MATLAB file I/O (loadmat, savemat) in SciPy - Mini Project: Build & Apply

MATLAB File I/O with loadmat and savemat

📖 Scenario: You have some data from a small experiment saved in Python. You want to save this data to a MATLAB file so your friend who uses MATLAB can open it. Later, you want to load the data back from the MATLAB file to check it.

🎯 Goal: Learn how to save data to a MATLAB .mat file using savemat and load data from a MATLAB .mat file using loadmat in Python.

📋 What You'll Learn

Create a Python dictionary with experiment data

Set a filename string for the MATLAB file

Save the dictionary to a MATLAB file using savemat

Load the data back from the MATLAB file using loadmat

Print the loaded data to verify it matches the original

💡 Why This Matters

🌍 Real World

Scientists and engineers often share data between Python and MATLAB. Saving data in MATLAB format allows easy collaboration.

💼 Career

Data scientists and engineers working in mixed software environments need to exchange data files. Knowing how to read and write MATLAB files in Python is a useful skill.

1

Create the experiment data dictionary

Create a dictionary called experiment_data with these exact entries: 'temperature': [22, 23, 21, 20], 'humidity': [30, 35, 33, 31], and 'pressure': [1012, 1013, 1011, 1010].

SciPy

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

Hint

Use curly braces {} to create a dictionary. Each key is a string and each value is a list of numbers.

2

Set the MATLAB filename

Create a string variable called mat_file and set it to 'experiment.mat'.

SciPy

experiment_data = {
    'temperature': [22, 23, 21, 20],
    'humidity': [30, 35, 33, 31],
    'pressure': [1012, 1013, 1011, 1010]
}
# Create the mat_file string with the filename 'experiment.mat'
# Your code here

Hint

Use quotes to create a string and assign it to mat_file.

3

Save the data to the MATLAB file

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

SciPy

experiment_data = {
    'temperature': [22, 23, 21, 20],
    'humidity': [30, 35, 33, 31],
    'pressure': [1012, 1013, 1011, 1010]
}
mat_file = 'experiment.mat'
# Import savemat from scipy.io
# Use savemat to save experiment_data to mat_file
# Your code here

Hint

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

4

Load and print the data from the MATLAB file

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

SciPy

from scipy.io import savemat

experiment_data = {
    'temperature': [22, 23, 21, 20],
    'humidity': [30, 35, 33, 31],
    'pressure': [1012, 1013, 1011, 1010]
}
mat_file = 'experiment.mat'

savemat(mat_file, experiment_data)
# Import loadmat from scipy.io
# Load data from mat_file into loaded_data
# Print loaded_data
# Your code here

Hint

Use from scipy.io import loadmat. Then call loadmat(filename) and print the result.

Practice

1.

What does the scipy.io.loadmat function do?

easy

A. Loads data from a MATLAB .mat file into a Python dictionary

B. Saves a Python dictionary as a MATLAB .mat file

C. Converts Python lists to MATLAB arrays

D. Executes MATLAB code from Python

Solution

1. Step 1: Understand the function purpose

   loadmat is designed to read MATLAB .mat files.

2. Step 2: Identify the output type

   It returns the data as a Python dictionary with variable names as keys.

3. Final Answer:

   Loads data from a MATLAB .mat file into a Python dictionary -> Option A

4. Quick Check:

   loadmat reads .mat files into dict [OK]

Hint: Remember: loadmat reads, savemat writes [OK]

Common Mistakes:

• Confusing loadmat with savemat
• Thinking loadmat executes MATLAB code
• Assuming loadmat converts data formats automatically

2.

Which of the following is the correct way to save a Python dictionary data to a MATLAB file named output.mat using savemat?

?

easy

A. savemat('output.mat', data)

B. savemat(data, 'output.mat')

C. savemat({'output.mat': data})

D. savemat('output.mat', {'data': data})

Solution

1. Step 1: Check savemat function signature

   savemat(filename, dict) requires a filename string and a dictionary of variables.

2. Step 2: Wrap data in a dictionary with a variable name

   To save data, it must be inside another dictionary like {'data': data}.

3. Final Answer:

   savemat('output.mat', {'data': data}) -> Option D

4. Quick Check:

   savemat needs filename and dict [OK]

Hint: savemat needs dict with variable names as keys [OK]

Common Mistakes:

• Passing data directly without wrapping in dict
• Swapping filename and data arguments
• Using incorrect argument types

3.

What will be the output of the following code?

from scipy.io import loadmat
mat_data = loadmat('sample.mat')
print(type(mat_data))

Assume sample.mat is a valid MATLAB file.

medium

A. <class 'numpy.ndarray'>

B. <class 'dict'>

C. <class 'list'>

D. FileNotFoundError

Solution

1. Step 1: Understand loadmat output

   loadmat returns a Python dictionary containing MATLAB variables.

2. Step 2: Check the printed type

   Printing type of mat_data shows <class 'dict'>.

3. Final Answer:

   <class 'dict'> -> Option B

4. Quick Check:

   loadmat returns dict [OK]

Hint: loadmat output is always a dict [OK]

Common Mistakes:

• Expecting a list or array directly
• Assuming loadmat raises error if file exists
• Confusing output type with variable inside dict

4.

Identify the error in this code snippet:

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

medium

A. Dictionary keys must be strings

B. List values cannot be saved in .mat files

C. Arguments to savemat are in wrong order

D. Missing import statement

Solution

1. Step 1: Check savemat argument order

   savemat expects filename first, then dictionary.

2. Step 2: Identify argument swap

   Code passes dictionary first, filename second, which is incorrect.

3. Final Answer:

   Arguments to savemat are in wrong order -> Option C

4. Quick Check:

   savemat(filename, dict) order matters [OK]

Hint: Filename is first argument in savemat [OK]

Common Mistakes:

• Swapping filename and data arguments
• Assuming lists can't be saved
• Forgetting to import savemat

5.

You want to save two variables, a = [1, 2, 3] and b = [[4, 5], [6, 7]], into a MATLAB file vars.mat. Which code correctly saves both variables so MATLAB can load them as a and b?

hard

A. savemat('vars.mat', {'a': a, 'b': b})

B. savemat('vars.mat', {'vars': [a, b]})

C. savemat('vars.mat', {'a': [a], 'b': [b]})

D. savemat('vars.mat', {'a': a}, {'b': b})

Solution

1. Step 1: Understand savemat input format

   savemat requires a dictionary mapping variable names to values.

2. Step 2: Check how to save multiple variables

   Pass a single dictionary with keys 'a' and 'b' mapped to their values.

3. Step 3: Identify correct option

   savemat('vars.mat', {'a': a, 'b': b}) correctly passes {'a': a, 'b': b} as one dictionary.

4. Final Answer:

   savemat('vars.mat', {'a': a, 'b': b}) -> Option A

5. Quick Check:

   Multiple variables saved in one dict [OK]

Hint: Use one dict with all variables as keys [OK]

Common Mistakes:

• Passing multiple dicts instead of one
• Wrapping variables in extra lists unnecessarily
• Using a single key for multiple variables