Saving and loading data (scipy.io) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save a NumPy array to a .mat file using scipy.io.

SciPy

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

Drag options to blanks, or click blank then click option'

Asavemat

Bload

Csave

Dloadmat

Attempts:

3 left

💡 Hint

Common Mistakes

Using loadmat instead of savemat to save data.

Using save or load which are not scipy.io functions.

The savemat function saves data to a MATLAB .mat file.

2fill in blank

medium

Complete the code to load data from a .mat file using scipy.io.

SciPy

from scipy import io
data = io.[1]('data.mat')
print(data['arr'])

Drag options to blanks, or click blank then click option'

Aload

Bsavemat

Cloadmat

Dsave

Attempts:

3 left

💡 Hint

Common Mistakes

Using savemat instead of loadmat to load data.

Using save or load which are not scipy.io functions.

The loadmat function loads data from a MATLAB .mat file.

3fill in blank

hard

Fix the error in the code to correctly save two arrays to a .mat file.

SciPy

import numpy as np
from scipy import io
x = np.array([1, 2])
y = np.array([3, 4])
io.savemat('data.mat', [1])

Drag options to blanks, or click blank then click option'

A{'x': x, 'y': y}

B[x, y]

Cx, y

D('x', x, 'y', y)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a list or tuple instead of a dictionary.

Passing variables without keys.

The data must be passed as a dictionary mapping variable names to arrays.

4fill in blank

hard

Fill both blanks to save a dictionary with arrays and then load it back.

SciPy

from scipy import io
import numpy as np
data = {'a': np.array([1, 2]), 'b': np.array([3, 4])}
io.[1]('file.mat', data)
loaded = io.[2]('file.mat')
print(loaded['a'])

Drag options to blanks, or click blank then click option'

Asavemat

Bloadmat

Csave

Dload

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up savemat and loadmat.

Using save or load which are not scipy.io functions.

Use savemat to save and loadmat to load data.

5fill in blank

hard

Fill all three blanks to save an array with a custom name, load it, and print the loaded array.

SciPy

import numpy as np
from scipy import io
arr = np.array([10, 20, 30])
io.[1]('mydata.mat', [2])
loaded = io.[3]('mydata.mat')
print(loaded['my_array'])

Drag options to blanks, or click blank then click option'

Asavemat

B{'my_array': arr}

Cloadmat

D{'array': arr}

Attempts:

3 left

💡 Hint

Common Mistakes

Not using a dictionary with the correct key name.

Using wrong function names for saving or loading.

Use savemat to save with a dictionary naming the array, then loadmat to load it back.

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