Converting to and from Python lists in NumPy - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert a NumPy array to a Python list.

NumPy

import numpy as np
arr = np.array([1, 2, 3])
py_list = arr.[1]()

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

Atolist()

Btolist

Cto_list

Dto_list()

Attempts:

3 left

💡 Hint

Common Mistakes

Using to_list() which does not exist.

Forgetting the parentheses after the method name.

The tolist() method converts a NumPy array to a Python list.

2fill in blank

medium

Complete the code to convert a Python list to a NumPy array.

NumPy

import numpy as np
py_list = [4, 5, 6]
arr = np.[1](py_list)

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

Afromlist

Basarray

Carray

Dtolist

Attempts:

3 left

💡 Hint

Common Mistakes

Using tolist() which converts arrays to lists.

Using fromlist which is not a NumPy function.

The np.array() function converts a Python list to a NumPy array.

3fill in blank

hard

Fix the error in the code to convert a nested Python list to a NumPy array.

NumPy

import numpy as np
nested_list = [[1, 2], [3, 4]]
arr = np.[1](nested_list)

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

Atolist

Barray

Casarray

Dfromlist

Attempts:

3 left

💡 Hint

Common Mistakes

Using tolist() which converts arrays to lists.

Using fromlist which is not a valid NumPy function.

Use np.array() to convert nested lists to multidimensional NumPy arrays.

4fill in blank

hard

Fill both blanks to create a list of lengths of strings from a list using list comprehension.

NumPy

words = ['apple', 'banana', 'cherry']
lengths = [[1] for [2] in words]

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

Alen(word)

Bword

Cw

Dlen(w)

Attempts:

3 left

💡 Hint

Common Mistakes

Using word as variable but len(word) as expression without matching names.

Using len(word) but loop variable is w (mismatch).

The list comprehension uses len(w) to get length and w as the loop variable.

5fill in blank

hard

Fill all three blanks to create a dictionary of word lengths for words longer than 5 characters.

NumPy

words = ['apple', 'banana', 'cherry', 'date']
length_dict = { [1]: [2] for [3] in words if len([3]) > 5 }

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

Aword

Blen(word)

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Not matching the loop variable with the condition.

The dictionary comprehension uses word as key, len(word) as value, and loops with word.

Practice

1. What does the function np.array() do when given a Python list?

easy

A. It deletes the Python list.

B. It converts the Python list into a Python dictionary.

C. It converts the Python list into a NumPy array.

D. It converts the Python list into a string.

Solution

1. Step 1: Understand np.array() purpose

   The function np.array() is designed to take a Python list and create a NumPy array from it.

2. Step 2: Compare options with function behavior

   Only It converts the Python list into a NumPy array. correctly states that it converts a list to a NumPy array. Other options describe unrelated actions.

3. Final Answer:

   It converts the Python list into a NumPy array. -> Option C

4. Quick Check:

   np.array(list) = NumPy array [OK]

Hint: np.array() always makes a NumPy array from a list [OK]

Common Mistakes:

• Thinking np.array() converts to dict or string
• Confusing np.array() with list methods
• Assuming np.array() deletes data

2. Which of the following is the correct syntax to convert a NumPy array arr back to a Python list?

easy

A. arr.tolist()

B. arr.to_list()

C. tolist(arr)

D. list(arr.to)

Solution

1. Step 1: Recall the method to convert NumPy array to list

   The correct method is tolist() called on the array object.

2. Step 2: Check each option's syntax

   arr.tolist() uses arr.tolist(), which is correct. arr.to_list() has a wrong method name. Options C and D are invalid syntax.

3. Final Answer:

   arr.tolist() -> Option A

4. Quick Check:

   array.tolist() converts to list [OK]

Hint: Use arr.tolist() to get a Python list from NumPy array [OK]

Common Mistakes:

• Using arr.to_list() instead of arr.tolist()
• Trying to call tolist() as a standalone function
• Incorrect method chaining or syntax errors

3. What is the output of this code?

import numpy as np
lst = [1, 2, 3]
arr = np.array(lst)
print(arr.tolist())

medium

A. [[1], [2], [3]]

B. array([1, 2, 3])

C. TypeError

D. [1, 2, 3]

Solution

1. Step 1: Convert list to NumPy array

   The list [1, 2, 3] is converted to a NumPy array arr with values [1 2 3].

2. Step 2: Convert NumPy array back to list

   Calling arr.tolist() converts the array back to a Python list [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   arr.tolist() = original list [OK]

Hint: arr.tolist() returns the original list values [OK]

Common Mistakes:

• Expecting array output instead of list
• Confusing print output with array representation
• Thinking it returns nested lists

4. Identify the error in this code snippet:

import numpy as np
lst = [1, 2, 3]
arr = np.array(lst)
new_list = arr.to_list()

medium

A. arr.to_list() is not a valid method

B. np.array() cannot convert lists

C. lst should be a NumPy array already

D. Missing import statement

Solution

1. Step 1: Check method names on NumPy arrays

   The method to convert a NumPy array to a list is tolist(), not to_list().

2. Step 2: Identify the error in the code

   Using arr.to_list() causes an AttributeError because the method does not exist.

3. Final Answer:

   arr.to_list() is not a valid method -> Option A

4. Quick Check:

   Use arr.tolist(), not arr.to_list() [OK]

Hint: Use arr.tolist(), not arr.to_list() [OK]

Common Mistakes:

• Typing to_list() instead of tolist()
• Assuming np.array() fails on lists
• Forgetting to import numpy

5. You have a nested Python list nested = [[1, 2], [3, 4]]. Which code correctly converts it to a NumPy array and then back to a nested Python list?

hard

A. arr = np.array(nested)
new_list = list(arr)

B. arr = np.array(nested)
new_list = arr.tolist()

C. arr = list(nested)
new_list = arr.tolist()

D. arr = np.array(nested)
new_list = arr.to_list()

Solution

1. Step 1: Convert nested list to NumPy array

   Using np.array(nested) converts the nested list into a 2D NumPy array.

2. Step 2: Convert NumPy array back to nested list

   Calling arr.tolist() converts the 2D array back to the original nested Python list structure.

3. Final Answer:

   arr = np.array(nested) new_list = arr.tolist() -> Option B

4. Quick Check:

   np.array() + tolist() preserves nested lists [OK]

Hint: Use np.array() then tolist() for nested list conversion [OK]

Common Mistakes:

• Using list(arr) which flattens the array
• Calling arr.to_list() which is invalid
• Trying to convert list before np.array()