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Iteration Statements (Loops)

Iteration statements, or loops, are used to execute a block of code repeatedly as long as a condition is met or for a sequence of elements.

1. The while Loop

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

count = 1
while count <= 3:
    print(count)
    count += 1

2. The for Loop and range()

A for loop is used for iterating over a sequence (such as a list, a tuple, a dictionary, a set, or a string).

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

The range() Function

To loop through a set of code a specified number of times, use the range() function. It returns a sequence of numbers, starting from 0 by default, and increments by 1 by default, ending before the specified number.

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

3. Loop Control Statements

  • break: Terminates the loop immediately.
  • continue: Stops the current iteration and jumps to the next one.
  • pass: Acts as a syntactic placeholder if you do not want to run any code in the loop body.
# break example
for x in range(5):
    if x == 3:
        break
    print(x)  # Prints 0, 1, 2

# continue example
for x in range(5):
    if x == 2:
        continue
    print(x)  # Prints 0, 1, 3, 4 (skips 2)

4. Idiomatic Looping Techniques

Looping with Index: enumerate()

Instead of creating a manual counter, use enumerate() to retrieve both the index and the item during a loop.

fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"Index {index}: {fruit}")

Looping in Parallel: zip()

Use zip() to iterate over two or more sequences of the same length in parallel.

names = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 92]

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

5. Loop else Clause

In Python, loops can have an else block. The else block runs only if the loop completed successfully without hitting a break statement.

for i in range(3):
    print(i)
else:
    print("Loop finished without break")  # This will print

for i in range(3):
    if i == 1:
        break
else:
    print("Loop finished")  # This will NOT print

6. List Comprehensions (Introductory)

A list comprehension provides a concise way to create lists using a single line of code instead of a full for loop.

# Creating squares using list comprehension
squares = [x ** 2 for x in range(1, 6)]
print(squares)  # Output: [1, 4, 9, 16, 25]

Further Reading