Loops (for and while)¶

Loops are used to execute a block of code repeatedly. Python provides two basic loop statements: for and while.

1. The for Loop¶

In Python, the for statement iterates over the items of any sequence (such as a list, tuple, or string) in the order that they appear.

Syntax¶

for item in sequence:
    # statements

languages = ["Python", "Java", "C++"]
for lang in languages:
    print(lang)

The range() Function¶

To iterate over a sequence of numbers, use the built-in range() function:

Syntax¶

range(stop)
range(start, stop[, step])

• start: Initial number (default 0).
• stop: Upper limit (exclusive; the sequence stops before this number).
• step: Increment value (default 1).

# 0 to 4
for i in range(5):
    print(i, end=" ")
print()

# 2 to 10 with step 2
for n in range(2, 11, 2):
    print(n, end=" ")

Looping Helper Functions: enumerate() and zip()¶

1. enumerate(): Index and Value Together¶

When looping through a sequence and tracking index position, use enumerate():

frameworks = ["Django", "FastAPI", "Flask"]
for index, item in enumerate(frameworks, start=1):
    print(f"{index}: {item}")

2. zip(): Multiple Sequences in Parallel¶

names = ["Alice", "Bob", "Charlie"]
scores = [90, 85, 95]

for name, score in zip(names, scores):
    print(f"{name} scored {score}")

2. The while Loop¶

A while loop executes as long as a specified condition remains True.

Syntax¶

while condition:
    # statements

# Fibonacci series: the sum of two preceding numbers
a, b = 0, 1
while a < 50:
    print(a, end=" ")
    a, b = b, a + b

Loop Control Statements: break, continue, and pass¶

• break: Terminates the current loop execution immediately.
• continue: Skips the remainder of the current iteration and advances to the next iteration.
• pass: Does nothing. It is used as a syntactic placeholder.

# break example
for n in range(1, 10):
    if n == 5:
        break
    print(n, end=" ")  # 1 2 3 4
print()

# continue example
for n in range(1, 6):
    if n == 3:
        continue  # Skip 3
    print(n, end=" ")  # 1 2 4 5

The else Clause on Loops¶

In Python, loops can have an else clause. The else block executes only if the loop completes without encountering a break statement:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(f"{n} equals {x} * {n // x}")
            break
    else:
        # Loop fell through without finding a factor
        print(f"{n} is a prime number")