Converting to and from Python lists in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of converting numpy array to list

What is the output of this code snippet converting a numpy array to a Python list?

NumPy

import numpy as np
arr = np.array([[1, 2], [3, 4]])
result = arr.tolist()
print(result)

A [1, [2, 3], 4]

B [1, 2, 3, 4]

C [[1, 2], [3, 4]]

D [[1, 2, 3, 4]]

💡 Hint

Think about how a 2D numpy array converts to nested lists.

The tolist() method converts a numpy array into a nested Python list preserving the shape. Here, a 2x2 array becomes a list of two lists, each with two elements.

❓ data_output

intermediate

Result of creating numpy array from list

What is the shape of the numpy array created from this list?

NumPy

import numpy as np
lst = [[1, 2, 3], [4, 5, 6]]
arr = np.array(lst)
print(arr.shape)

A (3, 2)

B (2, 3)

C (6,)

D (2,)

💡 Hint

Check how many rows and columns the list has.

The list has 2 sublists each with 3 elements, so the numpy array shape is (2, 3).

🔧 Debug

advanced

Identify the error when converting list to numpy array

What error does this code raise when converting a list with mixed types to a numpy array?

NumPy

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

A SyntaxError: invalid syntax

B TypeError: unsupported operand type(s) for +: 'int' and 'str'

C ValueError: could not convert string to float: 'two'

D No error, output: array(['1', 'two', '3'], dtype='<U21')

💡 Hint

Numpy tries to find a common data type for all elements.

Numpy converts all elements to strings to create a uniform array of strings, so no error occurs.

🚀 Application

advanced

Convert nested list to 1D numpy array

Which option correctly converts a nested list into a flat 1D numpy array?

NumPy

import numpy as np
nested_list = [[1, 2], [3, 4], [5, 6]]

A np.array(nested_list).flatten()

B np.array(nested_list).tolist()

C np.array(nested_list).reshape(2, 3)

D np.array(nested_list).ravel().tolist()

💡 Hint

Flattening means making all elements in one dimension.

The flatten() method returns a copy of the array collapsed into one dimension.

🧠 Conceptual

expert

Understanding difference between tolist() and list() on numpy arrays

What is the key difference between using tolist() and list() on a numpy array?

NumPy

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

A tolist() converts the entire array into nested Python lists; list() converts only the top-level array into a list of numpy arrays

B tolist() and list() produce identical outputs

C tolist() flattens the array; list() preserves the shape

D tolist() returns a numpy array; list() returns a Python list

💡 Hint

Think about how each method handles nested structures.

tolist() recursively converts all elements to Python lists, preserving nested structure. list() converts only the outer array to a list, keeping inner elements as numpy arrays.

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()