📊 2D Arrays (Nested Lists)

Learn how to work with data organised in rows and columns, like a table or grid.

What you'll learn:

  • What 2D arrays are and when to use them
  • Creating 2D arrays (lists of lists)
  • Accessing elements using two indices: [row][column]
  • Using nested loops to iterate through 2D arrays
  • Practical applications: grids, tables, matrices

Learning Path

Follow these activities in order.

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1. Predict: What's in the Grid?

Predict what 2D array operations will output.

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2. Study: Worked Example

See how to create and navigate 2D arrays step by step.

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3. Investigate: Trace Nested Loops

Track how nested loops iterate through a 2D array.

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4. Modify: Arrange the Code

Put code blocks in order to work with 2D arrays.

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5. Test: Knowledge Quiz

Check your understanding of 2D arrays.

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Key Vocabulary

2D array

An array of arrays - data organised in rows and columns, like a table or spreadsheet.

row

A horizontal line in a 2D array. The first index selects which row.

column

A vertical line in a 2D array. The second index selects which column.

nested loop

A loop inside another loop. Used to visit every element in a 2D array.

grid

Another name for a 2D array. Think of game boards, maps, or spreadsheets.

matrix

A mathematical term for a 2D array of numbers. Used in maths and graphics.

Quick Reference

arrays_2d_examples.py 
# Creating a 2D array (3 rows x 4 columns)
grid = [
    [1,  2,  3,  4],   # Row 0
    [5,  6,  7,  8],   # Row 1
    [9, 10, 11, 12]    # Row 2
]

# Accessing elements: array[row][column]
print(grid[0][0])  # First row, first column: 1
print(grid[1][2])  # Second row, third column: 7
print(grid[2][3])  # Third row, fourth column: 12

# Getting a whole row
print(grid[1])     # [5, 6, 7, 8]

# Number of rows and columns
rows = len(grid)           # 3 rows
cols = len(grid[0])        # 4 columns

# Nested loop - print every element
for row in grid:
    for item in row:
        print(item, end=" ")
    print()  # New line after each row

# Nested loop with indices
for r in range(len(grid)):
    for c in range(len(grid[r])):
        print(grid[r][c], end=" ")

Visualising a 2D Array

Think of a 2D array as a table with rows and columns:

Col 0 Col 1 Col 2
Row 0 grid[0][0]
"A"		 grid[0][1]
"B"		 grid[0][2]
"C"
Row 1 grid[1][0]
"D"		 grid[1][1]
"E"		 grid[1][2]
"F"