Iteration With The For Loop

The Python for loop is arguably the most-used construct in real production code. It walks over any iterable — lists, strings, dictionaries, tuples, files, generators — with a clean, readable syntax. In this lesson you'll go beyond range() and learn every pattern professional Python developers use every day.

• Basic form: for variable in iterable:
• Use range() for numeric sequences without building a list.
• Use enumerate() to get index and value together.
• Use zip() to walk through several iterables in parallel.
• Add an else block that runs when the loop completes without break.

# 1) Iterate over any sequence
for item in iterable:
    # work with item

# 2) Numeric range
for i in range(start, stop, step):
    print(i)

# 3) Index + value
for idx, val in enumerate(my_list):
    print(idx, val)

# 4) Parallel iteration
for x, y in zip(list_a, list_b):
    print(x, y)

• Iterables vs iterators: Anything that has an __iter__ method is iterable. A for loop hides this complexity — you simply write for x in collection and Python takes care of the rest.
• range(start, stop, step): range() produces lazily-evaluated integers. It is memory-efficient — range(1_000_000) uses just a few bytes. stop is exclusive.
• enumerate(): When you need both the index and the value, never write for i in range(len(lst)). Use enumerate(lst) — it's faster and far more idiomatic.
• zip() for parallel loops: Pair two or more iterables: for name, age in zip(names, ages). Stops at the shortest iterable; use itertools.zip_longest to keep going.
• Looping over dictionaries: By default, for k in d iterates over keys. Use d.values() or d.items() for values or key-value pairs.
• The for-else clause: Add else: at the same indent level as for. The else block runs when the loop ends without hitting a break — ideal for 'not found' messages.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

for i in range(0, 11, 2):
    print(i, end=' ')

students = ["Riya", "Ankit", "Meera"]
for index, name in enumerate(students, start=1):
    print(index, name)

marks = {"Math": 95, "Science": 88, "English": 92}
for subject, score in marks.items():
    print(subject, "=>", score)

names = ["Anu", "Vivek", "Sara"]
ages  = [21, 24, 19]
for n, a in zip(names, ages):
    print(f"{n} is {a} years old")

nums = [3, 7, 11, 19, 23]
target = 8
for n in nums:
    if n == target:
        print("Found!")
        break
else:
    print("Not found")

• Use direct iteration (for x in lst) — avoid for i in range(len(lst)).
• Combine enumerate() with start=1 for human-friendly numbering.
• zip() halts at the shortest sequence; use itertools.zip_longest if you need padding.
• Inside dict loops, .items() is the most explicit and self-documenting.
• List comprehensions are often faster and clearer than explicit for loops for simple transforms.
• Use break + the else clause to express 'search-and-fallback' logic.

• Modifying the list you're iterating: creates skipped items; iterate over list[:] or use a comprehension.
• Misunderstanding range: range(1, n) stops at n-1.
• Using indices needlessly: prefer enumerate over manual counters.
• Forgetting .items() on dicts: for k, v in d: raises ValueError.
• Inefficient lookups in the loop body: compute constants outside the loop.
• Misplaced else: it belongs at the for-block indent level, not inside it.

• Problem 1: Print all even numbers from 1 to 50 using range().
• Problem 2: Reverse a string using a for loop without slicing.
• Problem 3: Count how many times each letter appears in a given word.
• Problem 4: Compute the dot product of two equal-length lists using zip().
• Problem 5: Print a multiplication table from 1 to 10 in a clean grid using nested for loops.
• Problem 6: Read a list of student names and print 'Topper: ' for the highest scorer.

• Q1. How does Python's for loop differ from C-style for loops?
• Q2. What is the purpose of enumerate()? Show an example.
• Q3. When would you use zip() over a plain for?
• Q4. Can you iterate over a dictionary directly? What does it yield?
• Q5. What does the else block on a for loop mean?
• Q6. Why is iterating with for x in range(len(lst)) considered unidiomatic?

• Q1: Does range() create a list?
  No. range() returns a lazy range object. Convert it to a list with list(range(...)) only when you really need a list.
• Q2: How can I get both index and value?
  Use enumerate(my_list) — it produces (index, value) pairs.
• Q3: Can I loop over a string character by character?
  Yes. for c in 'hello' yields h, e, l, l, o one character at a time.
• Q4: What happens if I change the list while looping?
  Indices shift and items may be skipped. Iterate over my_list[:] (a copy) or build a new list with a comprehension.
• Q5: Is the for-else block really useful?
  Yes — especially for search loops where you want to do something only if no match was found. Place else at the for indent level.
• Q6: Are for loops faster than while loops?
  Usually yes for known iterables, because for is optimised and avoids extra condition checks each iteration.