Loops and conditional statements

Python Basics

In the previous post, we discussed different operator types, some of them are useful when performing mathematical computations, and others are useful when performing logical operations. If you feel like you need a refresher on that matter, you can find it by following the link. 👇👇👇

Now it's time to understand how to control program flow with loops and conditional statements.

Previously, we wrote a script that printed "Hi, PyCharm" three times.

def print_hi(name):
    greetings = f'Hi, {name}'
    print(greetings)
    print(greetings)
    print(greetings)

if __name__ == '__main__':
    print_hi('PyCharm')

To achieve that, we copied and pasted the print(...) function three times and extracted the greetings message into a variable. The solution works but is cumbersome. And if we wanted to print that message a million times, would we copy and paste that code a million times? The answer is no because Python has the means to address repetitive fragments of code. Those means are called loops.

Loops in programming are structures that allow you to execute a block of code repeatedly. They are used to automate repetitive tasks and perform operations multiple times without having to write the same code over and over again. Loops are essential for improving the efficiency and readability of your programs.

There are two main types of loops in Python: while loops and for loops.

While loops

A while loop is a control structure that repeatedly executes a block of code as long as a specified condition remains True. This type of loop is particularly useful when the number of iterations is not known in advance, and we want to keep executing a certain block of code until a condition is met.

Here's the basic structure of a while loop:

while condition:
    # Code to be executed

The loop will continue to execute the code block as long as the condition remains True. Once the condition becomes False, the loop stops and the program continues with the next statement after the loop.

Here's an example that demonstrates the concept of a while loop.

count = 0
while count < 5:
    print(count)
    count += 1

The condition is count < 5. We can see that we start with the count equal to 0 and increment it each time we execute the loop. Once the count becomes 5, the loop stops executing.

❯ python3 main.py 
0
1
2
3
4

For loops

A for loop is a control structure used to iterate over a sequence (such as a list, tuple, string, or range) and execute a block of code for each element in the sequence. It's particularly useful when we know the number of iterations we want to perform or when we want to process each item in a collection.

The basic structure of a for loop is as follows.

for element in sequence:
    # Code to be executed

The loop iterates over each element in the sequence, and for each iteration, the specified block of code is executed.

for loops are also often used with the range(...) function, which generates a sequence of numbers that we can iterate over:

for i in range(5):
    print(i)

The following example prints integers in sequence from 0 to 5.

❯ python3 main.py 
0
1
2
3
4

for loops are very useful when iterating over elements of complex types, such as lists.

💡 A list is a built-in data structure that allows you to store and organize a collection of items.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

In this example, the loop iterates through the fruits list and prints each fruit on a separate line. The output is:

❯ python3 main.py 
apple
banana
cherry

Knowing the loop mechanism in Python, we can easily improve our script that printed the "Hi, PyCharm" message three times. We will use a while loop to repeat a block of code and a variable to track the number of times the loop is executed.

def print_hi(name):
    greetings = f'Hi, {name}'
    count = 3
    while count > 0:
        print(greetings)
        count -= 1


if __name__ == '__main__':
    print_hi('PyCharm')

We've introduced counter to track the number of times the loop is executed. Initially, it is set to 3, and with each loop is it decreased by one using the -= operator. The loop is executed as long as the count > 0 condition is True. Once count becomes 0, loop execution stops.

When we run the script, it prints the "Hi, PyCharm" message three times as before.

❯ python3 main.py 
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm

Now if we wanted to print the message more times, 10 for instance, we don't have to copy the code but just update the counter value.

def print_hi(name):
  greetings = f'Hi, {name}'
  count = 10
  while count > 0:
    print(greetings)
    count -= 1

The message is printed 10 times now.

❯ python3 main.py 
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm
Hi, PyCharm

Since we know the number of times the loop is executed, the while loop can be easily transformed into a for loop.

def print_hi(name):
  greetings = f'Hi, {name}'
  for _ in range(10):
    print(greetings)

💡 Underscore (_) means the variable is irrelevant in this context, so we ignore it.

Conditional statements

Conditional statements are used to control the execution of our code based on certain conditions. They allow us to create branches in our code, ensuring that different blocks of code are executed depending on whether a condition is True or False.

The basic structure of a if condition is as follows.

if condition:
    # Code to be executed

The specified block of code is executed only if a condition is True. Let's run a simple example.

count = 3
if count > 2:
    print("Count is greater than 2")
print("Hello!")

We have a count variable set to 3. Therefore, the count > 2 condition is fulfilled. As a result "Count is greater than 2" is printed.

❯ python3 main.py 
Count is greater than 2
Hello!

When we set the count variable to 2, the condition is not fulfilled.

count = 2
if count > 2:
    print("Count is greater than 2")
print("Hello!")

When we run the script, we can quickly notice that the block of code under the if statement isn't executed.

❯ python3 main.py
Hello!

We can create a more sophisticated execution flow using the else statement.

if condition:
    # some code
else:
    # code to be executed if the condition is not met

The else statement is executed when the previous if condition isn't fulfilled. Let’s observe it in the following example.

count = 2
if count > 2:
    print("Count is greater than 2")
else:
    print("Count isn't greater than 2")
print("Hello!")

And execute it.

❯ python3 main.py 
Count isn't greater than 2
Hello!

As the count variable is set to 2, the count > 2 condition is not met, so the else branch is executed.

If the if-else statement is not enough, Python allows to create even more complex execution flow. The elif statement allows us to check multiple conditions if the initial if condition is not met.

if condition1:
    # some code
elif condition2:
    # some code
elif condition3:
    # some code
else:
    # code to be executed if conditions are not met

In this situation, the else statement is used to execute a block of code if none of the previous conditions (in if and elif statements) are fulfilled.

count = 1
if count > 2:
    print("Count is greater than 2")
elif count == 1:
    print("Count equals 1")
else:
    print("Count isn't greater than 2")
print("Hello!")

In the above example, we have the count == 1 condition. Since count is equal to 1, the first condition is not met, and the execution falls into the second one. Here is the result of running the script.

❯ python3 main.py 
Count equals 1
Hello!

Keep in mind that we can use elif multiple times in case of more demanding logic.

Now let's slightly modify our greetings script to print "Hello, PyCharm" every second time.

First of all, we rename the greetings variable to hiGreetings.

hiGreetings = f'Hi, {name}'

We introduce another variable called helloGreetings .

helloGreetings = f'Hello, {name}'

It will hold the new welcome text. Next, we add if statement into the while loop.

if (count % 2) == 0:
    print(hiGreetings)
else:
    print(helloGreetings)

We use (count % 2) == 0 as the condition. When running the loop, we will get the following values:

• count values: 10, 9, 7, 6, 5, 4, 3, 2, 1, and

• count % 2 values: 0, 1, 0, 1, 0, 1, 0, 1, 0, 1.

When count modulo 2 equals 0, we print "Hi, PyCharm". Otherwise, we print "Hello, PyCharm".

Here is the full code snippet.

def print_hi(name):
    hiGreetings = f'Hi, {name}'
    helloGreetings = f'Hello, {name}'
    count = 10
    while count > 0:
        if (count % 2) == 0:
            print(hiGreetings)
        else:
            print(helloGreetings)
        count -= 1

if __name__ == '__main__':
    print_hi('PyCharm')

Let's run the script and see the result.

❯ python3 main.py 
Hi, PyCharm
Hello, PyCharm
Hi, PyCharm
Hello, PyCharm
Hi, PyCharm
Hello, PyCharm
Hi, PyCharm
Hello, PyCharm
Hi, PyCharm
Hello, PyCharm

Works like a charm!

Controlling the execution of the loop

In the case of for loop, the number of times it repeats is derived from the sequences it iterates over and is known upfront. In the case of while loop, we don’t know the number of times it repeats, so we provide a kind of “stop” condition explicitly. These are the default mechanisms to control loops' repetition. However in some particular cases, they are not enough, and more fine-grained means are needed. Python provides continue, break and else keywords in addition to the default loop control flow.

continue statement

The continue statement is used within loops to skip the rest of the current iteration and move on to the next iteration. When the continue statement is encountered, the code block below it for the current iteration is skipped, and the loop proceeds with the next iteration.

Here's the basic structure of using the continue statement within a loop:

for item in sequence:
    if condition:
        # code to be executed if the condition is met
        continue
    # code to be executed for other cases

or

while condition:
    if condition:
        # code to be executed if the condition is met
        continue
    # code to be executed for other cases

Let's look at examples.

for num in range(5):
    if num == 3:
        print("Skipping number 3")
        continue
    print("Number:", num)

In this example, when the loop encounters the number 3, it prints "Skipping number 3" and moves on to the next iteration without executing the subsequent line that prints the number itself.

❯ python3 main.py 
Number: 0
Number: 1
Number: 2
Skipping number 3
Number: 4

A similar effect can be achieved using while loop.

num = 0
while num < 5:
    if num == 3:
        print("Skipping number 3")
        num += 1
        continue
    print("Number:", num)
    num += 1

The major difference here when compared to the previous example is the fact num is incremented in two places. Without the incrementation in the if block, the code would fall into an infinite loop.

break statement

The break statement is used within loops to abruptly terminate the loop's execution, regardless of whether the loop's condition or iterations are completed. When the break statement is encountered, the program exits the loop immediately and continues with the code that follows the loop.

Here's the basic structure of using the break statement within a loop:

for item in sequence:
    if condition:
        # code to be executed if the condition is met
        break
    # code to be executed for other cases

or

while condition:
    if condition:
        # code to be executed if the condition is met
        break
    # code to be executed for other cases

Let's see an example.

for num in range(5):
    if num == 3:
        print("Breaking at number 3")
        break
    print("Number:", num)

In this example, when the loop encounters the number 3, it prints "Breaking at number 3" and immediately exits the loop, without continuing with the rest of the iterations.

❯ python3 main.py
Number: 0
Number: 1
Number: 2
Breaking at number 3

Yet similar behavior can be achieved in the case of a while loop, let's see an example.

num = 0
while (True):
    if num == 3:
        print("Breaking at number 3")
        break
    print("Number:", num)
    num += 1

Here we have an interesting fact, we use a True as the condition in the while loop. If we hadn't used a break statement, the loop would have run infinitely.

❯ python3 main.py
Number: 0
Number: 1
Number: 2
Breaking at number 3

The output is of course the same as in the previous example.

else clause

The else clause in Python loops is used to specify a block of code that should be executed after the loop has completed its normal iterations. This code block is executed only if the loop completes its iterations without any break statement being encountered. It is optional to use the else clause with loops, but it can be useful in certain scenarios.

Here's the general structure of using the else clause with loops:

for item in sequence:
    # code to be executed for each item
else:
    # code to be executed after the loop completes (optional)

or

while condition:
    # code to be executed while the condition is true
else:
    # code to be executed after the loop completes (optional)

Let's see an example.

for num in range(5):
    if num == 3:
        print("Number 3 found!")
        break
else:
    print("Number 3 not found.")

In this example, the loop iterates through the range. If the loop encounters the number 3, it prints "Number 3 found!" and exits the loop using the break statement. If the loop completes all iterations without encountering the number 3, the else block gets executed, printing "Number 3 not found."

❯ python3 main.py
Number 3 found!

In the range from 0 to 4, number 3 has been found. Now, we slightly modify the script and shorten the range.

for num in range(2):
    if num == 3:
        print("Number 3 found!")
        break
else:
    print("Number 3 not found.")

And of course, run it.

❯ python3 main.py
Number 3 not found.

Obviously, number 3 hasn't been found in the range from 0 to 2.

Summary

In this post, we learned the basics of flow control which are loops and conditional statements.

If statements are crucial for implementing conditional logic, allowing your program to make decisions and take different actions based on various conditions.

Loops are essential for automating repetitive tasks, iterating through data, and solving various programming challenges. A for loop is used to iterate over a sequence of items, while a while loop is used to repeatedly execute a block of code as long as a specified condition is true.

Additionally, we can modify the loop flow using dedicated keywords. The continue statement is helpful when you want to exclude certain elements or cases from being processed within a loop while continuing with the remaining iterations. The break statement is useful when you want to prematurely exit a loop based on a certain condition. It’s particularly handy for stopping further iterations once a specific goal or condition is achieved. Using the else clause with loops can be particularly useful when you want to perform some actions if a certain condition is not met during the loop’s execution. It can help improve the readability and logic flow of your code.