Array indexing and slicing in Data Analysis Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the first element of the array.

Data Analysis Python

import numpy as np
arr = np.array([10, 20, 30, 40])
first_element = arr[[1]]
print(first_element)

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

A1

B0

C-1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 to access the first element.

Using an index outside the array range.

The first element in a Python array is accessed with index 0.

2fill in blank

medium

Complete the code to slice the array and get elements from index 1 to 4 (exclusive).

Data Analysis Python

import numpy as np
arr = np.array([5, 10, 15, 20, 25])
slice_arr = arr[[1]]
print(slice_arr)

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

A2:5

B1:3

C0:3

D1:4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1:3 which excludes index 3.

Using 2:5 which starts too late.

Slicing with 1:4 gets elements at indices 1, 2, and 3.

3fill in blank

hard

Fix the error in the code to correctly get the last two elements of the array.

Data Analysis Python

import numpy as np
arr = np.array([2, 4, 6, 8, 10])
last_two = arr[[1]]
print(last_two)

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A-2:

B3:5

C4:6

D-3:-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3:5 which works but is less flexible.

Using -3:-1 which excludes the last element.

Using -2: slices from the second last element to the end.

4fill in blank

hard

Fill both blanks to create a dictionary with words as keys and their lengths as values, but only for words longer than 3 characters.

Data Analysis Python

words = ['cat', 'house', 'tree', 'a', 'elephant']
lengths = {word: [1] for word in words if [2]
print(lengths)

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Alen(word)

Bword

Clen(word) > 3

Dword > 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using word instead of len(word) for length.

Using word > 3 which compares strings incorrectly.

Use len(word) to get length and filter words with length greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase words as keys and their lengths as values, but only for words longer than 4 characters.

Data Analysis Python

words = ['apple', 'bat', 'carrot', 'dog', 'elephant']
result = { [1]: [2] for w in words if [3] }
print(result)

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

Aw.upper()

Blen(w)

Clen(w) > 4

Dw.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using w.lower() instead of w.upper() for keys.

Filtering with wrong length condition.

Keys are uppercase words, values are lengths, filtered by length > 4.

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