Week 6 — basic collections

Quiz 2 scope: Weeks 1–8 per IITM May 2026 foundation courses. Source baseline: IITM BS admissions important-dates calendar · May 2026 cycle. Times on assessments are operational conventions — verify hall ticket.

Week map

Classify → Represent → Execute → Trap-check

• Recognize: Ask: What does seq[-2] return?
• Procedure: Last element is index -1. Slice [1:4] takes indices 1,2,3. Step 2 skips every other. Empty slice when start >= end with positive step.
• Variations / traps: Watch for: IndexError on len(seq) or -len-1.

Formula chain (compressed)

list mutable vs tuple immutable → indexing/slicing → append vs +.

1. List literal — [a, b, c] — ordered, mutable sequence
2. Tuple literal — (a, b, c) — ordered, immutable
3. Index / slice — L[i], L[a:b:c] — 0-based, stop exclusive
4. append — L.append(x) — mutate in place
5. Concatenate — L + M → new list — does not mutate L

Deep study

Programming in Python · Week 6 — Lists and tuples

Week map

List notation

• lst = [1, 2, 3] → ordered mutable sequence → elements accessible by index.
• lst[i] → element at index i → 0-based → lst[0] is first.
• lst[-1] → last element → negative index counts from end.
• lst[a:b] → slice from a inclusive to b exclusive → lst[1:3] gives two elements.
• len(lst) → number of elements.

python
Run Inline
colors = ["red", "green", "blue"]
colors[1] = "yellow"   # mutable
colors.append("black") # [red, yellow, blue, black]

Common list methods

• .append(x) → add one item at end.
• .insert(i, x) → insert at index i, shift right.
• .pop() → remove and return last; .pop(i) at index.
• .remove(x) → remove first occurrence of value x.
• lst + other → concatenate → new list.
• lst * n → repeat list n times.

Tuple notation

• t = (1, 2, 3) → ordered immutable sequence → cannot reassign elements.
• t = (42,) → single-element tuple needs trailing comma.
• () → empty tuple.
• Tuples useful for fixed records: (name, score), coordinates (x, y).

python
Run Inline
point = (3, 4)
x, y = point          # unpacking
# point[0] = 5       # TypeError — immutable

List vs tuple

Pattern families

Easy — Index and slice

Medium — Mutate and traverse

Hard — Nested lists and aliasing

Worked mini-examples

python
Run Inline
nums = [10, 20, 30, 40, 50]
nums[1:4]    # [20, 30, 40]
nums[-2:]     # [40, 50]

python
Run Inline
a = [1, 2]
a.append([3])     # [1, 2, [3]]
b = [1, 2]
b.insert(1, 99)   # [1, 99, 2]

python
Run Inline
vals = [1, 2, 3]
doubled = []
for v in vals:
    doubled.append(v * 2)
# [2, 4, 6]

python
Run Inline
pair = ("Alice", 92)
name, score = pair

python
Run Inline
row = [0, 0]
grid = [row, row]
grid[0][0] = 1
# grid is [1, 0], [1, 0](/courses/may26-python/notes/1%2C%200%5D%2C%20%5B1%2C%200) — both rows share row

Traps

• t = (5) is int 5, not tuple — need (5,).
• append([x]) nests list; extend([x]) adds element.
• Slice copy is shallow — inner mutable objects still shared.
• Modifying list while iterating forward can skip elements.
• lst.sort() sorts in place; sorted(lst) returns new list.

Diagnostic (try yourself)

1. What is ["a", "b", "c"][1]? What is [-1]?
2. After x = [1, 2]; x.append(3); x.insert(0, 0), what is x?
3. Can you change t[0] if t = (10, 20)? Why or why not?
4. What does a = [1, 2] + [3] produce? How is it different from a.append(3) starting from [1, 2]?
5. row = [0, 0]; m = [row, row]; m[1][1] = 9. What is m[0]?

ChatGPT prep archive

• Chatty reference archive

Archived import for extra depth — complements the notes above, not official IITM material.

Core concepts

• List []: ordered, mutable; tuple (): ordered, immutable.
• Indexing: 0-based; negative indices from end; seq[i] one element.
• Slicing: seq[start:end:step] copies subsequence; end exclusive.
• Methods: append, pop; + concatenates sequences.

Notation & vocabulary

Pattern families

Easy — Index and slice

Medium — Mutate vs share

Hard — List algorithms trace

Drill these on the pattern atlas — filter to week 6.

Traps

• IndexError on len(seq) or -len-1.
• Confusing slice copy with alias mutation.
• Using tuple where list needed for append.
• Step 0 in slice is invalid.

Retrieval prompts

1. What does seq[-2] return?
2. Can you append to a tuple?
3. Difference between a = b for lists vs a = b[:]?

Practice loop

1. Read Deep study (if present) or core concepts once.
2. Recite the formula chain without looking.
3. Open one easy pattern on the interactive atlas for week 6.
4. Attempt without solutions; mark studied after an honest try.
5. Say one trap aloud before closing the tab.

Related

• Full weeks 1–8 prep guide
• Pattern atlas note
• Formula chain desk