Saving and loading data (scipy.io) - Mini Project: Build & Apply

Saving and loading data (scipy.io)

📖 Scenario: You have collected some simple data from a small experiment. You want to save this data to a file so you can use it later without typing it again. Then, you want to load the saved data back into your program to check it.

🎯 Goal: Learn how to save a dictionary of data to a file using scipy.io.savemat and then load it back using scipy.io.loadmat.

📋 What You'll Learn

Create a dictionary with specific data arrays

Save the dictionary to a .mat file using scipy.io.savemat

Load the data back from the .mat file using scipy.io.loadmat

Print the loaded data to verify it matches the original

💡 Why This Matters

🌍 Real World

Scientists and engineers often save experimental data in .mat files to share and analyze later without re-running experiments.

💼 Career

Knowing how to save and load data files is essential for data scientists and researchers to manage data efficiently and reproduce results.

1

Create a dictionary with experiment data

Create a dictionary called experiment_data with two keys: 'temperature' and 'pressure'. Set 'temperature' to the list [22.1, 23.4, 21.8] and 'pressure' to the list [101.2, 100.8, 101.5].

SciPy

# Create the dictionary experiment_data with temperature and pressure lists
# Your code here

Hint

Use curly braces {} to create a dictionary. The keys are strings and the values are lists of numbers.

2

Set the filename to save the data

Create a variable called filename and set it to the string 'experiment.mat' which will be the name of the file to save the data.

SciPy

experiment_data = {'temperature': [22.1, 23.4, 21.8], 'pressure': [101.2, 100.8, 101.5]}
# Create the filename variable with 'experiment.mat'
# Your code here

Hint

Use quotes to create a string for the filename.

3

Save the dictionary to a .mat file

Import savemat from scipy.io. Then use savemat to save experiment_data to the file named filename.

SciPy

experiment_data = {'temperature': [22.1, 23.4, 21.8], 'pressure': [101.2, 100.8, 101.5]}
filename = 'experiment.mat'
# Import savemat from scipy.io
# Use savemat to save experiment_data to filename
# Your code here

Hint

Use from scipy.io import savemat to import. Then call savemat(filename, experiment_data).

4

Load the data back and print it

Import loadmat from scipy.io. Load the data from filename into a variable called loaded_data. Then print loaded_data.

SciPy

from scipy.io import savemat

experiment_data = {'temperature': [22.1, 23.4, 21.8], 'pressure': [101.2, 100.8, 101.5]}
filename = 'experiment.mat'
savemat(filename, experiment_data)
# Import loadmat from scipy.io
# Load the data from filename into loaded_data
# Print loaded_data
# Your code here

Hint

Use loaded_data = loadmat(filename) and then print(loaded_data). The output will include extra keys like '__header__'.

Practice

1. What is the primary purpose of scipy.io.savemat in data science?

easy

A. To load data from a CSV file

B. To save Python variables into a MATLAB .mat file

C. To visualize data in plots

D. To convert Python code to MATLAB code

Solution

1. Step 1: Understand the function purpose

   scipy.io.savemat is designed to save Python data into MATLAB's .mat file format.

2. Step 2: Compare options with function use

   Only To save Python variables into a MATLAB .mat file correctly describes saving Python variables into a .mat file. Other options describe unrelated tasks.

3. Final Answer:

   To save Python variables into a MATLAB .mat file -> Option B

4. Quick Check:

   savemat saves data = A [OK]

Hint: Remember: savemat saves, loadmat loads [OK]

Common Mistakes:

• Confusing savemat with loadmat
• Thinking savemat loads data
• Assuming it works with CSV files

2. Which of the following is the correct way to load a .mat file named data.mat using scipy?

easy

A. data = io.loadmat('data.mat')

B. data = io.savemat('data.mat')

C. data = io.load('data.mat')

D. data = io.readmat('data.mat')

Solution

1. Step 1: Identify the correct function for loading .mat files

   The function to load MATLAB files in scipy is loadmat.

2. Step 2: Check syntax correctness

   data = io.loadmat('data.mat') uses io.loadmat('data.mat'), which is the correct syntax. Other options use incorrect function names.

3. Final Answer:

   data = io.loadmat('data.mat') -> Option A

4. Quick Check:

   loadmat loads .mat files = A [OK]

Hint: Load with loadmat, save with savemat [OK]

Common Mistakes:

• Using savemat to load data
• Using non-existent functions like readmat
• Missing parentheses or quotes

3. What will be the output of this code snippet?

import numpy as np
from scipy import io
arr = np.array([1, 2, 3])
io.savemat('test.mat', {'array': arr})
data = io.loadmat('test.mat')
print(data['array'])

medium

A. [[1 2 3]]

B. Error: KeyError

C. [[1]
[2]
[3]]

D. [1 2 3]

Solution

1. Step 1: Understand how savemat stores arrays

   When saving a 1D numpy array, savemat stores it as a 2D array with shape (1, n) by default.

2. Step 2: Check the loaded data shape

   Loading back with loadmat returns a 2D array with shape (1, 3), so printing shows [[1 2 3]].

3. Final Answer:

   [[1 2 3]] -> Option A

4. Quick Check:

   1D array saved as 2D row = [[1 2 3]] [OK]

Hint: Loaded arrays from .mat are often 2D, not 1D [OK]

Common Mistakes:

• Expecting 1D array output
• Confusing row vs column shape
• Assuming KeyError due to wrong key

4. Identify the error in this code snippet:

from scipy import io
my_data = {'x': [1, 2, 3]}
io.savemat('file.mat', my_data)
loaded = io.loadmat('file.mat')
print(loaded['my_data'])

medium

A. FileNotFoundError when loading file.mat

B. SyntaxError in savemat call

C. KeyError because 'my_data' is not a key in loaded dict

D. TypeError because list cannot be saved

Solution

1. Step 1: Check keys saved in .mat file

   The dictionary key 'x' is saved, not the variable name 'my_data'. So loaded dict has key 'x', not 'my_data'.

2. Step 2: Understand the cause of KeyError

   Trying to access loaded['my_data'] causes KeyError because that key does not exist.

3. Final Answer:

   KeyError because 'my_data' is not a key in loaded dict -> Option C

4. Quick Check:

   Access saved keys, not variable names [OK]

Hint: Access keys used in savemat dict, not variable names [OK]

Common Mistakes:

• Assuming variable name is key in loaded dict
• Confusing syntax errors with runtime errors
• Expecting automatic key renaming

5. You want to save multiple numpy arrays a and b into a single .mat file and later load them back. Which code correctly saves and loads these arrays so you can access them by their variable names?

hard

A. io.savemat('multi.mat', [a, b])
data = io.loadmat('multi.mat')
print(data['a'], data['b'])

B. io.savemat('multi.mat', a, b)
data = io.loadmat('multi.mat')
print(data['a'], data['b'])

C. io.savemat('multi.mat', {'a': a, 'b': b})
data = io.loadmat('multi.mat')
print(data['multi']['a'], data['multi']['b'])

D. io.savemat('multi.mat', {'a': a, 'b': b})
data = io.loadmat('multi.mat')
print(data['a'], data['b'])

Solution

1. Step 1: Save multiple arrays with a dictionary

   Use a dictionary with keys as variable names and values as arrays in savemat. io.savemat('multi.mat', {'a': a, 'b': b}) data = io.loadmat('multi.mat') print(data['a'], data['b']) does this correctly.

2. Step 2: Load and access arrays by keys

   After loading, access arrays by their keys 'a' and 'b' in the loaded dictionary. io.savemat('multi.mat', {'a': a, 'b': b}) data = io.loadmat('multi.mat') print(data['a'], data['b']) prints them correctly.

3. Final Answer:

   io.savemat('multi.mat', {'a': a, 'b': b}) data = io.loadmat('multi.mat') print(data['a'], data['b']) -> Option D

4. Quick Check:

   Save/load dict with keys = variable access [OK]

Hint: Save dict with names, load dict and access keys [OK]

Common Mistakes:

• Passing list instead of dict to savemat
• Trying to access nested keys incorrectly
• Passing multiple args to savemat instead of one dict