Week 2 — iteration & selection

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: Initial value for product accumulation?
• Procedure: Initialize total 0. For each x, add to total. Result after last item.
• Variations / traps: Watch for: Max of empty without check.

Formula chain (compressed)

accumulator → max/min scan → AND predicate over loop → selection diamond.

1. Accumulator — sum ← sum + x — running total
2. Max/min — if x > best: best ← x — linear extremum
3. AND over items — all satisfy P(x) — flag stays true
4. Selection — if condition → branch — at most one path
5. Counter — count ← count + 1 — how many match

Deep study

Computational Thinking · Week 2 — Iteration and selection

Week map

Accumulation notation

• Sum accumulator → start 0 → total ← total + x.
• Product accumulator → start 1 → prod ← prod * x.
• Count accumulator → start 0 → increment when condition holds.
• Max scan → start first item or sentinel → replace when x > best.

AND filter

Max and min scans

text
best ← first item
for each x in data:
    if x > best:
        best ← x

Pattern families

Easy — Sum or count list

• Sum all elements.
• Count how many equal a target.
• Product of list (start 1).

Medium — Max with AND filter

• Largest among entries meeting two tests.
• Count items where both predicates true.
• Sum only negatives in list.

Hard — Compound loop logic

• Multiple accumulators in one pass (sum and count together).
• Empty list: max undefined; count returns 0.
• Index loop vs for-each — same totals if bounds correct.

Worked mini-examples

• 4 yes, 6 yes → count 2.

Traps

• Product accumulator starting at 0 (always 0).
• OR filter when problem says “all conditions”.
• Max of empty without check.
• Off-by-one manual index: 0..len-1 not 0..len.
• Updating max before checking filter — polluted by ineligible items.

Diagnostic (try yourself)

1. List [7, -2, 4, 0, 11]. What is the sum of positive entries only?
2. Same list: how many entries are both positive and less than 10?
3. List [3, 9, 1, 9, 2]. What is the maximum value? If we only consider values ≥ 5, what is the max?
4. Why does product accumulation start at 1, not 0?
5. One pass: for [5, 10, 15, 20], find sum and count of multiples of 5.

ChatGPT prep archive

• Chatty reference archive

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

Core concepts

• Accumulation: start neutral (0 for sum, 1 for product) then combine each item.
• Max/min scan: keep best-so-far; compare each element.
• Selection with AND: all conditions must pass—think checklist.
• Loop guard: ensure collection non-empty before picking first as max.

Notation & vocabulary

Pattern families

Easy — Sum list

Medium — Max with AND filter

Hard — Count satisfying all

Drill these on the pattern atlas — filter to week 2.

Traps

• Max of empty without check.
• OR used when problem says all conditions.
• Accumulator wrong neutral element.
• Off-by-one in manual index loop.

Retrieval prompts

1. Initial value for product accumulation?
2. How does AND filter differ from OR?
3. What if max is sought on empty data?

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 2.
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