How to Convert a Dictionary to a NumPy Array

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Problem Formulation and Solution Overview

In this article, you’ll learn how to convert a Dictionary into a NumPy array in Python.

Example:

Input Dictionary: 
{2017: 74, 
 2018: 80, 
 2019: 84, 
 2020: 73, 
 2021: 79}

Output NumPy Array:
[[2017 74]
 [2018 80]
 [2019 84]
 [2020 73]
 [2021 79]]

To make it more fun, we have the following running scenario:

The Museum of Natural History has been researching volcanic activity over a five (5) year period. They have the yearly total eruptions saved in a Dictionary format. However, they need you to convert this data into a NumPy array for analysis purposes.

💬 Question: How would we write Python code to convert a Dictionary into a NumPy array?

We can accomplish this task by one of the following options:


Preparation

Before any data manipulation can occur, two (2) new libraries will require installation.

  • The Pandas library enables access to/from a DataFrame.
  • The NumPy library supports multi-dimensional arrays and matrices in addition to a collection of mathematical functions.

To install these libraries, navigate to an IDE terminal. At the command prompt ($), execute the code below. For the terminal used in this example, the command prompt is a dollar sign ($). Your terminal prompt may be different.


$ pip install pandas

Hit the <Enter> key on the keyboard to start the installation process.

$ pip install numpy

Hit the <Enter> key on the keyboard to start the installation process.

If the installations were successful, a message displays in the terminal indicating the same.


Feel free to view the PyCharm installation guide for the required libraries.


Add the following code to the top of each code snippet. This snippet will allow the code in this article to run error-free.

import pandas as pd
import numpy as np 

Method 1: Use NumPy Array and List

This example uses np.array() and a list to convert the data from a Dictionary format into a NumPy array format. A great one-liner!

v_totals = {2017: 74, 2018: 80, 2019: 84, 2020: 73, 2021: 79}
v_info  = np.array(list(v_totals.items()))
print(v_info)

This code takes the contents of v_totals and converts this into a list, v_list. This list then converts into a NumPy array, and the results save to v_info. Finally, the contents are output to the terminal.

Output

[[2017 74]
[2018 80]
[2019 84]
[2020 73]
[2021 79]]

Method 2: Use np.fromiter()

In this example, the np.fromiter() function creates two (2) new arrays from the keys and values extracted from the Dictionary format.

v_totals = {2017: 74, 2018: 80, 2019: 84, 2020: 73, 2021: 79}
keys = np.fromiter(v_totals.keys(), dtype=np.int64)
vals = np.fromiter(v_totals.values(), dtype=np.int64)

print('Type Keys:', type(keys))
print('Type Vals:', type(vals))
print(keys, vals)

This code takes the contents of v_totals and extracts the keys into one np.array() and values into another np.array(). The results save to keys, and vals respectively.

The following two (2) lines confirm the data was successfully converted.

Type Keys: <class 'numpy.ndarray'>
Type Vals: <class 'numpy.ndarray'>

The output from the final line is sent to the terminal.

Output

[2017 2018 2019 2020 2021] [74 80 84 73 79]

Method 3: Use Pandas Series

According to documentation, the Pandas Series is built on top of NumPy. Therefore, applying the values method on a series will be a NumPy array.

v_totals = {2017: 74, 2018: 80, 2019: 84, 2020: 73, 2021: 79}
series = pd.Series(v_totals, index=[2017, 2018, 2019, 2020, 2021])
print(series)

This code uses Pandas to take the Dictionary v_totals, extract the values, and assign the keys as the index parameter. The result saves to series. The output is sent to the terminal.

Output

201774
201880
2019 84
202073
202179

Method 4: Convert Nested Dictionary to NumPy array

For this example, let’s say your boss would like additional details about Volcanic eruptions for June-July, 2020. A nested dictionary below has been created to accommodate.

v_dict = {2018: 80,
          2019: 84,
          2020: {'June 1':  'Cleveland',
                 'June 16': 'Copahue',
                 'June 18': 'Turrialba',
                 'July 16': 'Indonesia',
                 'July 20': 'Telica'}}
      
v_items = v_dict.items()
v_data  = list(v_items)
results = np.array(v_data)
print(results)

This code declares a nested dictionary. The items (key:value pairs) are then extracted and saved to v_items.

This converts to a list and saves to v_data. Next, v_data is converted to a NumPy array and saved to results.

Finally, the output from results is sent to the terminal.

Output

[[2018 80]
[2019 84]
[2020
{'June 1': 'Cleveland', 'June 16': 'Copahue', 'June 18': 'Turrialba', 'July 16': 'Indonesia', 'July 20': 'Telica'}]]

Summary

These four (4) methods to convert a Dictionary to a NumPy array should give you enough information to select the best one for your coding requirements.

Good Luck & Happy Coding!