Array indexing and slicing in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is array indexing?

Array indexing is the way to access a single element in an array using its position number, starting from zero.

beginner

How do you slice an array in Python?

Slicing means taking a part of an array by specifying a start and end position like array[start:end]. It gives a new array with elements from start up to but not including end.

intermediate

What does negative indexing do in arrays?

Negative indexing counts from the end of the array. For example, -1 means the last element, -2 means the second last, and so on.

intermediate

Explain the slice notation array[start:end:step].

This notation lets you pick elements from start to end (not including end) by skipping elements according to step. For example, step=2 picks every second element.

beginner

What happens if you omit start or end in slicing?

If start is omitted, slicing starts from the beginning. If end is omitted, slicing goes to the last element. For example, array[:3] means from start to index 2.

What is the index of the first element in a Python array?

A 0

B 1

C -1

D Depends on the array

Python arrays start counting at zero, so the first element is at index 0.

What does array[2:5] return?

A Elements at positions 2, 3, 4, and 5

B Elements at positions 2, 3, and 4

C Elements at positions 3, 4, and 5

D Elements at positions 0 to 5

Slicing includes the start index but excludes the end index.

What does negative index -1 mean in an array?

A Second last element

B First element

C Second element

D Last element

Negative indices count from the end, so -1 is the last element.

What will array[::2] do?

A Select every second element starting from the second

B Select every element

C Select every second element starting from the first

D Select elements in reverse order

The step of 2 means pick every second element starting at index 0.

If you want to slice from the start to index 4, which is correct?

A array[:5]

B array[0:4]

C array[1:5]

D array[5:]

Slicing excludes the end index, so to include index 4, end should be 5.

Describe how to access the last three elements of an array using slicing.

Think about counting from the end with negative numbers.

Explain the difference between array indexing and slicing with examples.

Indexing gets one item, slicing gets a part of the array.

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