2D array indexing (row, col) in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the expression array[2, 3] mean in a 2D NumPy array?

It means accessing the element at row 2 and column 3 of the 2D array. Rows and columns start counting from 0.

beginner

How do you select the entire 3rd row from a 2D NumPy array named arr?

Use arr[2, :]. The colon : means all columns in row 2.

beginner

How do you select the entire 2nd column from a 2D NumPy array named arr?

Use arr[:, 1]. The colon : means all rows in column 1.

intermediate

What happens if you use negative indices like arr[-1, -2] in a 2D NumPy array?

Negative indices count from the end. -1 is the last row, -2 is the second last column.

intermediate

How can you select a submatrix from rows 1 to 3 and columns 2 to 4 in a 2D NumPy array arr?

Use slicing: arr[1:4, 2:5]. The end index is exclusive, so it selects rows 1,2,3 and columns 2,3,4.

In a 2D NumPy array, what does arr[0, 0] access?

A First column, last row element

B Last row, last column element

C First row, first column element

D Last column, first row element

Indexing starts at 0, so arr[0, 0] is the element in the first row and first column.

How do you select all rows but only the 2nd column from a 2D array arr?

A arr[2, :]

B arr[1, :]

C arr[:, 2]

D arr[:, 1]

The colon : means all rows, and 1 selects the second column.

What does arr[-1, :] select in a 2D NumPy array?

A Last row

B First row

C Last column

D First column

Negative index -1 means the last row, and : means all columns.

Which slice selects rows 0 to 2 and columns 1 to 3 in arr?

A arr[0:3, 1:4]

B arr[1:3, 0:4]

C arr[0:2, 1:3]

D arr[1:4, 0:3]

Slicing is exclusive at the end, so 0:3 means rows 0,1,2 and 1:4 means columns 1,2,3.

What is the shape of arr[:, 2] if arr is 5x5?

A (5, 5)

B (5,)

C (1, 5)

D (5, 1)

Selecting a single column returns a 1D array with length equal to the number of rows.

Explain how to access a single element, a full row, and a full column in a 2D NumPy array.

Think about how rows and columns are indexed and how slicing works.

Describe how negative indices work in 2D array indexing and give an example.

Negative numbers start counting backwards from the last element.