Loops#

Loops in Python allow you to execute a block of code repeatedly, either a known number of times or until a condition is met.

For Loop#

The for loop iterates over an iterable (like a list, tuple, string, range, or dictionary).

Syntax:

for item in iterable:
    # do something with item

Looping through common iterables:

# List
for fruit: str in ["apple", "banana", "cherry"]:
    print(fruit)

# Tuple
for coord: int in (0, 1, 2):
    print(coord)

# String
for char:str in "hello":
    print(char)

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

# Dictionary
ages: dict[str, int] = {"Alice": 30, "Bob": 25}

# iterates over keys
for name in ages:
    print(name, ages[name])

# or using items() to get key-value pairs
for name, age in ages.items():
    print(name, age)

Break and Continue#

  • break exits the loop immediately.

  • continue skips to the next iteration.

for i: int in range(10):
    if i == 5:
        break       # stops the loop when i == 5
    if i % 2 == 0:
        continue    # skips even numbers
    print(i)        # prints 1, 3

For Loop with Else#

The else block runs only if the loop completes without encountering break.

for i: int in range(3):
    print(i)
else:
    print("Done without break")

List Comprehension#

A concise way to build lists from iterables.

Syntax:

[ expression for item in iterable if condition ]

Example:

squares: list[int] = [x*x for x in range(6)]      # [0, 1, 4, 9, 16, 25]
evens: list[int]  = [x for x in range(10) if x%2==0]  # [0, 2, 4, 6, 8]

While Loop#

Repeats a block while a condition is True.

Syntax:

while condition:
    # do something

Loop termination condition: when condition becomes False.

count: int = 3
while count > 0:
    print(count)
    count -= 1

Break and Continue in While#

n: int = 0
while True:
    n += 1
    if n == 3:
        continue    # skip printing 3
    if n > 5:
        break       # exit when n > 5
    print(n)        # prints 1, 2, 4, 5

While Loop with Else#

else executes if the loop terminates without break:

i: int = 0
while i < 3:
    print(i)
    i += 1
else:
    print("Finished cleanly")

Nested Loops#

You can place loops inside loops for multi-dimensional iteration:

for i: int in range(3):
    for j: int in range(2):
        print(f"i={i}, j={j}")

Applications of Loops#

  • Iterating lists:

    names: list[str] = ["Anna", "Ben", "Cara"]
for name: str in names:
    print(name.upper())

  • Calculating sum:

    total: int = 0
for num: int in [1, 2, 3, 4]:
    total += num
print(total)  # 10

  • Searching:

    target: int = 7
found: bool = False
for x: int in range(10):
    if x == target:
        found = True
        break
print("Found!" if found else "Not found")

  • Processing strings:

    text: str = "Hello World"
vowels: list[str] = [ch for ch in text if ch.lower() in "aeiou"]
print(vowels)  # ['e', 'o', 'o']

Additional Resources#