Saving and loading data (scipy.io)

Introduction

We save data to keep it safe and use it later. Loading data helps us continue work without starting over.

You want to save results of a calculation to use later.

You need to share data with a teammate in a file.

You want to load data from a previous session to analyze again.

You want to save arrays or matrices in a file format that Python can read easily.

You want to store data in a format compatible with MATLAB.

Syntax

SciPy

from scipy import io

# Save data to a .mat file
io.savemat('filename.mat', {'var_name': data})

# Load data from a .mat file
data = io.loadmat('filename.mat')

The data is saved in MATLAB .mat file format.

Data is stored as a dictionary with variable names as keys.

Examples

This saves a numpy array named 'array' into 'mydata.mat'.

SciPy

from scipy import io
import numpy as np

arr = np.array([1, 2, 3])
io.savemat('mydata.mat', {'array': arr})

This loads the file and prints the saved array.

SciPy

from scipy import io

loaded = io.loadmat('mydata.mat')
print(loaded['array'])

Sample Program

This program saves a 2x2 matrix to a file and then loads it back to print it.

SciPy

from scipy import io
import numpy as np

# Create some data
matrix = np.array([[10, 20], [30, 40]])

# Save the matrix to a file
io.savemat('matrix_data.mat', {'matrix': matrix})

# Load the data back
loaded_data = io.loadmat('matrix_data.mat')

# Print the loaded matrix
print(loaded_data['matrix'])

Important Notes

When loading, the result is a dictionary with extra metadata keys like '__header__'.

Use the variable name keys to access your saved data.

Files saved with savemat can be opened in MATLAB software.

Summary

Use io.savemat to save data as .mat files.

Use io.loadmat to load data back into Python.

Data is stored and accessed as dictionary entries by variable names.

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