← Python

Lists – Indexing

Square bracket notation accesses a list item by its zero-based integer index

What it does: Unlike strings, list items can be changed by assigning to their index. -1 always gives the last element of a list. Index 0 is always the first item. Indices go from 0 to len-1 — [3] doesn't exist in a 3-item list. in checks if the value exists anywhere in the list. for...in is the standard way to iterate through a list. Assigning to nums[1] replaces the item at index 1 (the second item). The loop goes through items in order — 1 is first.

Common mistakes: List has indices 0,1,2: index 3 is out of range. Use square brackets [] for indexing, not parentheses ().

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Learning

Duo

colors = ['red','blue','green']\ncolors[0] returns?

'red''blue'

Matches

❌ nums = [10, 20, 30] print(nums[3])

↓

Error: Index 3 out of range

Applying

Meanings

Term

list[0]

Meaning

Access the first item in the list.

Practice

colors = ['red', 'blue', 'green']

Testing

Gaps

print(colors[])

Check Answer

Spots

2first = colors[0]

3last = colors[3]

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Quick Questions

1. colors = ['red','blue','green']\ncolors[0] returns?

A. 'red' ✓

B. 'blue'

Index 0 is always the first item

2. colors = ['red','blue','green']\ncolors[-1] returns?

A. 'red'

B. 'green' ✓

[-1] is always the last item in a list or string

3. Can you change a list item by index?

A. Yes lists are mutable ✓

B. No lists are immutable

Unlike strings, lists are mutable — you can assign new values by index

4. nums = [10,20,30]\nnums[3] does what?

A. Returns 30

B. IndexError ✓

Indices go from 0 to len-1 — [3] doesn't exist in a 3-item list

5. 'cat' in ['cat','dog'] returns?

A. True ✓

B. False

in checks if the value exists anywhere in the list

Match the Pairs

nums = [10, 20, 30] print(nums[3]) → Error: Index 3 out of range

nums = [1, 2, 3] print(nums(0)) → Error: Use [] not () for indexing

nums = [5, 10, 15] nums[1] = 99 → Valid: Lists are mutable

nums[-1] → Ok: Last item in list

c = ['r','g','b'] print(c[0]) → Output: r

c = ['r','g','b'] print(c[-1]) → Output: b

Key Term

list[0]

Access the first item in the list.~~list[-1]

Fill in the Blanks

1. colors = ['red', 'blue'] print(colors[0])

2. nums = [10, 20, 30] nums[1] = 99

3. pets = ['cat', 'dog'] print('cat' in pets)

4. for fruit in fruits: print(fruit)

5. colors = ['red', 'blue', 'green'] print(colors[-1])

Spot the Error

Question 1

A colors = ['red', 'blue', 'green']

B first = colors[0]

C last = colors[3]

D print(first, last)

Line C — List has indices 0,1,2: index 3 is out of range

Question 2

A colors = ['red', 'blue']

B first = colors(0)

C print('First:', first)

D print('Done')

Line B — Use square brackets [] for indexing, not parentheses ()

Question 3

A colors = ['red', 'blue']

B first = colors['0']

C print('First:', first)

D print('Done')

Line B — Index must be an integer, not a string

Question 4

A nums = [10, 20, 30]

B nums(1) = 99

C print('Updated:', nums)

D print('Done')

Line B — Use square brackets for assignment: nums[1] = 99

Question 5

A colors = ['red', 'blue']

B for color in colors

C print(color)

D print('Done')

Line B — Missing colon after the for statement

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