Array indexing and slicing in Data Analysis Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of slicing a 2D NumPy array

What is the output of the following code snippet?

Data Analysis Python

import numpy as np
arr = np.array([[10, 20, 30], [40, 50, 60], [70, 80, 90]])
sliced = arr[1:, :2]
print(sliced)

A

[[50 60]
 [80 90]]

B

[[40 50]
 [70 80]]

C

[[20 30]
 [50 60]]

D

[[40 60]
 [70 90]]

💡 Hint

Remember that arr[1:, :2] means rows from index 1 to end, columns from index 0 up to 2 (not including 2).

The slice arr[1:, :2] selects rows starting from index 1 (second row) to the end, and columns from index 0 up to but not including 2 (first two columns). So it picks rows [[40,50,60],[70,80,90]] and columns [[40,50],[70,80]].

❓ data_output

intermediate

Number of elements after slicing a 1D array

Given the array arr = [5, 10, 15, 20, 25, 30], how many elements are in arr[2:5]?

Data Analysis Python

arr = [5, 10, 15, 20, 25, 30]
sliced = arr[2:5]
print(len(sliced))

A 3

B 4

C 2

D 5

💡 Hint

Remember slicing includes the start index but excludes the end index.

arr[2:5] includes elements at indices 2, 3, and 4, which are 15, 20, and 25. So the length is 3.

🔧 Debug

advanced

Identify the error in slicing a NumPy array

What error does the following code raise?

Data Analysis Python

import numpy as np
arr = np.array([1, 2, 3, 4, 5])
sliced = arr[::2, 1]
print(sliced)

A SyntaxError: invalid syntax

B TypeError: unhashable type: 'slice'

C IndexError: too many indices for array

D No error, outputs [1 3 5]

💡 Hint

Check how many dimensions the array has and how many indices are used.

The array arr is 1D, but the code tries to index it with two indices (arr[::2, 1]), which causes an IndexError for too many indices.

🧠 Conceptual

advanced

Effect of negative step in slicing

What is the output of this code? import numpy as np arr = np.array([10, 20, 30, 40, 50]) sliced = arr[4:1:-1] print(sliced)

A [50 40 30 20]

B [40 30 20]

C [10 20 30]

D [50 40 30]

💡 Hint

Negative step means slicing backwards, start index is included, end index is excluded.

arr[4:1:-1] starts at index 4 (value 50), goes backwards up to but not including index 1, so it includes indices 4,3,2 which are 50,40,30.

🚀 Application

expert

Extracting a diagonal from a 2D NumPy array using slicing

Given a 3x3 NumPy array, which slicing expression extracts the main diagonal elements as a 1D array?

Data Analysis Python

import numpy as np
arr = np.array([[1,2,3],[4,5,6],[7,8,9]])

A arr[range(3), range(3)]

B arr[:, 0]

C arr[0:3, 0:3]

D arr[::2, ::2]

💡 Hint

Think about how to select elements where row and column indices are the same.

Using arr[range(3), range(3)] selects elements where row and column indices match: (0,0), (1,1), (2,2), which is the main diagonal.

Practice

1. What does the expression arr[2] do when arr is a Python list?

easy

A. Returns the first two elements of the list

B. Removes the element at index 2 from the list

C. Returns a new list with elements from index 0 to 2

D. Returns the element at index 2 (third element) of the list

Solution

1. Step 1: Understand list indexing

   In Python, list indexing starts at 0, so index 2 means the third element.

2. Step 2: Identify what arr[2] returns

   This expression returns the single element at position 2, not a range or removal.

3. Final Answer:

   Returns the element at index 2 (third element) of the list -> Option D

4. Quick Check:

   Index 2 means third element [OK]

Hint: Indexing gets one element by position [OK]

Common Mistakes:

• Thinking slicing syntax is used for single element
• Confusing index 2 with first two elements
• Assuming it removes an element

2. Which of the following is the correct syntax to get a slice of a list lst from index 1 to 4 (excluding 4)?

easy

A. lst(1:4)

B. lst[1:4]

C. lst[1,4]

D. lst[1-4]

Solution

1. Step 1: Recall Python slice syntax

   Slicing uses square brackets with colon: start:stop, where stop is excluded.

2. Step 2: Check each option

   Only lst[1:4] uses correct slice syntax to get elements at indices 1, 2, 3.

3. Final Answer:

   lst[1:4] -> Option B

4. Quick Check:

   Slice syntax uses colon [OK]

Hint: Use colon inside brackets for slicing [OK]

Common Mistakes:

• Using comma instead of colon
• Using parentheses instead of brackets
• Using dash instead of colon

3. Given arr = [10, 20, 30, 40, 50], what is the output of arr[1:4]?

medium

A. [20, 30, 40]

B. [10, 20, 30, 40]

C. [30, 40, 50]

D. [20, 30, 40, 50]

Solution

1. Step 1: Understand slicing indices

   The slice arr[1:4] includes elements from index 1 up to but not including index 4.

2. Step 2: Extract elements at indices 1, 2, 3

   These are 20 (index 1), 30 (index 2), and 40 (index 3).

3. Final Answer:

   [20, 30, 40] -> Option A

4. Quick Check:

   Slice excludes stop index [OK]

Hint: Slice includes start, excludes stop index [OK]

Common Mistakes:

• Including element at stop index
• Confusing indices with values
• Off-by-one errors in slicing

4. What is wrong with this code snippet?

arr = [1, 2, 3, 4, 5]
print(arr[2:6])

medium

A. It will print elements from index 2 to 6 including index 6

B. It will cause an IndexError because 6 is out of range

C. It will print elements from index 2 to 5 without error

D. It will print only the element at index 6

Solution

1. Step 1: Understand slicing behavior with out-of-range stop

   Python slicing does not raise error if stop index is beyond list length; it stops at the list end.

2. Step 2: Check what arr[2:6] returns

   It returns elements from index 2 to the end (index 4), which are [3, 4, 5].

3. Final Answer:

   It will print elements from index 2 to 5 without error -> Option C

4. Quick Check:

   Slicing stop can exceed length safely [OK]

Hint: Slicing stop index can exceed list length safely [OK]

Common Mistakes:

• Expecting IndexError for slicing beyond length
• Thinking slice includes stop index
• Confusing slicing with indexing

5. You have a list data = [5, 10, 15, 20, 25, 30]. How do you get every second element starting from index 1 up to index 5?

hard

A. data[1:6:2]

B. data[1:5:2]

C. data[1:5]

D. data[::2]

Solution

1. Step 1: Understand slicing with step

   Slicing syntax is start:stop:step. To get every second element, step=2.

2. Step 2: Identify correct start and stop

   Start is 1, stop is 6 to include element at index 5 (stop excluded).

3. Step 3: Verify the slice

   data[1:6:2] returns elements at indices 1, 3, 5 -> [10, 20, 30].

4. Final Answer:

   data[1:6:2] -> Option A

5. Quick Check:

   Use step in slice for skipping elements [OK]

Hint: Use third slice value for step size [OK]

Common Mistakes:

• Using wrong stop index excluding last element
• Omitting step and getting all elements
• Confusing step with start or stop