python-week5

Week map

Nested loop notation

• Outer index → often row or first coordinate → runs its full range once per “round.”
• Inner index → completes entire range for each outer value → total body runs = outer count × inner count.
• break → exits innermost loop only → outer continues unless restructured.

python
Run Inline
count = 0
for i in range(3):
    for j in range(2):
        count += 1
# count = 6  (3 × 2)

Search pattern

python
Run Inline
found = False
for item in seq:
    if item == target:
        found = True
        break

Min/max pattern

python
Run Inline
best = seq[0]
for x in seq[1:]:
    if x > best:
        best = x

Pattern families

Easy — Nested count

Medium — Search and index

Hard — Min/max with updates

Worked mini-examples

python
Run Inline
pairs = []
for i in range(2):
    for j in range(3):
        pairs.append((i, j))
# 6 pairs: (0,0)..(1,2)

python
Run Inline
nums = [4, 7, 2, 9]
target = 7
found = False
for n in nums:
    if n == target:
        found = True
        break
# found is True

python
Run Inline
data = [-3, -8, -1, -5]
m = data[0]
for x in data:
    if x > m:
        m = x
# m = -1

python
Run Inline
for i in range(3):
    for j in range(3):
        if j == 1:
            break
    # inner breaks at j=1; outer continues
# outer runs 3 times; inner runs 2 times each → 6 inner bodies

Traps

• break only leaves inner loop in nested structure.
• Max initialized to 0 fails for all-negative data.
• Off-by-one: range(len) vs direct iteration.
• Inner variable shadows outer (for i inside for i).
• Counting iterations as n + m instead of n × m.

Diagnostic (try yourself)

1. How many times does the body run?

python
Run Inline
for a in range(4):
    for b in range(3):
        pass

1. After nested loops below, what is j?

python
Run Inline
for i in range(2):
    for j in range(5):
        pass

1. Write a loop that sets found = True and breaks when target appears in list items.
2. Find max of [-10, -3, -7] — why should best not start at 0?
3. In a 3×3 nested loop with break when j == 0 in the inner loop, how many inner bodies execute per outer step?