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Sets – Basics

A set is an unordered collection of unique values defined with curly braces or set()

What it does: g[0] is [1,2], then [1] gives the second item: 2. A nested for loop visits every element in every inner list. Any list item can itself be a list — nesting creates multi-dimensional structures. grid[0] gets the first inner list — a second index is needed to get a number. grid[1] is [3,4], then [0] picks the first item — which is 3. len() counts the top-level items — there are 2 inner lists, not 4 numbers. Rows and columns map naturally to a list of lists. Lists are mutable — you can assign to any index including nested ones.

Common mistakes: Use two separate brackets grid[0][1], not a comma grid[0, 1]. grid has indices 0 and 1. Index 2 is out of range.

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Learning

Duo

What is a nested list?

A list inside another listA sorted list

Matches

❌ g = [[1,2],[3,4]] print(g[2][0])

↓

Error: Row index 2 out of range

Applying

Meanings

Term

Nested list

Meaning

A list that contains other lists as elements.

Practice

grid = [[1, 2], [3, 4]]

Testing

Gaps

print(grid[][1])

Check Answer

Spots

1grid = [[1, 2], [3, 4]]

2print(grid[0, 1])

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Quick Questions

1. What is a nested list?

A. A list inside another list ✓

B. A sorted list

Any list item can itself be a list — nesting creates multi-dimensional structures

2. grid = [[1,2],[3,4]]\ngrid[0] returns?

A. 1

B. [1, 2] ✓

grid[0] gets the first inner list — a second index is needed to get a number

3. grid = [[1,2],[3,4]]\ngrid[0][1] returns?

A. 2 ✓

B. 1

grid[0] is [1,2], then [1] picks index 1 of that inner list — which is 2

4. grid = [[1,2],[3,4]]\ngrid[1][0] returns?

A. 4

B. 3 ✓

grid[1] is [3,4], then [0] picks the first item — which is 3

5. How do you loop through all items in a nested list?

A. Two nested for loops ✓

B. One for loop

The outer loop gets each inner list; the inner loop gets each item within it

Match the Pairs

g = [[1,2],[3,4]] print(g[2][0]) → Error: Row index 2 out of range

g = [[1,2],[3,4]] print(g[0,1]) → Error: Use g[0][1] not g[0,1]

g = [[1,2],[3,4]] g[0][1] → Valid: Returns 2

for row in g: for item in row: print(item) → Ok: Nested loops to traverse

g = [[1,2],[3,4]] print(g[1][0]) → Output: 3

g = [[1,2],[3,4]] print(len(g)) → Output: 2

Key Term

Nested list

A list that contains other lists as elements.~~grid[row][col]

Fill in the Blanks

1. grid = [[1, 2], [3, 4]] print(grid[0][1])

2. grid = [[1, 2], [3, 4]] print(grid[1][1])

3. for row in grid: for item in row: print(item)

4. grid = [[1, 2], [3, 4]] print(len( grid))

5. grid = [[1, 2], [3, 4]] grid[0][0] = 99 print(grid)

Spot the Error

Question 1

A grid = [[1, 2], [3, 4]]

B print(grid[0, 1])

Line B — Use two separate brackets grid[0][1], not a comma grid[0, 1]

Question 2

A grid = [[1, 2], [3, 4]]

B print(grid[2][0])

Line B — grid has indices 0 and 1. Index 2 is out of range.

Question 3

A grid = [[1, 2], [3, 4]]

B for item in grid:

C print(item) # expects individual numbers

Line B — One loop gives inner lists [1,2] and [3,4]. Need nested loops for individual items.

Question 4

A grid = [1, 2], [3, 4]

B first_row = grid[0]

C print('First row:', first_row)

D print('Item:', grid[0][1])

Line A — Missing outer brackets: this creates a tuple of lists, not a list of lists

Question 5

A grid = [[1, 2], [3, 4]]

B print(grid[0][1][0])

Line B — grid[0][1] is 2 (an integer). Can't index into a number.

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