🧭 Control Flow

Why it matters

Control flow is how you translate problem statements into steps.

The idea

Conditionals choose paths; loops repeat work without copy-paste. Prefer for over while whenever you know the iterable up front — for is harder to misread and harder to make infinite.

Try it

if / elif / else ladder, with a small game classifier:

def grade(score):
  if score >= 90: return "A"
  elif score >= 80: return "B"
  elif score >= 70: return "C"
  elif score >= 60: return "D"
  else: return "F"

for s in [95, 82, 71, 64, 30]:
  print(s, grade(s))

for over a range, with enumerate to get both index and value:

names = ["Ada", "Linus", "Margaret", "Grace"]
for i, n in enumerate(names, start=1):
  print(f"{i:>2}. {n}")

while is best when the stop condition is a state, not a count. break exits early; continue skips to the next iteration.

# Find the first square > 100
n = 1
while True:
  if n * n > 100:
      print("first n with n^2 > 100:", n)
      break
  n += 1

Quick check

• Q: When do you use continue? A: Skip the rest of the loop body and go to the next iteration.

Mini drills

• Sum all even numbers from 1 to 20.
• Print numbers 10 to 1 using a while loop.

Common mistakes

• Mistake: Infinite loop in while. Fix: Update the loop variable.
• Mistake: Off-by-one in range. Fix: Remember range is end-exclusive.

Key takeaways

• Use for over an iterable when you can; while when the stop is state-based.
• enumerate(seq) beats range(len(seq)) for index+value loops.
• break exits, continue skips — both are fine in moderation.
• Indentation IS the block; no {} to forget.