Single element access in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you access a single element in a NumPy array?

You use square brackets with the index of the element, like array[index]. For multi-dimensional arrays, use commas to separate indices, like array[row, column].

beginner

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

The first element has index 0. NumPy arrays use zero-based indexing, so counting starts at 0.

beginner

How do you access the element in the second row and third column of a 2D NumPy array named arr?

Use arr[1, 2]. Remember, counting starts at 0, so row 1 is the second row and column 2 is the third column.

beginner

What happens if you try to access an index that is out of range in a NumPy array?

NumPy will raise an IndexError because the index does not exist in the array.

beginner

Can you use negative indices to access elements in a NumPy array? What does array[-1] return?

Yes, negative indices count from the end. array[-1] returns the last element of the array.

What does arr[0] return for a NumPy array arr?

A The first element

B The last element

C An error

D The middle element

NumPy arrays use zero-based indexing, so arr[0] returns the first element.

How do you access the element in the third row and first column of a 2D NumPy array matrix?

A <code>matrix[0, 2]</code>

B <code>matrix[3, 1]</code>

C <code>matrix[1, 2]</code>

D <code>matrix[2, 0]</code>

Indexing starts at 0, so third row is index 2 and first column is index 0.

What will happen if you try to access arr[10] when arr has only 5 elements?

A Raises an <code>IndexError</code>

B Returns the last element

C Returns <code>None</code>

D Returns the first element

Accessing an index outside the array size raises an IndexError.

What does arr[-2] return in a NumPy array?

A Second element

B Second last element

C Last element

D An error

Negative indices count from the end, so -2 is the second last element.

Which of these is the correct way to access a single element in a 3D NumPy array arr?

A <code>arr[1, 2, 3]</code>

B <code>arr[1][2][3]</code>

C Both A and B

D Neither A nor B

You can use either comma-separated indices or chained brackets to access elements in multi-dimensional arrays.

Explain how to access a single element in a 2D NumPy array and what indexing rules apply.

Think about how rows and columns are counted starting from zero.

Describe what happens if you try to access an element outside the bounds of a NumPy array.

What error does Python give when you ask for something that does not exist?