Loops - `for`

Introduction

In this section, you’ll learn how to loop through elements using just a few lines of code. Looping allows you to take the same action, or set of actions with every item in an iterable. Among iterables are for example, lists or dictionaries. As a result, you'll be able to streamline tedious tasks. First, we'll loop over lists.

Looping over lists

You’ll often want to run through all entries in a list, performing the same task with each item. In the below example, we loop through a list of passwords and print the length of each one.

passwords = ["1234", "password", "admin", "123456"]
for password in passwords:
    print(f"Password: {password} is {len(password)} characters long.")

>>> Output

Password: 1234 is 4 characters long.
Password: password is 8 characters long.
Password: admin is 5 characters long.
Password: 123456 is 6 characters long.

A loop is written with the for statement. The password is a temporary variable that holds the current item in the list. You can choose any name you want for the temporary variable that holds each value. However, it’s helpful to choose a meaningful name that represents a single item from the list. For example:

for experiment in experiments:
    ...
for user in users:
    ...

Info

When you’re using loops for the first time, keep in mind that the set of steps is repeated once for each item in the list, no matter how many items are in the list. If you have a million items in your list, Python repeats these steps a million times.

Scope

Python uses indentation (whitespace) to indicate, what is part of the loop. With an indentation being four characters of whitespace. For a faster way to intend, use the tab key Tab.

Let's extend the example from above:

passwords = ["1234", "password", "admin", "123456"]
for password in passwords:
    print(f"Password: {password} is {len(password)} characters long.")

print("All passwords have been checked.")

>>> Output

Password: 1234 is 4 characters long.
Password: password is 8 characters long.
Password: admin is 5 characters long.
Password: 123456 is 6 characters long.
All passwords have been checked.

You can easily see that only the first print statement is part of the loop, simply because it is indented. The second print statement is executed after the loop has finished as it is outside the loop.

`IndentationError`

In longer programs, you’ll notice blocks of code indented at a few different levels. These indentation levels help you gain a general sense of the overall program’s organization.

As you begin to write code that relies on proper indentation, you’ll need to watch for a few common indentation errors.

Expected indentation

for number in [1, 2, 3]:
print(number)

  Cell In[4], line 2
    print(number)
    ^
IndentationError: expected an indented block after 'for' statement on line 1

As the IndentationError states, Python expects an indented block of code after the for statement.

Unexpected indentation

message = "Hello"
    print(message)

  Cell In[9], line 2
    print(message)
    ^
IndentationError: unexpected indent

In this case, the code snippet contains an unnecessary indentation.

Square numbers

Square each number in a given list and print the result. First, initialize a list of numbers from 1 to 10. Square each number and print it. Use a for loop.

`range()`

The range() function makes it easy to generate a series of numbers. For example, you can use range() to print a series of numbers like this:

for value in range(3):
  print(value)

>>> Output

0
1
2

Remember, that Python 'starts counting at 0'. 3 is not included in the output, as range() generates a sequence up to, but not including, the number you provide. You can also pass two arguments to range(), the first and the last number of the sequence. In this case, the sequence will start at the first number and end at the last number minus one.

for value in range(3, 6):
    print(value)

>>> Output

3
4
5

range()

Use range() to build a list which holds the numbers from 15 to 20 - including 20.

Savings account growth

Write a for loop to calculate the growth of savings over a period of time. Use following formula to calculate the future value of savings in year \(t\):

\[ \text{A} = \text{P} \times \left(1 + \frac{\text{r}}{100} \right)^{\text{t}} \]

where:

\(\text{A}\) is the future value of the savings account or investment.
\(\text{P}\) is the present value of the savings account or investment.
\(\text{r}\) is the annual interest rate.
\(\text{t}\) is the number of years the money is invested for.

Given values:

\(\text{P} = 1000\)
\(\text{r} = 5\)

Print the future value of the savings account over a period of 10 years. Skip each second year. Use Python's documentation on range() as a starting point.

Detour: Simple statistics on lists with numbers

A few functions are specific to lists of numbers. For example, you can easily find the minimum, maximum, and sum of a list of numbers:

numbers = [1.0, 8.38, 3.14, 7.0, 2.71]
print(
    f"Minimum: {min(numbers)}",
    f"Maximum: {max(numbers)}",
    f"Sum: {sum(numbers)}", sep="\n"
)

>>> Output

Minimum: 1.0
Maximum: 8.38
Sum: 22.23

Calculate the average

Calculate the average of the following list:

numbers = [4.52, 3.14, 2.71, 1.0, 8.38]

List comprehensions

... are a concise way to create lists.

A list comprehension combines a for loop to create a new list in a single line.

Rewrite a list comprehension

Rewrite the following list comprehension in a regular for-loop to achieve the same result:

squares = [value**2 for value in range(1,11)]
print(squares)

>>> Output

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Looping over dictionaries

As previously discussed, you can not only loop over a list, but also iterate over a variety of different data types, such as dictionaries. You can loop over a dictionary’s key-value pairs, solely over the keys or just the values.

`items()`

Using the .items() method, we can loop over the key-value pairs. Take note, that the method returns two values, which we store in two separate variables (key and value).

We can freely choose the variable names in the for-loop. It does not have to be key and value respectively.

parts = {
    "P100": "Bolt",
    "P200": "Screw",
    "P300": "Hinge",
}

for key, value in parts.items():
    print(key, value)

>>> Output

P100 Bolt
P200 Screw
P300 Hinge

`values()`, `keys()`

Dictionary methods

Define a (non-empty) dictionary of your choice and use both methods .values() and .keys() to access solely values and keys respectively.

Recap

With the introduction of the for loop, you can now start to automate re-occurring tasks. We have covered:

Looping over lists
Indentation and possible resulting IndentationError
range() to generate a series of numbers
Simple statistics on lists of numbers
List comprehensions
Specific methods to loop over dictionaries

Loops - for

Introduction

Looping over lists

Scope

IndentationError

Expected indentation

Unexpected indentation

range()

Detour: Simple statistics on lists with numbers

List comprehensions

Looping over dictionaries

items()

values(), keys()

Recap

Loops - `for`

`IndentationError`

`range()`

`items()`

`values()`, `keys()`