More control structures

Introduction

In this section, we will cover additional control structures. First, we discuss the if statement, which allows us to execute code based on a condition. Followed by the elif, else and while statements.

`if`

The if statement lets you evaluate conditions. The simplest kind of if statement has one condition and one action. Here is some pseudocode:

if condition is True:
    do something

You can put any condition in the first line and just about any action in the indented block following the test. If the condition evaluates to True, Python executes the indented code following the if statement. If the test evaluates to False, the indented code block (following the if) is ignored.

user = "admin"

if user == "admin":
  print(f"Welcome {user}!")

>>> Output

Welcome admin!

First, the condition user == "admin" is evaluated. If it evaluates to True, the indented print is executed. If the condition evaluates to False, the indented code block is ignored.

Indentation plays the same role in if statements as it did in for loops (see the previous section).

Password strength: Part 1

In the section on comparisons and logical operators, you had to check whether a password meets certain criterias. The following example expands on this task as you are given a list of passwords. You have to check if each password exceeds a length of 12 characters.

Execute the first code cell to generate some random passwords (note every time you rerun the code snippet, different passwords will be generated).

# generate passwords - simply execute the code to generate some random
# passwords
import random
import string

passwords = []
for i in range(10):
    length = random.randint(3, 25)
    password = "".join(random.choices(string.ascii_letters, k=length))
    passwords.append(password)

The list passwords should look something like this:

['PWgOYxQgnxgXm',
 'gpOMVTmCSjAcndowkUd',
 'ADKIEthzsGBr',
 'VRLzOIZtEz',
 'uOckmTJjeonUyMlnG',
 'gjOpWuHrIbG',
 'doxIylbRkNLdvdLNgVgYsDGzd',
 'KvUdsgZhPIrS',
 'LrdpffEqlBVQYr',
 'ncyqXNLnVstVxlx']

Now, loop over the passwords and check if each password exceeds the character limit of 12. If so, print the password.

`else`

Previously, every time the condition in the if statement evaluated to False, no action was taken. Hence, the else clause is introduced which allows you to define a set of actions that are executed when the conditional test fails.

user = "random_user"

if user == "admin":
  print(f"Welcome {user}!")
else:
  print("Only admins can enter this area!")

>>> Output

Only admins can enter this area!

Password strength: Part 2

Let's expand on our previous example. Re-use your code to check the length of the generated passwords. Now, we would like to store all passwords that did not meet our criteria in the empty list invalid_passwords.

Hint: Introduce an else statement to save the invalid passwords.

`elif`

Often, you’ll need to test more than two possible situations, and to evaluate these, you can use an if-elif-else syntax. Python executes only one block in an if-elif-else chain. It runs each conditional test in order until one passes. When a test passes, the code following that test is executed and the rest is skipped.

user = "xX_user_Xx"
registered_users = [
    "admin",
    "guest",
    "SwiftShark22",
    "FierceFalcon66",
    "BraveWolf11"
]

if user == "admin":
  print(f"Welcome {user}!")
elif user not in registered_users:
  print("Please create an account first!")
else:
  print("Only admins can enter this area!")

>>> Output

Please create an account first!

Info

As you might have noticed, you can use a single if statement or if in combination with else. For multiple conditions you can add as many elif parts as you wish.

`while`

The for loop takes an iterable and executes a block of code once for each element. In contrast, the while loop runs as long as a certain condition is True.

For instance, you can use a while loop to count up through a series of numbers. Here is an example:

# set a counter
current_number = 1

while current_number <= 5:
  print(current_number)
  # increment the counter value by one
  current_number += 1

>>> Output

Note, that the variable, that is checked in the while-condition must be defined prior to the loop, otherwise we will encounter a NameError.

Infinite loops

Moreover, the variable must be updated within the loop to avoid an infinite loop. For example, if current_number is not incremented by one, the condition current_number <= 5 will always evaluate to True, leaving us stuck in an infinite loop. In such cases, simply click the Stop button (on the left-hand side of the respective code cell) to interrupt the execution.

Addition assignment

In the above example, we used the += operator, referred to as addition assignment. It is a shorthand for incrementing a variable by a certain value.

a = 10
a += 5
print(a)

>>> Output

The above code is equivalent to a = a + 5. This shorthand assignment can be used with all arithmetic operators, such as subtraction -= or division /=.

While loop

Write some code, to print all even numbers up to 42 using a while loop.

Detour: User input

Most programs are written to solve an end user’s problem. To do so, usually we need to get some information from the user. For a simple example, let’s say someone wants to enter a username.

You can store the user input in the variable user_name like in the example below.

user_name = input("Please enter your username:")

However, the input() function always returns a string.

age = input("Please enter your age:")
print(type(age))

>>> Output

<class 'str'>

... use casting to convert the input to the desired type.

`break`

To exit any loop immediately without running any remaining 'loop code', use the break statement. The break statement directs the flow of your program; you can use it to control which lines of code are executed and which aren’t, so the program only executes code that you want it to, when you want it to.

for i in range(5):
    if i == 3:
        break
    print(i)

print("Continue running the program...")

>>> Output

0
1
2
Continue running the program...

`continue`

Rather than breaking out of a loop entirely, you can use the continue statement to return to the beginning of the loop based on the result of a condition.

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

>>> Output

Recap

In this section we have expanded on control structures. We discussed:

if statements and how to use them
else clauses
elif statements for multiple conditions
while loops
break and continue statements for more 'fine-grained' control