Control Flow

Control flow is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The control flow of a Python program is regulated by conditional statements, loops, and function calls.

Conditional Statements

Conditional statements are used to perform different actions based on different conditions.

If Statement

An if statement is a conditional statement that runs or skips code based on whether a condition is True or False.

x = 10
if x > 5:
    print("x is greater than 5")

If-Else Statement

An if-else statement is a conditional statement that runs one block of code if the condition is True and another block of code if it is False.

x = 10
if x > 5:
    print("x is greater than 5")
else:
    print("x is less than or equal to 5")

If-Elif-Else Statement

An if-elif-else statement is a conditional statement that runs different code for different conditions.

x = 10
if x > 5:
    print("x is greater than 5")
elif x == 5:
    print("x is equal to 5")
else:
    print("x is less than 5")

Match Case Statement

Also known as switch-case in other languages. Python 3.10 introduced the match statement, which is similar to a switch statement in other languages. This is useful when you have multiple conditions to check. Instead of writing a very long if-elif-else statement, you can use the match statement.

def match_example(x): # we will cover functions in the next section
    match x:
        case 1:
            print("one")
        case 2:
            print("two")
        case _:
            print("other")

match_example(1)
# Output:
# one

match_example(2)
# Output:
# two

match_example(3)
# Output:
# other

Loops

Loops are used to iterate over a sequence or perform a task repeatedly.

For Loop

A for loop is used to iterate over a sequence (list, tuple, dictionary, set, or string).

fruits = ["apple", "banana", "cherry"]

for fruit in fruits: # 'fruit' is the name we give to each element in the list as we go through the loop
    print(fruit)

# This is equivalent to above:
for x in fruits:
    print(x)

# Output:
# apple
# banana
# cherry

Tip

When constructing your loops, try and name the variable a sensible name that helps you understand what the loop is doing. For example, if you were looping through a dataframe:

for row in dataframe:
    print(row)

This makes it easy to read and understand.

While Loop

A while loop is used to execute a block of code repeatedly as long as a condition is True.

i = 1
while i < 6:
    print(i)
    i += 1

# Output:
# 1
# 2
# 3
# 4
# 5

Break and Continue Statements

break and continue are used to alter the flow of a loop.

Break Statement

A break statement is used to exit a loop when a certain condition is met.

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

# Output:
# apple
# banana

Continue Statement

A continue statement is used to skip the current block and move to the next iteration of the loop.

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

# Output:
# apple
# cherry

Nested Loops

A nested loop is a loop inside a loop.

adjectives = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]


for adjective in adjectives:
    for fruit in fruits:
        print(adjective, fruit)

# Output:
# red apple
# red banana
# red cherry
# big apple
# big banana
# big cherry
# tasty apple
# tasty banana
# tasty cherry

List Comprehension

Just to make you aware that list comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.

Warning

I tend not to recommend this. It is better to be more verbose and explicit in your code, because it is easier for everybody to read and understand. I strongly suggest keeping things as simple as possible. Code is read much more than it is written.

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]

print(newlist)
# Output:
# ['apple', 'banana', 'mango']

# Here is how I would do this instead

newlist = [] # declare you are making a list
for fruit in fruits:
    if "a" in fruit:
        newlist.append(fruit)     

# I think these 3 lines are more readable 
# compared to the shorthand syntax above.