Loops in Python | Python Interview Questions

What You'll Learn

For loops and iteration patterns
While loops and when to use them
Loop control: break, continue, else
Useful built-in functions: range, enumerate, zip
Performance considerations

Understanding Python Loops

Loops allow you to execute code repeatedly. Python's loop syntax is clean and readable, emphasizing iteration over sequences rather than manual index management.

For Loop

The for loop iterates over any iterable (lists, strings, ranges, etc.):

code.pyPython

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

# Iterate over string
for char in "Python":
    print(char)

# Iterate over dictionary
person = {"name": "John", "age": 30}
for key in person:
    print(key)  # Keys only

for key, value in person.items():
    print(f"{key}: {value}")  # Key-value pairs

The range() Function

Generate sequences of numbers:

code.pyPython

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

# range(start, stop)
for i in range(2, 6):
    print(i)  # 2, 3, 4, 5

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

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

enumerate() - Get Index and Value

code.pyPython

fruits = ["apple", "banana", "cherry"]

# Without enumerate (not Pythonic)
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# With enumerate (Pythonic!)
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# Start from different index
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}: {fruit}")  # 1: apple, 2: banana, 3: cherry

zip() - Parallel Iteration

code.pyPython

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]

# Iterate two lists together
for name, age in zip(names, ages):
    print(f"{name} is {age}")

# Zip three lists
cities = ["NYC", "LA", "Chicago"]
for name, age, city in zip(names, ages, cities):
    print(f"{name}, {age}, {city}")

# Unequal lengths - stops at shortest
list1 = [1, 2, 3]
list2 = [4, 5]
for a, b in zip(list1, list2):
    print(a, b)  # (1,4), (2,5) - stops at 2 pairs

While Loop

Execute while a condition is true:

code.pyPython

# Basic while
count = 0
while count < 5:
    print(count)
    count += 1

# Infinite loop with break
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

# Common pattern: processing until condition
data = [1, 2, 3, 4, 5]
while data:  # While list is not empty
    item = data.pop()
    print(item)

Loop Control Statements

code.pyPython

# break - exit loop immediately
for i in range(10):
    if i == 5:
        break
    print(i)  # 0, 1, 2, 3, 4

# continue - skip to next iteration
for i in range(5):
    if i == 2:
        continue
    print(i)  # 0, 1, 3, 4

# pass - do nothing (placeholder)
for i in range(5):
    if i == 2:
        pass  # TODO: handle this case
    print(i)  # 0, 1, 2, 3, 4

The else Clause (Unique to Python)

Executes when loop completes without break:

code.pyPython

# else runs after loop completes
for i in range(3):
    print(i)
else:
    print("Loop completed normally")

# else doesn't run if break occurs
for i in range(5):
    if i == 3:
        break
else:
    print("This won't print")

# Practical use: search pattern
def find_item(items, target):
    for item in items:
        if item == target:
            print(f"Found {target}")
            break
    else:
        print(f"{target} not found")

Nested Loops

code.pyPython

# 2D iteration
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for num in row:
        print(num, end=" ")
    print()  # New line after each row

# With indices
for i in range(3):
    for j in range(3):
        print(f"({i},{j})", end=" ")
    print()

Interview Tip

When asked about Python loops:

Use for for iterating over sequences
Use while for condition-based loops
Always prefer enumerate() over range(len())
Use zip() for parallel iteration
Understand the else clause on loops
Avoid modifying a list while iterating over it