python-week4

Week map

Loop notation

• for i in range(n): → counted loop → i takes each value produced by range → body runs once per value.
• range(stop) → integers 0, 1, …, stop-1 → range(4) → 0,1,2,3 (four values).
• range(start, stop) → from start inclusive to stop exclusive → range(2,6) → 2,3,4,5.
• range(start, stop, step) → step size → range(0,10,2) → 0,2,4,6,8.

Accumulator

python
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total = 0          # neutral for sum
for k in range(5):
    total = total + k
# total = 0+1+2+3+4 = 10

for vs while

• for — known iteration count or explicit sequence.
• while cond: — repeat until cond false; must ensure progress toward termination.

python
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n = 8
while n > 0:
    n = n - 3
# n goes 8→5→2→-1, stops; loop ran 3 times

Pattern families

Easy — Count iterations

• How many values does range(a,b) produce? → max(0, b-a) for step 1.
• List indices visited: range(len(s)) for string s.
• Final loop variable value after for (last assigned value remains).

Medium — Accumulator trace

• Sum 1..n, sum of squares, count positives in fixed list.
• Product accumulator starts at 1, not 0.
• Print once after loop vs each iteration.

Hard — Combined condition in loop

• Sum only entries meeting predicate (if x % 2 == 0 inside loop).
• while with compound update; detect infinite loop risk if condition never changes.
• Nested setup for week 5 preview: outer i, inner j — count pairs briefly.

Worked mini-examples

python
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s = 0
for i in range(1, 11):
    s += i
# s = 55

python
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data = [4, 7, 2, 9, 0, 6]
c = 0
for x in data:
    if x % 2 == 0:
        c += 1
# c = 4

python
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val = 20
steps = 0
while val >= 10:
    val = val - 6
    steps += 1
# val = 8, steps = 2

python
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t = 0
for j in range(0, 15, 3):
    t += j
# j: 0,3,6,9,12 → t = 30

Traps

• range(1, n) vs range(n) — classic off-by-one.
• Forgetting accumulator initialization (total = 0 missing).
• Modifying loop variable expecting to change how many iterations — range already fixed.
• while True without break path.
• Using = instead of += in accumulation (total = x resets each time).

Diagnostic (try yourself)

1. How many numbers does range(5, 15) generate? List them.
2. What is the final value of sum?

python
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sum = 0
for i in range(2, 8):
    sum = sum + i

1. What is printed?

python
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prod = 1
for k in range(1, 5):
    prod = prod * 2
print(prod)

1. Write a loop that prints integers 10 down to 1 (inclusive) using range with a negative step.
2. After this loop, what is n?

python
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n = 50
while n > 1:
    n = n // 2