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.