[Tut] Python One Line Sum List

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Python One Line Sum List

Article summary: Here’s a quick visual overview of the contents of this tutorial.

• Flat List: To sum over a list of numbers in a single line of Python code, use Python’s built-in function
  Code:

  sum(list)
  .
  
  
• Nested List: To sum over a list of lists in one line Python, use a generator expression to flatten the list and pass the result into the function:
  Code:

  sum(x for y in list for x in y)
  .
  

Method 1: Sum over a Flat List in One Line

Problem: How to sum over all values in a given Python list?

Example: Given the following list.

Code:

a = [1, 2, 3]

You want to calculate the sum of all values in the list—using only a single line of Python code!

Code:

# RESULT: 6

Solution: Python’s built-in

Code:

sum()

function helps you to sum over all values in an iterable, such as a Python list.

Summing up a list of numbers appears everywhere in coding. Fortunately, Python provides the built-in

Code:

sum()

function to sum over all elements in a Python list—or any other iterable for that matter. (Official Docs)

Code: Here’s the minimal code example.

Code:

a = [1, 2, 3] print(sum(a)) # 6

How does it work? The syntax is

Code:

sum(iterable, start=0)

:

Argument |
Description

Code:

iterable

|
Sum over all elements in the

Code:

iterable

. This can be a list, a tuple, a set, or any other data structure that allows you to iterate over the elements.
Example:

Code:

sum([1, 2, 3])

returns

Code:

1+2+3=6

.

Code:

start

|
(Optional.) The default start value is 0. If you define another start value, the sum of all values in the

Code:

iterable

will be added to this start value.
Example:

Code:

sum([1, 2, 3], 9)

returns

Code:

9+1+2+3=15

.



Exercise: Try to modify the sequence so that the sum is 30 in our interactive Python shell:

https://repl.it/@finxter/sumpythonlist?lite=true

Method 2: Sum over a Nested List of Lists in One Line

Problem: Given multiple lists in a list of lists. How can you sum over all values in a list of lists such as

Code:

[[1, 2], [3, 4], [5, 6]]

in Python?

Solution: Use a generator expression to flatten the values in the nested list and pass the resulting iterable into the

Code:

sum()

function.

Code: The following code creates a list of lists:

Code:

a = [[1, 2], [3, 4], [5, 6]]

To sum over the values in the list of lists, use the following one-liner:

Code:

print(sum(x for y in a for x in y))

The output is printed on the shell:

Code:

# OUTPUT: 21

But how does it work? The main part of the code is the generator expression

Code:

x for y in a for x in y

that flattens the list.

• The part
  Code:

  x for y in a for x in y
  iterates over all elements
  Code:

  y
  in the nested list
  Code:

  a
  .
  
  
• The part
  Code:

  x for y in a for x in y
  iterates over all elements
  Code:

  y
  in the inner list
  Code:

  y
  .
  
  
• The part
  Code:

  x for y in a for x in y
  stores the inner element in the iterable.
  

Here’s a recap on the technique of list comprehension.



To learn more about different ways to sum() elements in a list, check out my detailed blog tutorial:

Related Tutorial: Python sum() List — Ultimate Guide

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