Saving and loading data (scipy.io) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the scipy.io module?

The scipy.io module helps save and load data files in different formats, making it easy to share and reuse data in Python programs.

beginner

Which function in scipy.io is used to save data in MATLAB .mat file format?

The function scipy.io.savemat() saves Python data structures like dictionaries into MATLAB .mat files.

beginner

How do you load data from a MATLAB .mat file using scipy.io?

Use scipy.io.loadmat() to read data from a .mat file. It returns a dictionary with variable names as keys and data as values.

intermediate

What data types can scipy.io.savemat() handle when saving to a .mat file?

It can save dictionaries containing arrays, lists, numbers, and strings. The keys become variable names in the MATLAB file.

intermediate

Why might you want to use scipy.io to save and load data instead of plain text files?

Because scipy.io supports complex data formats like MATLAB files, it preserves data types and structure better than plain text, making data exchange easier and more reliable.

Which function saves data to a MATLAB .mat file?

A save()

B loadmat()

C savemat()

D load()

savemat() saves data to .mat files, while loadmat() loads data from them.

What does loadmat() return when loading a .mat file?

A A dictionary with variable names as keys

B A list of arrays

C A single array

D A string with file content

loadmat() returns a dictionary where keys are variable names and values are the data.

Which data structure is commonly used with savemat() to organize data?

A List

B Tuple

C Set

D Dictionary

savemat() expects a dictionary where keys become variable names in the saved file.

If you want to save a NumPy array to a .mat file, what should you do?

A Put the array inside a dictionary and pass it to <code>savemat()</code>

B Pass the array directly to <code>savemat()</code>

C Convert the array to a list first

D Use <code>loadmat()</code>

You should put the array inside a dictionary with a variable name as key before passing to savemat().

Which of these is NOT a benefit of using scipy.io for saving/loading data?

A Preserves data types and structure

B Automatically visualizes data

C Eases data exchange between Python and MATLAB

D Supports MATLAB file format

scipy.io does not visualize data; it only saves and loads data files.

Explain how to save a NumPy array to a MATLAB .mat file using scipy.io. Include the key steps and functions.

Think about how MATLAB variables are represented and how Python dictionaries map to them.

Describe how loadmat() helps when working with MATLAB data files in Python.

Consider how you access MATLAB variables after loading.

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