for Loops

One of the defining features of computers is their ability to perform repetitive tasks with precision and speed. Python, like many other languages, offers powerful construct known as the for loop to automate and simplify such repetitive actions.

What is a for Loop?

A for loop is a programming structure that enables you to execute a specific code block repeatedly. It iterates over a sequence of items, performing the same actions for each item. Whether you want to process a list of values, manipulate strings, or iterate over the elements of a dictionary, the for loop is a go-to tool.

Basic Structure of a For Loop

In Python, a for loop is defined using the for keyword, followed by a variable (often called an iterator) that represents each item in the sequence, the in keyword, and the sequence itself. The indented code block immediately following the for statement defines what you want to do with each item in the sequence.

for i in range(1,3):
    # Code to be executed for each item
    print(i)

Would return:

1
2

Using For Loops with Lists

See Also

Lists in Python

One common use case for for loops is iterating over lists. Lists are collections of data elements, and you can effortlessly process each element in the list using a for loop. Here’s an example:

fav_books = ['mastery', 'the signal and the noise', 'the organized mind']

print("My favourite books include:")
for book in fav_books:
    print("   - " + book.title())

Results in:

My favourite books include:
   - Mastery
   - The Signal And The Noise
   - The Organized Mind

In the above code, the for loop iterates over the fav_books list, converting each book title to title case and printing it.

For Loops and the range() Function

for loops can also work in tandem with the range() function, which generates a sequence of numbers. This is particularly useful when you must repeat an action a specific number of times. However, it’s essential to remember that range() generates numbers up to, but not including the specified end value. Here’s an example:

for number in range(1,11):
    print(number)

Returns:

1
2
3
4
5
6
7
8
9
10

This code snippet will print numbers from 1 to 10, emphasizing the importance of being mindful of the common “off by one” error.

Looping Over Dictionaries

See Also

Dictionaries in Python

Python’s for loop can efficiently iterate over dictionaries (the dict object)as well. Dictionaries are collections of key-value pairs, and you can loop over their keys, values, or both. Lets start with a basic dict of names (keys) and phone numbers (values):

phone_numbers = {
    "Jim": "+37682929928",
    "Dwight": "+423998200919",
    "Michael": "+876230123654"
    }

Looping over the keys is the default behaviour, so it is possible to use for name in phone_numbers:. However, being a bit more explicit is recommended:

for name in phone_numbers.keys():
    print("## KEYS")
    print(name)
    print(' ')

Returns:

Jim
Dwight
Michael

It is also possible to loop over the values of a dict object:

# looping over values
for number in phone_numbers.values():
    print("## VALUES")
    print(number)
    print(' ')

Which would return:

+37682929928
+423998200919
+876230123654

Depending on the use case, it might also be necessary to return both the keys and the values of a dict object:

for name, number in phone_numbers.items():
    print(name, number)

Resulting with:

Jim +37682929928
Dwight +423998200919
Michael +876230123654

Tip

Note that dict.items() returns a tuple so we unpack it by using two variables in the for loop.