of List: 66

Explanation

Here, you can see that the sum() method takes two parameters – numlist, the iterable and 5 as the start value. The final value is 61 (without the start value) and 66 (with the start value 5 added to it).

2) Using for Loop

# Python code to calculate sum of integer list # Using for loop # Declare list of numbers numlist = [2,4,2,5,7,9,23,4,5] # Calculate sum of list numsum=0 for i in numlist: numsum+=i print('Sum of List: ',numsum)

Output

Explanation

Here, a for loop is run over the list called numlist. With each iteration, the elements of the list are added. The result is 61 which is printed using the print statement.

3) Sum of List Containing String Value

# Python code to calculate sum of list containing integer as string # Using for loop # Declare list of numbers as string numlist = ['2','4','2','5','7','9','23','4','5'] # Calculate sum of list numsum=0 for i in numlist: numsum+=int(i) print('Sum of List: ',numsum)

Output

Here, the list called numlist contains integers as strings. Inside the for loop, these string elements are added together after converting them into integers, using the int() method.

4) Using While Loop

# Python code to calculate sum of list containing integer as string # Using While loop # Declare list of numbers as string numlist = [2,4,2,5,7,9,23,4,5] # Declare function to calculate sum of given list def listsum(numlist): total = 0 i = 0 while i < len(numlist): total = total + numlist[i] i = i + 1 return total # Call Function # Print sum of list totalsum = listsum(numlist); print('Sum of List: ', totalsum)

Explanation

In this program, elements of the numlist array are added using a while loop. The loop runs until the variable i is less than the length of the numlist array. The final summation is printed using the value assigned in the totalsum variable.

Conclusion

Using a for loop or while loop is great for summing elements of a list. But the sum() method is faster when you are handling huge lists of elements.

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How to compute the sum of a list in python

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When given a list of integers, how do you find the sum of all the elements in the list?

Algorithms

Let’s have a look at a few of the algorithms used to compute the sum of a list in Python.

1. Using a simple loop

The most basic solution is to traverse the list using a for/while loop, adding each value to the variable total . This variable ​will hold the sum of the list at the end of the loop. See the code below:

def sum_of_list(l):
 
total = 0
 
for val in l:
 
total = total + val
 
return total
 
my_list = [1,3,5,2,4]
print "The sum of my_list is", sum_of_list(my_list)

2. Computing the sum recursively

In this approach, instead of using loops, we will calculate the sum recursively. Once the end of the list is reached, the function will start to roll back. SumOfList takes two arguments as parameters: the list and the index of the list ( n ). Initially, n is set at the maximum possible index in the list and decremented at each recursive call. See the code below:

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How to Sum Elements of Two Lists in Python: Comprehensions and More

Welcome back to another edition of the How to Python series. This time I want to sum elements of two lists in Python. I got the inspiration for this topic while trying to do just this at work the other day. In short, one of the best ways to sum elements of two lists in Python is to use a list comprehension in conjunction with the addition operator. For example, we could perform an element-wise sum of two lists as follows: `[x + y for x, y in zip(list_a, list_b)]`python. But, as always, we’ll take a look at other options.

Table of Contents

Video Summary

Are there any better ways to get this working?

A Little Recap

ethernet_devices = [1, [7], [2], [8374163], [84302738]] usb_devices = [1, [7], [1], [2314567], [0]] # The long way all_devices = [ ethernet_devices[0] + usb_devices[0], ethernet_devices[1] + usb_devices[1], ethernet_devices[2] + usb_devices[2], ethernet_devices[3] + usb_devices[3], ethernet_devices[4] + usb_devices[4] ] # Some comprehension magic all_devices = [x + y for x, y in zip(ethernet_devices, usb_devices)] # Let's use maps import operator all_devices = list(map(operator.add, ethernet_devices, usb_devices)) # We can't forget our favorite computation library import numpy as np all_devices = np.add(ethernet_devices, usb_devices)

As we can see, there are a lot of ways to run an element-wise sum of two lists. Take your pick. As always, thanks for stopping by! If you liked this article, I have a massive list of code snippets just like this for your perusal. If you’re interested in learning more about Python, consider subscribing to The Renegade Coder—or at least hop on our mailing list, so you’ll never miss another article. Tune in next time to learn how to check if a file exists in Python. While you’re here, you might be interested in some these other Python articles:

• How to Automate Your GitHub Wiki
• How I Automated My Grading Responsibilities
• Reverse a String in Python

Once again, thanks for stopping by. I appreciate it!

How to Python (42 Articles)—Series Navigation

The How to Python tutorial series strays from the usual in-depth coding articles by exploring byte-sized problems in Python. In this series, students will dive into unique topics such as How to Invert a Dictionary, How to Sum Elements of Two Lists, and How to Check if a File Exists.

Each problem is explored from the naive approach to the ideal solution. Occasionally, there’ll be some just-for-fun solutions too. At the end of every article, you’ll find a recap full of code snippets for your own use. Don’t be afraid to take what you need!

If you’re not sure where to start, I recommend checking out our list of Python Code Snippets for Everyday Problems. In addition, you can find some of the snippets in a Jupyter notebook format on GitHub,

If you have a problem of your own, feel free to ask. Someone else probably has the same problem. Enjoy How to Python!

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