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Python for Loop (With range, enumerate, zip)

Modified: | Tags: Python

This article provides an overview of for loops in Python, including basic syntax and examples of using functions like range(), enumerate(), zip(), and more within for loops.

Contents

Python also has a while statement that repeats the process as long as the condition is True.

Basic syntax of for loops in Python: for ... in ... :

Python's for is similar to foreach in other programming languages

In many programming languages like C, the for loop uses a counter variable and a continuation condition:

for (int i = 0; i < 10; i++) {
    ...
}

In Python, the for loop doesn't require a counter variable, and you can use any name for the loop variable:

for variable in iterable_object:
    ...

This is similar to the foreach loop in other languages, where elements of iterable objects, such as lists, are assigned to variables in sequence.

Here is an example:

l = ['Alice', 'Bob', 'Charlie']

for name in l:
    print(name)
# Alice
# Bob
# Charlie
source: for_usage.py

Note that blocks are expressed with indentation (usually four spaces) rather than brackets in Python.

Break a for loop: break

You can break a for loop using break.

l = ['Alice', 'Bob', 'Charlie']

for name in l:
    if name == 'Bob':
        print('!!BREAK!!')
        break
    print(name)
# Alice
# !!BREAK!!
source: for_usage.py

Use an if statement for conditional branching.

Continue to the next iteration: continue

You can skip the current iteration and proceed to the next one using continue.

While break terminates the entire for loop, continue skips the remaining code in the current iteration and moves on to the next.

l = ['Alice', 'Bob', 'Charlie']

for name in l:
    if name == 'Bob':
        print('!!SKIP!!')
        continue
    print(name)
# Alice
# !!SKIP!!
# Charlie
source: for_usage.py

Execute code after normal termination: else

The else clause can be used to execute code after the for loop completes its normal execution.

l = ['Alice', 'Bob', 'Charlie']

for name in l:
    print(name)
else:
    print('!!FINISH!!')
# Alice
# Bob
# Charlie
# !!FINISH!!
source: for_usage.py

If the for loop is terminated by break, the code in the else clause is not executed.

for name in l:
    if name == 'Bob':
        print('!!BREAK!!')
        break
    print(name)
else:
    print('!!FINISH!!')
# Alice
# !!BREAK!!
source: for_usage.py

When using continue, the for loop completes normally, so the code in the else clause is executed.

for name in l:
    if name == 'Bob':
        print('!!SKIP!!')
        continue
    print(name)
else:
    print('!!FINISH!!')
# Alice
# !!SKIP!!
# Charlie
# !!FINISH!!
source: for_usage.py

By using else and continue, you can break out of nested loops. See the following article for details.

Extract specific elements: slicing

To extract specific elements, use slicing with the syntax [start:stop], where start and stop are indices starting at 0. Note that the stop index is excluded.

l = ['A', 'B', 'C', 'D', 'E', 'F', 'G']

for c in l[2:5]:
    print(c)
# C
# D
# E
source: for_usage.py

You can omit start and stop, or use [start:stop:step] to extract elements at every step. Here are examples of extracting only odd-numbered and even-numbered elements.

for c in l[::2]:
    print(c)
# A
# C
# E
# G

for c in l[1::2]:
    print(c)
# B
# D
# F
source: for_usage.py

For more details on slicing, see the following article.

Counter (index): range()

You can use the range() function to create a counter (index) for a for loop.

for i in range(3):
    print(i)
# 0
# 1
# 2
source: for_range.py

In Python 3, range() creates a range object, and its content is not displayed when printed with print(). For explanation purposes, the following example uses list() to convert the range object to a list. You don't need to convert it to a list in a for loop.

The range() function accepts different numbers of arguments:

  • range(stop): consecutive numbers 0 <= i < stop
  • range(start, stop): consecutive numbers start <= i < stop
  • range(start, stop, step): integer values at every step in the range start <= i < stop
    • Specifying a negative value for step will result in a reverse order
print(list(range(6)))
# [0, 1, 2, 3, 4, 5]

print(list(range(10, 13)))
# [10, 11, 12]

print(list(range(0, 10, 3)))
# [0, 3, 6, 9]

print(list(range(10, 0, -3)))
# [10, 7, 4, 1]
source: for_range.py

When used in a for loop, it looks like the following:

for i in range(10, 0, -3):
    print(i)
# 10
# 7
# 4
# 1
source: for_range.py

See the following article for details of range().

Elements and their indices: enumerate()

To simultaneously retrieve elements and their respective indices from an iterable object, such as a list, within a for loop, use the enumerate() function.

l = ['Alice', 'Bob', 'Charlie']

for name in l:
    print(name)
# Alice
# Bob
# Charlie

for i, name in enumerate(l):
    print(i, name)
# 0 Alice
# 1 Bob
# 2 Charlie
source: enumerate_start.py

You can specify a starting value for the second argument of enumerate(), allowing you to start from an arbitrary integer value instead of 0.

for i, name in enumerate(l, 1):
    print(i, name)
# 1 Alice
# 2 Bob
# 3 Charlie

for i, name in enumerate(l, 42):
    print(i, name)
# 42 Alice
# 43 Bob
# 44 Charlie
source: enumerate_start.py

Unlike range(), enumerate() does not have an argument for specifying a step value. However, you can achieve the same effect as follows.

step = 3
for i, name in enumerate(l):
    print(i * step, name)
# 0 Alice
# 3 Bob
# 6 Charlie
source: enumerate_start.py

Multiple lists: zip()

To retrieve elements from multiple iterable objects, such as lists, and assign them to multiple variables within a for loop, use the zip() function.

names = ['Alice', 'Bob', 'Charlie']
ages = [24, 50, 18]

for name, age in zip(names, ages):
    print(name, age)
# Alice 24
# Bob 50
# Charlie 18
source: zip_example.py

You can also combine three or more iterable objects.

points = [100, 85, 90]

for name, age, point in zip(names, ages, points):
    print(name, age, point)
# Alice 24 100
# Bob 50 85
# Charlie 18 90
source: zip_example.py

For details on how it behaves when the number of elements is different, as well as additional information, see the following article.

As demonstrated in the example above, zip() allows you to retrieve elements from multiple iterable objects in sequence. If you want to obtain all possible combinations of elements from multiple iterable objects, you can use the itertools.product() function, which will be described later.

Multiple lists with indices: enumerate(), zip()

To iterate through multiple lists with their indices, you can combine the enumerate() and zip() functions. When doing so, make sure to enclose the variable names for the zip() part in parentheses ().

names = ['Alice', 'Bob', 'Charlie']
ages = [24, 50, 18]

for i, (name, age) in enumerate(zip(names, ages)):
    print(i, name, age)
# 0 Alice 24
# 1 Bob 50
# 2 Charlie 18
source: for_enumerate_zip.py

Reverse order: reversed()

Use the reversed() function to iterate through elements of an iterable object, such as a list, in reverse order.

l = ['Alice', 'Bob', 'Charlie']

for name in reversed(l):
    print(name)
# Charlie
# Bob
# Alice
source: for_reversed.py

range objects can be reversed by either using the reversed() function or by specifying a negative value for step.

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

for i in range(2, -1, -1):
    print(i)
# 2
# 1
# 0
source: for_reversed.py

enumerate objects cannot be reversed directly; you need to convert them to lists using list(). If you want to reverse only the iterable and not the indices, you can use reversed() within enumerate().

# for i, name in reversed(enumerate(l)):
#     print(i, name)
# TypeError: 'enumerate' object is not reversible

for i, name in reversed(list(enumerate(l))):
    print(i, name)
# 2 Charlie
# 1 Bob
# 0 Alice

for i, name in enumerate(reversed(l)):
    print(i, name)
# 0 Charlie
# 1 Bob
# 2 Alice
source: for_reversed.py

zip objects cannot be reversed either. You need to convert them to lists with list().

l2 = [24, 50, 18]

# for name, age in reversed(zip(l, l2)):
#     print(name, age)
# TypeError: 'zip' object is not reversible

for name, age in reversed(list(zip(l, l2))):
    print(name, age)
# Charlie 18
# Bob 50
# Alice 24
source: for_reversed.py

Nested loops: itertools.product()

In Python, nested loops are created by adding more indentation levels to represent nested code blocks.

l1 = [1, 2, 3]
l2 = [10, 20, 30]

for i in l1:
    for j in l2:
        print(i, j)
# 1 10
# 1 20
# 1 30
# 2 10
# 2 20
# 2 30
# 3 10
# 3 20
# 3 30
source: break_nested_loops.py

You can get the same result using itertools.product().

import itertools

l1 = [1, 2, 3]
l2 = [10, 20, 30]

for i, j in itertools.product(l1, l2):
    print(i, j)
# 1 10
# 1 20
# 1 30
# 2 10
# 2 20
# 2 30
# 3 10
# 3 20
# 3 30
source: break_nested_loops_itertools_product.py

It is also possible to pass three or more iterables as arguments to itertools.product(). See the following article for details.

If you want to break out of a loop inside nested loops using break, consider using itertools.product() to simplify the process.

dict objects in for loop

When using a for loop to iterate over a dict object, you can access the dictionary keys.

d = {'key1': 1, 'key2': 2, 'key3': 3}

for k in d:
    print(k)
# key1
# key2
# key3
source: dict_keys_values_items.py

If you want to get values or key-value pairs, use the values() or items() methods.

for v in d.values():
    print(v)
# 1
# 2
# 3
source: dict_keys_values_items.py
for k, v in d.items():
    print(k, v)
# key1 1
# key2 2
# key3 3
source: dict_keys_values_items.py

See the following article for details.

List comprehensions

List comprehensions provide a simpler way to create a new list by processing the elements of iterable objects, compared to using a for loop.

List comprehensions are written as follows.

[expression for variable_name in iterable]

Here are some examples along with equivalent for loops.

squares = [i**2 for i in range(5)]
print(squares)
# [0, 1, 4, 9, 16]

squares = []
for i in range(5):
    squares.append(i**2)

print(squares)
# [0, 1, 4, 9, 16]
source: list_comprehension.py

See the following article for details on list comprehensions.

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