np.union1d() for union in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of np.union1d() with integer arrays

What is the output of this code snippet using np.union1d()?

NumPy

import numpy as np
arr1 = np.array([1, 3, 5, 7])
arr2 = np.array([3, 4, 5, 6])
result = np.union1d(arr1, arr2)
print(result)

A [1 3 4 5 6 7]

B [1 3 5 7 3 4 5 6]

C [3 4 5 6]

D [1 3 5 7]

💡 Hint

np.union1d() returns sorted unique elements from both arrays combined.

The function combines both arrays, removes duplicates, and sorts the result. So all unique elements from both arrays appear once in sorted order.

❓ data_output

intermediate

Number of elements in union of two arrays

Given two arrays, how many elements are in the union computed by np.union1d()?

NumPy

import numpy as np
arr1 = np.array([10, 20, 30, 40])
arr2 = np.array([30, 40, 50, 60, 70])
union = np.union1d(arr1, arr2)
print(len(union))

A 8

B 5

C 7

D 6

💡 Hint

Count unique elements after merging both arrays.

The union contains unique elements: 10, 20, 30, 40, 50, 60, 70. That's 7 elements.

🔧 Debug

advanced

Identify the error in using np.union1d() with lists

What error will this code raise?

NumPy

import numpy as np
list1 = [1, 2, 3]
list2 = [3, 4, 5]
result = np.union1d(list1, list2)
print(result)

A No error, prints [1 2 3 4 5]

B TypeError: unhashable type: 'list'

C ValueError: operands could not be broadcast together

D [1 2 3 4 5]

💡 Hint

np.union1d() accepts array-like inputs, including lists.

np.union1d() converts lists to arrays internally and returns the sorted unique union without error.

🧠 Conceptual

advanced

Behavior of np.union1d() with string arrays

What is the output of this code?

NumPy

import numpy as np
arr1 = np.array(['apple', 'banana', 'cherry'])
arr2 = np.array(['banana', 'date', 'fig'])
result = np.union1d(arr1, arr2)
print(result)

A ['apple' 'banana' 'banana' 'cherry' 'date' 'fig']

B ['apple' 'banana' 'cherry' 'date' 'fig']

C ['banana' 'date' 'fig']

D ['apple' 'cherry' 'date' 'fig']

💡 Hint

np.union1d() works with strings and sorts them alphabetically.

The function returns all unique strings from both arrays sorted alphabetically.

🚀 Application

expert

Using np.union1d() to find unique categories from two datasets

You have two datasets with category labels as arrays. Which code snippet correctly finds all unique categories combined?

A unique_categories = np.setdiff1d(dataset1['category'], dataset2['category'])

B unique_categories = np.concatenate((dataset1['category'], dataset2['category']))

C unique_categories = np.intersect1d(dataset1['category'], dataset2['category'])

D unique_categories = np.union1d(dataset1['category'], dataset2['category'])

💡 Hint

np.union1d() returns all unique elements from both arrays combined.

Option D correctly uses np.union1d() to find all unique categories from both datasets. Others either concatenate without uniqueness or find intersection/difference.

Practice

1. What does the function np.union1d() do when applied to two arrays?

easy

A. Finds the common elements between two arrays

B. Combines two arrays and returns unique sorted elements

C. Concatenates two arrays without removing duplicates

D. Sorts a single array in descending order

Solution

1. Step 1: Understand the purpose of np.union1d()

   This function merges two arrays and removes any duplicate values.

2. Step 2: Check the output characteristics

   The result is sorted and contains only unique elements from both arrays.

3. Final Answer:

   Combines two arrays and returns unique sorted elements -> Option B

4. Quick Check:

   Union = unique sorted merge [OK]

Hint: Think of union as merging without duplicates [OK]

Common Mistakes:

• Confusing union with intersection
• Assuming duplicates remain
• Thinking it sorts in descending order

2. Which of the following is the correct syntax to find the union of arrays a and b using numpy?

easy

A. np.union1d[a, b]

B. np.union(a, b)

C. np.union_1d(a, b)

D. np.union1d(a, b)

Solution

1. Step 1: Recall the correct function name

   The correct numpy function is np.union1d() with parentheses.

2. Step 2: Check syntax details

   Arguments are passed inside parentheses, not square brackets, and spelling must be exact.

3. Final Answer:

   np.union1d(a, b) -> Option D

4. Quick Check:

   Correct function call syntax [OK]

Hint: Use parentheses and exact function name np.union1d() [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Misspelling the function name
• Using a non-existent function np.union

3. What is the output of the following code?

import numpy as np
x = np.array([1, 3, 5])
y = np.array([3, 4, 5, 6])
result = np.union1d(x, y)
print(result)

medium

A. [1 3 4 5 6]

B. [1 3 5]

C. [3 4 5 6]

D. [1 3 5 3 4 5 6]

Solution

1. Step 1: Identify unique elements from both arrays

   Array x has [1, 3, 5], array y has [3, 4, 5, 6]. The union combines all unique values.

2. Step 2: Sort and remove duplicates

   Unique elements combined are [1, 3, 4, 5, 6], sorted in ascending order.

3. Final Answer:

   [1 3 4 5 6] -> Option A

4. Quick Check:

   Union = unique sorted merge [OK]

Hint: Union merges unique sorted elements from both arrays [OK]

Common Mistakes:

• Forgetting to remove duplicates
• Not sorting the result
• Printing only one array

4. The following code throws an error. What is the mistake?

import numpy as np
arr1 = [1, 2, 3]
arr2 = [3, 4, 5]
result = np.union1d(arr1 arr2)
print(result)

medium

A. Missing comma between arr1 and arr2 in function call

B. np.union1d() does not accept lists as input

C. np.union1d() requires arrays to be sorted first

D. print() function is used incorrectly

Solution

1. Step 1: Check function call syntax

   The function call np.union1d(arr1 arr2) is missing a comma between arguments.

2. Step 2: Confirm input types and print usage

   np.union1d accepts lists or arrays, and print() is correctly used.

3. Final Answer:

   Missing comma between arr1 and arr2 in function call -> Option A

4. Quick Check:

   Comma separates arguments [OK]

Hint: Check commas between function arguments [OK]

Common Mistakes:

• Omitting commas between arguments
• Thinking input must be numpy arrays only
• Assuming print() causes error

5. You have two datasets of customer IDs:

dataset1 = np.array([101, 102, 103, 104])
dataset2 = np.array([103, 104, 105, 106])

You want to create a combined list of all unique customer IDs sorted in ascending order. Which code snippet correctly achieves this?

hard

A. combined = np.intersect1d(dataset1, dataset2)

B. combined = np.concatenate((dataset1, dataset2))

C. combined = np.union1d(dataset1, dataset2)

D. combined = np.sort(np.append(dataset1, dataset2))

Solution

1. Step 1: Understand the goal

   We want all unique customer IDs from both datasets, sorted ascending.

2. Step 2: Evaluate each option

   combined = np.union1d(dataset1, dataset2) uses np.union1d which merges and sorts unique elements. combined = np.concatenate((dataset1, dataset2)) concatenates but keeps duplicates. combined = np.intersect1d(dataset1, dataset2) finds only common IDs. combined = np.sort(np.append(dataset1, dataset2)) appends and sorts but does not remove duplicates.

3. Final Answer:

   combined = np.union1d(dataset1, dataset2) -> Option C

4. Quick Check:

   Union merges unique sorted elements [OK]

Hint: Use np.union1d to merge unique sorted values [OK]

Common Mistakes:

• Using concatenate without removing duplicates
• Using intersect1d which finds only common elements
• Appending and sorting without removing duplicates