Repetition With Loops

Computers shine when they do the same job thousands of times without complaint. Loops are how Python tells the machine to repeat. In this chapter we introduce the for and while loops, the break, continue and else clauses, and the iteration patterns you'll meet in every real-world Python project.

• Use for x in iterable: to walk through every element.
• Use while condition: to repeat while a condition stays true.
• break exits the nearest enclosing loop immediately.
• continue skips the rest of this iteration and moves on to the next.
• pass is a no-op — useful as a placeholder.

# for-loop syntax
for item in iterable:
    # do something with item

# while-loop syntax
while condition:
    # do something
    # remember to update the condition

# loop control
for x in data:
    if x is None:
        continue           # skip this round
    if x == 'STOP':
        break              # exit the loop
    process(x)

• Why we use loops: Without loops you would copy-paste the same block over and over. Loops let you write one statement that runs many times, varying inputs each round.
• The for loop: for iterates over an iterable — anything that produces items one at a time (list, tuple, string, dict, set, file, range, generator). Python automatically stops when the iterable is exhausted.
• The while loop: while keeps running its body as long as a boolean expression is true. You must change something inside the body so the condition eventually becomes false — otherwise you get an infinite loop.
• break, continue and pass: break jumps out of the loop entirely. continue short-circuits to the next iteration. pass does nothing — useful as a placeholder while you sketch logic.
• The else clause: Python loops accept an else block that runs only if the loop finishes without break. Great for search algorithms.
• Nested loops: Loops inside loops let you scan grids, multiplication tables, or matrices. Watch the time complexity — a 100×100 nested loop runs 10,000 times.

for i in range(1, 6):
    print(i)

n = 5
while n > 0:
    print(n)
    n -= 1
print("Liftoff!")

n = 10
total = 0
for i in range(1, n + 1):
    total += i
print("Sum:", total)

data = [3, 7, 11, 15, 4, 9]
for x in data:
    if x % 2 == 0:
        print("First even:", x)
        break

nums = [4, -2, 7, -9, 6]
positive_sum = 0
for n in nums:
    if n < 0:
        continue
    positive_sum += n
print("Sum of positives:", positive_sum)

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i}x{j}={i*j}", end="  ")
    print()

• Prefer for when you know the iterable in advance; use while when the stopping condition is dynamic.
• Always update the loop variable inside a while body to avoid infinite loops.
• Use range(start, stop, step) for numeric sequences without storing the full list.
• Use enumerate(iterable) when you need both index and value.
• Use zip(a, b) to walk through two sequences in parallel.
• Avoid modifying a list while iterating over it — iterate over a copy instead.

• Infinite loop: forgetting to update the condition in a while.
• Off-by-one error: range(1, 10) stops at 9, not 10.
• Mutating during iteration: deleting list items inside a for-loop changes indices unexpectedly.
• Misplaced break: breaks out of the innermost loop only, not all nested ones.
• Heavy work inside the loop: compute constants once outside the loop, not on every iteration.
• Using range(len(x)) when not needed: iterate directly with for item in x.

• Problem 1: Print the multiplication table of any number entered by the user.
• Problem 2: Count how many vowels are present in a given sentence.
• Problem 3: Read 10 numbers and print the largest and the smallest.
• Problem 4: Use a while-loop to find the factorial of a given number.
• Problem 5: Print all prime numbers between 1 and 100.
• Problem 6: Build a simple password retry script — give the user 3 chances to log in.

• Q1. What is the difference between for and while loops?
• Q2. How does the loop else clause behave in Python?
• Q3. Explain break, continue and pass with examples.
• Q4. What is an infinite loop and how can you intentionally create one safely?
• Q5. What is the time complexity of nested loops?
• Q6. How would you iterate over two lists in parallel?

• Q1: When should I use for vs while?
  Use for when you know what to iterate over (a list, a range, a file). Use while when the loop ends based on a condition that changes during execution.
• Q2: What does the loop's else block do?
  It runs when the loop finishes naturally (without a break). It is commonly used to indicate that no match was found in a search loop.
• Q3: How do I exit two nested loops at once?
  Wrap them in a function and use return, or use a flag variable, or raise/catch an exception — Python has no labelled break.
• Q4: What is the difference between break and continue?
  break exits the loop entirely; continue skips the remainder of the current iteration and starts the next one.
• Q5: Can I iterate a dictionary?
  Yes — for key in d, for value in d.values(), or for key, value in d.items().
• Q6: Why is my while loop never ending?
  Most likely the variable used in the condition is never updated inside the body. Print the value during the loop to debug.