Saving and loading data (scipy.io) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of loading a saved .mat file?

Consider the following Python code that saves a dictionary to a .mat file and then loads it back. What will be the output of the print statement?

SciPy

import numpy as np
from scipy.io import savemat, loadmat

my_dict = {'array': np.array([1, 2, 3])}
savemat('test.mat', my_dict)
loaded = loadmat('test.mat')
print(loaded['array'])

A [1 2 3]

B

[[1]
 [2]
 [3]]

C [[1 2 3]]

D KeyError

💡 Hint

Remember how numpy arrays are stored and loaded in MATLAB format. The shape might change.

When saving a 1D numpy array with savemat, it is stored as a 2D array with shape (n, 1). Loading it back returns a 2D array. Here, it is loaded as a column vector with shape (3,1), so printing shows [[1] [2] [3]].

❓ data_output

intermediate

How many keys are in the loaded .mat file dictionary?

After running the following code, how many keys does the loaded dictionary contain?

SciPy

from scipy.io import savemat, loadmat

data = {'x': [1, 2], 'y': [3, 4]}
savemat('data.mat', data)
loaded_data = loadmat('data.mat')
print(len(loaded_data.keys()))

A 2

B 3

C 5

D 4

💡 Hint

loadmat adds some default keys starting with '__'.

loadmat returns a dictionary with the saved keys plus some metadata keys like '__header__', '__version__', and '__globals__'. So total keys are original 2 plus 3 metadata keys = 5 keys.

🔧 Debug

advanced

Why does this code raise a FileNotFoundError?

Examine the code below. Why does it raise a FileNotFoundError?

SciPy

from scipy.io import loadmat

loaded = loadmat('nonexistent_file.mat')

A The loadmat function is deprecated and cannot open files.

B loadmat requires the file to be in .npy format, not .mat.

C The file path must be absolute, relative paths cause errors.

D The file 'nonexistent_file.mat' does not exist in the current directory.

💡 Hint

Check if the file path is correct and the file exists.

FileNotFoundError occurs because the specified file does not exist in the directory where the code runs. loadmat expects the file to exist.

🚀 Application

advanced

Which option correctly saves multiple numpy arrays to a .mat file?

You have two numpy arrays, a and b. Which code snippet correctly saves both arrays into a single .mat file named 'arrays.mat'?

SciPy

import numpy as np
from scipy.io import savemat

a = np.array([1, 2, 3])
b = np.array([4, 5, 6])

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

B savemat('arrays.mat', [a, b])

C savemat('arrays.mat', a=a, b=b)

D savemat('arrays.mat', {'arrays': [a, b]})

💡 Hint

savemat expects a dictionary mapping variable names to arrays.

savemat requires a dictionary where keys are variable names and values are numpy arrays. Option A correctly passes a dictionary with keys 'a' and 'b'. Option A passes a list, which is invalid. Option A uses keyword arguments which savemat does not accept. Option A nests arrays inside a list under one key, which saves only one variable.

🧠 Conceptual

expert

What is the main limitation of using scipy.io.loadmat for loading MATLAB files?

Which of the following is a key limitation when using scipy.io.loadmat to load MATLAB .mat files?

A It cannot load MATLAB files saved in version 7.3 or later because they use HDF5 format.

B It only loads files smaller than 1MB due to memory constraints.

C It converts all numeric arrays to Python lists automatically.

D It requires MATLAB to be installed on the system to work.

💡 Hint

Check the file format versions supported by scipy.io.loadmat.

scipy.io.loadmat cannot read MATLAB .mat files saved in version 7.3 or later because those files use the HDF5 format, which loadmat does not support. For those files, h5py or MATLAB engine is needed.

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