Iteration

For Loop

The for loop allows iteration over elements in a collection (such as lists, strings, or ranges).

Iterating Over a List

Example:

colors = ['Red', 'Black', 'White']
for color in colors:
    print(f'Is {color} your favorite?')

Output:

Is Red your favorite?
Is Black your favorite?
Is White your favorite?

Iterating Over Numbers and Calculations

Example:

for number in [1, 2, 3, 4]:
    double = number * 2
    print(f'Twice {number} is {double}')

Output:

Twice 1 is 2
Twice 2 is 4
Twice 3 is 6
Twice 4 is 8

Iterating Over Characters of a String

Example:

for character in 'hello':
    print(character)

Output:

h
e
l
l
o

Iterating Over a Range of Numbers

Example:

for i in range(0, 50, 5):
    print(i, end=' ')

Output:

0 5 10 15 20 25 30 35 40 45

Skipping Items in a List

You can use slicing to iterate over every other item in the list

Example:

colors = ['Red', 'Black', 'Yellow', 'Green', 'Blue', 'Purple']
for color in colors[::2]:
    print(color, end=' ')

Output:

Red Yellow Blue

While Loop

The while loop repeats a block of code as long as a given condition is True.

Example:

countdown = 10
while countdown >= 0:
    print(countdown, end=' ')
    countdown -= 1

Output:

10 9 8 7 6 5 4 3 2 1 0

Iterators

An iterator allows you to traverse through a collection, such as a list, tuple, set, or string.

Once an iterator has exhausted its values, it will raise an error if you attempt to retrieve another element.

Example:

colors = ['Red', 'Black', 'Yellow', 'Green', 'Blue', 'Purple']
iter_color = iter(colors)
print(next(iter_color))
print(next(iter_color))

Output:

Red
Black

Behind the scenes, a for loop implicitly calls iter() and next() to iterate over the elements.

Range

range() generates an immutable sequence of numbers and is commonly used for looping a specific number of times in a for loop.

Syntax:

range(start, stop, step)

start: Starting value (inclusive)
stop: Ending value (exclusive)
step: Increment (default is 1)

Example:

list(range(0, 10, 2))

Output:

[0, 2, 4, 6, 8]

The range object only stores the values for start, stop, and step, and does not hold all the numbers in memory.

Using range with For Loop

Example:

for i in range(0, 10, 2):
    print(i, end=' ')

Output:

0 2 4 6 8

Range Slicing and Negative Indices

You can perform slicing and index lookups on range objects.

Example:

# Containment test
print(11 in range(0, 20, 2))  # False
print(10 in range(0, 20, 2))  # True

# Index lookup
print(range(0, 20, 2).index(10))  # 5

# Accessing elements by index
print(range(0, 20, 2)[5])  # 10

# Accessing elements by negative index
print(range(0, 20, 2)[-1])  # 18

Output:

False
True
5
10
18

Reversing a Range

Example:

for i in range(0, 10)[::-1]:
    print(i, end=', ')

Output:

9, 8, 7, 6, 5, 4, 3, 2, 1, 0,

continue in Loops

The continue statement skips the current iteration and continues with the next iteration of the loop.

Example:

for i in range(5):
    if i == 3:
        continue
    print(i, end=' ')

Output:

0 1 2 4

break in Loops

The break statement exits the loop.

Example:

for i in range(5):
    if i == 2:
        print('Breaking the loop')
        break
    print(i)

Output:

0
1
Breaking the loop

else in Loops

An else block can be used with for and while loops. It will run if the loop completes normally, without hitting a break statement.

Example with for:

for number in range(5):
    print(number, end=' ')
else:
    print('Loop completed without interruption.')

Output:

0 1 2 3 4 Loop completed without interruption.

Example with while:

countdown = 5
while countdown > 0:
    print(countdown)
    countdown -= 1
else:
    print('Countdown finished!')

Output:

5
4
3
2
1
Countdown finished!

Example with break:

for number in range(5):
    if number == 3:
        break
    print(number)
else:
    print('Loop completed without interruption.')

0
1
2

Enumerating over a collection with enumerate

The enumerate() function is useful when you need both the index and the value in a loop.

Example:

colors = ['Red', 'Black', 'Yellow', 'Green']
for index, color in enumerate(colors):
    print(f'{index}: {color}')

Output:

Red
Black
Yellow
Green

Using zip() to iterate over multiple collections

The zip() function allows to iterate over several collections at the same time.

Example:

names = ['Alice', 'Bob', 'Charlie']
scores = [85, 92, 78]

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

Output:

Alice: 85
Bob: 92
Charlie: 78

Last modified: 17 March 2025