Structured arrays vs DataFrames in NumPy - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a structured array with fields 'name' and 'age'.

NumPy

import numpy as np

people = np.array([('Alice', 25), ('Bob', 30)], dtype=[1])

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

A[('name', 'U10'), ('age', 'i4')]

B[('name', 'S10'), ('age', 'f4')]

C[('name', 'U5'), ('age', 'i2')]

D[('name', 'U10'), ('age', 'f8')]

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect data types like float for age.

Not using a list of tuples for dtype.

The dtype must define the field names and their data types. 'U10' means a Unicode string of length 10, and 'i4' means a 4-byte integer, suitable for age.

2fill in blank

medium

Complete the code to convert the structured array 'people' to a pandas DataFrame.

NumPy

import pandas as pd

df = pd.DataFrame([1])

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

Apeople.tolist()

Bpeople.astype(object)

Cpeople.view(np.recarray)

Dpeople

Attempts:

3 left

💡 Hint

Common Mistakes

Converting to list loses field names.

Using .view(np.recarray) unnecessarily.

You can pass the structured array directly to pd.DataFrame to create a DataFrame with columns matching the fields.

3fill in blank

hard

Fix the error in accessing the 'age' field from the structured array 'people'.

NumPy

ages = people[1]

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

A.age

B[1]

C['age']

D['Age']

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation which does not work for numpy structured arrays.

Using incorrect capitalization in field names.

To access a field in a structured array, use square brackets with the exact field name as a string.

4fill in blank

hard

Fill all four blanks to create a DataFrame from a dictionary comprehension that filters people older than 25.

NumPy

df_filtered = pd.DataFrame([1]: [2] for [3] in people if [4] > 25)

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

Aperson['name']

Bperson['age']

Cperson

Dp

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field for keys or values.

Not filtering correctly with the age condition.

The dictionary comprehension uses the person's name as keys and age as values for those older than 25.

5fill in blank

hard

Fill all three blanks to create a structured array from a list of tuples and then convert it to a DataFrame.

NumPy

data = [('John', 28), ('Jane', 32)]

arr = np.array(data, dtype=[[1], [2]])
df = pd.DataFrame([3])

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

A('name', 'U10')

B('age', 'i4')

Carr

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the original list to pd.DataFrame instead of the structured array.

Incorrect dtype format.

The dtype is a list of tuples defining fields. Then the structured array is passed directly to pd.DataFrame.

Practice

1. What is a key difference between a numpy structured array and a pandas DataFrame?

easy

A. Structured arrays automatically handle missing data, DataFrames do not.

B. Structured arrays can only store numbers, DataFrames can only store text.

C. DataFrames do not support named columns, structured arrays do.

D. Structured arrays have fixed data types per column, while DataFrames allow mixed types and more flexible operations.

Solution

1. Step 1: Understand data type handling in structured arrays

   Structured arrays in numpy require fixed data types for each named column, meaning each column's type is set and consistent.

2. Step 2: Compare with DataFrame flexibility

   DataFrames from pandas allow columns to have different data types and provide many flexible operations like handling missing data and complex indexing.

3. Final Answer:

   Structured arrays have fixed data types per column, while DataFrames allow mixed types and more flexible operations. -> Option D

4. Quick Check:

   Data type flexibility = D [OK]

Hint: Remember: structured arrays fix types, DataFrames are more flexible [OK]

Common Mistakes:

• Thinking structured arrays can handle missing data like DataFrames
• Assuming DataFrames cannot have mixed data types
• Believing structured arrays only store numbers

2. Which of the following is the correct way to create a numpy structured array with fields 'name' (string) and 'age' (integer)?

easy

A. np.array([('Alice', 25), ('Bob', 30)], dtype=[('name', 'int'), ('age', 'str')])

B. np.array([{'Name': 'Alice', 'age': 25}, {'Name': 'Bob', 'age': 30}])

C. np.array([('Alice', 25), ('Bob', 30)], dtype=[('name', 'U10'), ('age', 'i4')])

D. np.array([['Alice'], ['Bob']], dtype=[('name', 'U10'), ('age', 'i4')])

Solution

1. Step 1: Check dtype specification for structured arrays

   The dtype must be a list of tuples with field names and valid numpy data types, e.g., 'U10' for string and 'i4' for 4-byte integer.

2. Step 2: Verify the data matches the dtype

   np.array([('Alice', 25), ('Bob', 30)], dtype=[('name', 'U10'), ('age', 'i4')]) uses tuples matching the dtype fields correctly. np.array([{'name': 'Alice', 'age': 25}, {'name': 'Bob', 'age': 30}]) uses dicts which numpy does not accept directly for structured arrays. np.array([('Alice', 25), ('Bob', 30)], dtype=[('name', 'int'), ('age', 'str')]) swaps types incorrectly. np.array([['Alice', 25], ['Bob', 30]], dtype=[('name', 'U10'), ('age', 'i4')]) uses lists instead of tuples, which is invalid here.

3. Final Answer:

   np.array([('Alice', 25), ('Bob', 30)], dtype=[('name', 'U10'), ('age', 'i4')]) -> Option C

4. Quick Check:

   Correct dtype and tuple data = A [OK]

Hint: Use tuples and correct dtype list for structured arrays [OK]

Common Mistakes:

• Using dicts instead of tuples for structured array data
• Mixing up data types in dtype list
• Using lists instead of tuples for records

3. Given the code below, what will be the output?

import numpy as np
import pandas as pd

arr = np.array([(1, 'A'), (2, 'B')], dtype=[('id', 'i4'), ('label', 'U1')])
df = pd.DataFrame(arr)
print(df['label'][1])

medium

A. B

B. A

C. 1

D. Error: KeyError

Solution

1. Step 1: Understand conversion from structured array to DataFrame

   Creating a DataFrame from a structured array converts named fields into columns with the same names.

2. Step 2: Access the 'label' column and index 1

   df['label'] is a Series with values ['A', 'B']. Index 1 corresponds to 'B'.

3. Final Answer:

   B -> Option A

4. Quick Check:

   DataFrame column access = B [OK]

Hint: Structured array fields become DataFrame columns [OK]

Common Mistakes:

• Confusing index 0 and 1 values
• Expecting error due to structured array
• Mixing up field names and indices

4. What is wrong with this code snippet that tries to convert a pandas DataFrame to a numpy structured array?

import pandas as pd
import numpy as np

df = pd.DataFrame({'name': ['Tom', 'Jerry'], 'age': [5, 7]})
arr = np.array(df, dtype=[('name', 'U10'), ('age', 'i4')])
print(arr)

medium

A. The dtype should use 'S10' instead of 'U10' for strings.

B. The dtype argument is ignored; conversion does not create a structured array as expected.

C. The DataFrame must be converted to a list of tuples before creating the structured array.

D. There is no error; the code works correctly.

Solution

1. Step 1: Check how numpy.array handles DataFrame input with dtype

   Passing a DataFrame directly to np.array with dtype does not convert it into a structured array; dtype is ignored and a 2D array of objects is created.

2. Step 2: Identify correct conversion method

   To get a structured array, convert DataFrame to records (e.g., df.to_records()) before calling np.array.

3. Final Answer:

   The dtype argument is ignored; conversion does not create a structured array as expected. -> Option B

4. Quick Check:

   Direct np.array(df, dtype=...) ignores dtype [OK]

Hint: Convert DataFrame to records before numpy structured array [OK]

Common Mistakes:

• Assuming dtype works directly on DataFrame in np.array
• Not converting DataFrame to records first
• Confusing string dtype codes

5. You have a numpy structured array with fields 'city' (string) and 'temperature' (float). You want to convert it to a pandas DataFrame, filter rows where temperature > 20, then convert back to a structured array with the same fields. Which code snippet correctly does this?

hard

A. df = pd.DataFrame(arr); filtered = df.query('temperature > 20'); result = np.array(filtered.to_records(index=False), dtype=arr.dtype)

B. df = pd.DataFrame(arr); filtered = df[df.temperature > 20]; result = np.array(filtered, dtype=arr.dtype)

C. df = pd.DataFrame(arr); filtered = df[df['temperature'] > 20]; result = np.array(filtered.to_records())

D. df = pd.DataFrame(arr); filtered = df[df['temperature'] > 20]; result = np.array(filtered.to_dict())

Solution

1. Step 1: Convert structured array to DataFrame

   Creating a DataFrame from the structured array is straightforward: df = pd.DataFrame(arr).

2. Step 2: Filter rows where temperature > 20

   Using df.query('temperature > 20') or df[df['temperature'] > 20] both work, but query is concise and clear.

3. Step 3: Convert filtered DataFrame back to structured array with original dtype

   Use filtered.to_records(index=False) to get a structured array-like record array, then convert to numpy array with original dtype to keep field types consistent.

4. Final Answer:

   df = pd.DataFrame(arr); filtered = df.query('temperature > 20'); result = np.array(filtered.to_records(index=False), dtype=arr.dtype) -> Option A

5. Quick Check:

   Filter with query + to_records + dtype = A [OK]

Hint: Use to_records() and specify dtype when converting back [OK]

Common Mistakes:

• Not using to_records() before np.array conversion
• Forgetting to specify dtype on conversion back
• Using to_dict() which is incorrect here