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Hello Python
4/8

Control Flow

Topic 4 of 8, with 4 concept checks. Conditionals, loops, and Python's loop-else quirk

Make every branch and loop condition explicit

Execution paths

Trace which condition controls each path, what changes on every iteration, and how break, continue, and loop else clauses affect the route through a Python program.

Core lesson 01

break exits the loop. continue skips to the next iteration. pass does nothing — a syntactic placeholder.

These three keywords control loop execution differently. `break` stops the nearest enclosing loop immediately, skipping any remaining iterations. `continue` ends just the current iteration early and moves to the next one. `pass` isn't loop-specific at all — it's a no-op statement used anywhere Python requires a statement syntactically (empty function bodies, stub classes, placeholder branches).

Python example
for i in range(5):
    if i == 3:
        break
    print(i)          # 0 1 2

for i in range(5):
    if i == 3:
        continue
    print(i)          # 0 1 2 4

if True:
    pass              # placeholder, does nothing

What to remember

What's the difference between break, continue, and pass?

Common footguns

  • `break`/`continue` inside a nested loop only affect the innermost loop — there's no labeled break in Python.

Core lesson 02

The else block runs only if the loop completes without a break — a clean way to express 'search and not found'.

`for...else` (and `while...else`) is one of Python's more obscure but genuinely useful constructs. The `else` block executes when the loop finishes normally, i.e. the iterable was exhausted or the while-condition became false — but is *skipped* if the loop was exited via `break`. This maps naturally onto search loops where `break` means 'found it, stop early'.

Python example
def has_factor(n, candidates):
    for c in candidates:
        if n % c == 0:
            print(f"{c} divides {n}")
            break
    else:
        print("no factor found")

has_factor(15, [2, 4, 6])   # no factor found
has_factor(15, [2, 3, 6])   # 3 divides 15

What to remember

What does the else clause on a for loop do?

Common footguns

  • Assuming `else` runs when the loop 'fails' or is empty — it actually runs by default and is skipped only by break, which is the opposite of what the name suggests to most people.
for item in items:
    if item == target:
        break
else:
    print("not found")   # only runs if break never happened

Core lesson 03

result = value_if_true if condition else value_if_false — reads almost like English.

Python deliberately ordered the ternary expression to read left-to-right like prose, unlike C's `condition ? a : b`. It's an expression, not a statement, so it can be used anywhere a value is expected — assignments, function arguments, list comprehensions.

Python example
n = 7
label = 'even' if n % 2 == 0 else 'odd'
print(label)                       # odd

vals = [x if x > 0 else 0 for x in [-2, 3, -1, 5]]
print(vals)                        # [0, 3, 0, 5]

What to remember

How do you write a one-line conditional expression (ternary) in Python?

Common footguns

  • Chaining several ternaries on one line for multi-branch logic — technically legal but hurts readability fast; use if/elif/else instead.

Core lesson 04

It simulates a do-while: run the body at least once, then decide whether to exit — useful when the exit check needs data from inside the loop.

Some languages have a do-while loop that guarantees at least one execution before checking the condition. Python doesn't, so `while True:` combined with a conditional `break` is the idiomatic substitute — common for input-validation loops where you don't know if input is valid until after you've read it.

Python example
while True:
    raw = input("Enter a positive number: ") if False else "7"  # simulated input
    if raw.isdigit() and int(raw) > 0:
        n = int(raw)
        break
    print("invalid, try again")
print("got", n)

What to remember

Why might you use while True: with an internal break?

Common footguns

  • Forgetting the break entirely, or putting it behind a condition that can never become true — an accidental infinite loop.

Python lab

Browser Python lab

Runtime · idle

Python loads on your first run. Your code stays in this browser.

Best practices

  • Avoid deep if/else nesting — prefer early returns or guard clauses.
  • Use `for item in iterable` instead of `for i in range(len(iterable))` when the index isn't needed.
  • Use enumerate() instead of manually tracking an index counter.
  • Keep loop bodies short — extract logic into a function if a loop body grows large.

References

Python Docs — Control FlowReal Python — Python 'for' Loops

Apply the concept in Interview practice

FizzBuzzeasyHackerRank · O(n) time

Loop 1..n, checking divisibility by 15, then 3, then 5, printing accordingly, else the number itself.

Open problem
Climbing StairseasyLeetCode #70 · O(n) time, O(1) space

Fibonacci in disguise: ways(n) = ways(n-1) + ways(n-2). Build iteratively with two variables.

Open problem
Search Insert PositioneasyLeetCode #35 · O(log n) time

Binary search with low/high pointers; when the loop ends, low is the correct insertion index.

Open problem
Jump GamemediumLeetCode #55 · O(n) time, O(1) space

Track the farthest index reachable so far while scanning left to right; if the current index ever exceeds that farthest reach, return False.

Open problem
Missing NumbereasyLeetCode #268 · O(n) time, O(1) space

XOR all indices 0..n with all array values (every number cancels except the missing one), or use sum(range(n+1)) - sum(nums).

Open problem

Concept checks

Q01

What's the difference between break, continue, and pass?

Q02

What does the else clause on a for loop do?

Q03

How do you write a one-line conditional expression (ternary) in Python?

Q04

Why might you use while True: with an internal break?