🐍 Python📅 Day 7

Loops in Python

for and while loops, range(), break, continue, else on loops and enumerate.

🎯

Learning objectives

→Use for over any iterable
→Use while for condition-driven loops
→Master range(), enumerate() and zip()

💡 Key points

for iterates over an iterable (list, str, dict, range, file, generator).
while runs as long as a condition is True.
range(start, stop, step) generates numbers lazily — great for counted loops.
break exits early; continue skips to the next iteration.
for/while can have an else clause — runs when the loop completes without break.

💻 Code examples(5)

#1for + range

python

for i in range(5):          # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):       # 1..5
    print(i)

for i in range(0, 10, 2):   # 0, 2, 4, 6, 8
    print(i)

range is upper-exclusive. range(n) starts at 0. Step can be negative for countdown: range(10, 0, -1).

#2for over collections

python

names = ["Amit", "Priya", "Ravi"]
for name in names:
    print(name.upper())

# need index too? use enumerate
for i, name in enumerate(names, start=1):
    print(f"{i}. {name}")

Output

AMIT
PRIYA
RAVI
1. Amit
2. Priya
3. Ravi

Never write for i in range(len(x)) — always prefer enumerate.

#3while loop

python

n = 10
while n > 0:
    print(n)
    n -= 1

# infinite loop with break
while True:
    cmd = input("cmd> ")
    if cmd == "quit":
        break

while true is idiomatic. Always have a break or condition that flips — else infinite loop.

#4break, continue, else

python

for n in [3, 5, 7, 11]:
    if n % 2 == 0:
        print("Even found!")
        break
    print(n)
else:
    print("All odd.")

Output

3
5
7
11
All odd.

The else on a for loop runs only when the loop completes without hitting break. Great for search patterns.

#5Parallel iteration with zip

python

names = ["Amit", "Priya", "Ravi"]
marks = [87, 92, 76]

for name, mark in zip(names, marks):
    print(f"{name}: {mark}")

zip pairs items element-wise. Stops at the shortest iterable.

🎯 Practice

Q1. Print numbers 1 to 20 skipping multiples of 3.+

for n in range(1, 21): if n % 3 == 0: continue print(n)

Q2. When does the for/else block execute?+

When the loop finishes without executing break. Useful for 'not found' checks.

Q3. Difference between range(5) and range(0, 5)?+

Identical — both yield 0, 1, 2, 3, 4. range(5) is shorthand when start is 0.

📝 Notes

List comprehensions (loop shorthand)

Instead of:

squares = []
for x in range(1, 6):
    squares.append(x * x)

Write:

squares = [x * x for x in range(1, 6)]      # [1, 4, 9, 16, 25]
evens   = [x for x in range(20) if x % 2 == 0]

More Pythonic and faster. Same idea works with {} for sets/dicts and () for generators.

Avoid modifying the collection you iterate

# ❌ risky
for item in nums:
    if item < 0: nums.remove(item)

# ✅ iterate over a copy or build a new list
nums = [x for x in nums if x >= 0]