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Summing a 2D Array in Python

For many languages such as Java and C++, arrays and lists are different objects. C++ doesn’t even technically have “lists” and Java has a hybrid object called an ArrayList. While there are arrays in Python, such as numpy arrays, Python’s most common sequence or series collection is a list object. 

Python list objects may contain entries of any type from numbers to strings to dictionaries, and may even contain multiple types. I’ll be using “array” and “list” interchangeably in this post because they are used in almost the exact same way in Python. In this post we’re going to cover how to sum a 2D list (or array) of numbers in Python. We’ll cover the following sections:

  • What is a 2D Array or List in Python
  • How Do You Sum an Array?
  • How Do You Sum a 2D List?
  • Summary of Summing a 2D Array in Python

What is a 2D Array or List in Python?

In many other languages, such as Java, you’d need to declare a 2D array type before assigning it. In Python, we can just make a list and make each entry in the list another list. A 2D array represents a two-dimensional space. These are sometimes referred to as “nested” lists. In our example, we’ll create a list of three lists, each consisting of two numbers. The below code shows how we can instantiate a simple 2D list in Python.

example_2d = [[1, 2], [3, 4], [5, 6]]

How Do You Sum an Array in Python?

There are multiple ways to sum a list in Python. We can do it iteratively, we can break the list down, or we can use the built-in sum() method. In this post, we’re going to sum a list the simplest way, by using the sum() method. In the example below we create a one-dimensional list of three numbers and then sum them.

example_1d =[1, 2, 3]

This should output 6.

How Do You Sum a 2D List in Python?

Summing a 2D list or array object in Python is much more difficult. You can’t call the sum() function on a 2D array. If you try, you will get a TypeError of  unsupported operand type(s) for +: 'int' and 'list'. So, how can we sum a 2D array in Python? I’ll go over three examples here, first an iterative example, and then two Pythonic one liners.

Sum the 2D List with a For Loop

Summing a 2D array with a for loop is the most straightforward approach, especially for people new to Python. This is also the most intuitive approach for people coming from other programming languages. We start off with a sum of 0, notice that I named this variable _sum because the word sum is the name of the function. Then, we loop through each entry in the 2D list and add the sum() of that entry to our _sum variable. Finally, we print out our _sum variable to make sure that we got it.

for x in example_2d:
    _sum += sum(x)

This should print 21.

Pythonically Sum The 2D Array in One Line

Now let’s take a look at the first “Pythonic” way of summing a 2D array. We can do this in one line. The most “inner” function of our line will use list comprehension and create a list from the sums of each entry in the 2D list. Then, we’ll call sum() on that and print it out. 

The inner list should be [3, 7, 11]. The printed answer should once again be 21.

print(sum([sum(x) for x in example_2d]))

Pythonically Sum the 2D List in One Line (More Obscure)

Finally, let’s take a look at another Pythonic way to sum a 2D array. We can take advantage of the sum() function’s optional second parameter. Usually, sum() expects to be summing numbers, so if you just call sum() on a 2D list, you’ll get a TypeError as we saw earlier. However, the optional second start parameter allows us to bypass this. If we pass an empty list object, it will expect the individual entries of the 2D array to be list objects like they are. This allows us to then call sum() again on the list and sum a 1D array.

The output of sum(example_2d, []) is [1, 2, 3, 4, 5, 6] just as if we added the lists together. The print statement prints 21.

print(sum(sum(example_2d, [])))

Summary of How to Sum a 2D List or Array

In this post we went over what a 2D list is and how you can sum arrays in Python. We also went over three separate methods to sum 2D lists. First, we went over an iterative method using for loops. Then, we went over a classic Pythonic method. Finally, we went over a more obscure approach using the optional start parameter of the sum() function.

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