Introduction to Control Flows

What are Control Flows?

Control flows are programming structures that control the order in which your code runs. Without control flows, your program would just run once from top to bottom, line by line.

With control flows, your program can:

• Make decisions based on conditions
• Repeat actions multiple times
• Skip certain parts of code based on conditions

E.g.

age = int(input("How old are you? "))
if age >= 18:
    print("You can vote!")
else:
    print("You're too young to vote.")

Here, the program makes a decision based on the user’s age.

• If the user entered 15, the output would be “You’re too young to vote.”
• If the user entered 35, the output would be “You can vote!”

Control flow syntax and code blocks

Conditions

A condition is a statement that can be either True or False. We use comparison operators to create conditions.

Examples:

• age >= 18 (True if age is 18 or more)
• score == 100 (True if score equals exactly 100)
• password != "admin" (True if password is not “admin”)

Remember that the = symbol assigns a value, while == compares values.

Code Blocks

A code block is a group of statements that belong together. In Python, code blocks are created using indentation (four spaces are recommended, but some people use two spaces or a tab at the beginning of lines).

if temperature > 30:
    print("It's hot!")        # Indented
    print("Wear light clothes!")  # Also indented
    if wind > 60:
        print("It's windy!") # Indented again and only reached if temperature > 30
print("Have a great day!")   # Not indented

Note that this is different from:

if temperature > 30:
    print("It's hot!")        # Indented
    print("Wear light clothes!")  # Also indented
if wind > 60:
    print("It's windy!") # Indented again but always reached regardless of temperature
print("Have a great day!")   # Not indented

Nested Control Flows

Control flows are most powerful when you put them inside each other, as specified by the different levels of indentation.

for day in range(1, 8):
    print(f"Day {day}:")
    
    if day <= 5:
        print("  It's a weekday - time to work!")
    else:
        print("  It's the weekend - time to relax!")
    
    if day == 7:
        print("  Tomorrow is Monday!")

if Statements

The if statement lets your program make decisions.

Basic if statement

The indented code is only executed if the condition tests true

score = 85
if score >= 60:
    print("You passed!") # Only executed if score is greater or equal to 60

This gives the output

You passed!

While the code

score = 35
if score >= 60:
    print("You passed!") # Only executed if score is greater or equal to 60

gives no output since score is less than 60.

if-else statement

Choose between two options.

weather = "sunny"
if weather == "rainy":
    print("Take an umbrella!")
else:
    print("Enjoy the nice weather!")

This gives the output

Enjoy the nice weather!

since weather is “sunny” (and not “rainy”).

if-elif-else statement

Choose between multiple options.

grade = 85

if grade >= 90:
    print("Grade: A")
elif grade >= 80:
    print("Grade: B")
elif grade >= 70:
    print("Grade: C")
elif grade >= 60:
    print("Grade: D")
else:
    print("Grade: F")

This gives output:

Grade: B

since 85 is greater than 80 (and less than 90).

Logical Operators

You can use the and and or operators to combine multiple conditions to make more complex decisions.

and operator

and requires both (all) conditions to be True.

score = 85
passed_coursework = True

# Both conditions must be true
if score >= 90 and passed_coursework:
    print("Pass")
else:
    print("Fail")

This gives the output

Fail

since score = 85, which does not satisfy the condition score >= 90, even though passed_coursework = True (which satisfies the passed_coursework condition).

or operator

or only requires that at least one of the conditions must be True.

score = 85
passed_coursework = True

# At least one of the conditions must be true
if score >= 90 or passed_coursework:
    print("Pass")
else:
    print("Fail")

This gives the output

Pass

since passed_coursework = True (which satisfies the passed_coursework condition). It doesn’t matter that score = 85 and the condition score >= 90 is not satisfied.

not operator

not reverses the condition.

score = 85
passed_coursework = True

if not score < 90 and not passed_coursework==False:
    print("Pass")
else:
    print("Fail")

This gives the output

Fail

since score = 85, which does not satisfy the condition not score < 90 (since 85 is less than 90), even though passed_coursework = True satisfies the not passed_coursework==False condition.

while Loops

The while loop repeats code as long as a condition is True.

count = 1
while count <= 5:
    print(f"Count: {count}")
    count = count + 1  # Don't forget to update the variable!

This gives the output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

When using while loops, make sure there is a way for your conditions to become False so you can break out of them. It is very rarely the case that you will want an infinite loop.

for loops

The for loop repeats code a specific number of times.

for loop with lists and tuples (collections)

list_of_colours = ['blue', 'red', 'green']
# Would also work with a tuple, e.g. ('blue', 'red', 'green')
for colour in list_of_colours:
    print(colour)

This gives the output:

blue
red
green

• for item in collection: means “do this for each item”, e.g. for colour in list_of_colours means “do this for each item in list_of_colours”.
• colour is just a variable name given to each item in the list (you could have used any name. For example, the line for a in list_of_colours would work exactly the same, although you would then need to write print(a)).
• The code inside the loop runs once for each element in the list and the order is respected.

for loop with range()

Starting from 0:

for i in range(5):
    print(f"Count: {i}")

This gives the output:

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4

With both start and end of range specified:

# Start at 1, end before 6
for i in range(1, 6):
    print(f"Count: {i}")

This gives the output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

With step size specified:

# Count by 2s from 0 to 4 (5 is not included)
for i in range(0, 5, 2):
    print(f"Even number: {i}")

This gives the output:

Even number: 0
Even number: 2
Even number: 4

break and continue

The break and continue commands are used to stop and exit looping behaviour.

break`

break stops the loop completely.

for i in range(10):
    if i == 5:
        break  # Stop when i equals 5
    print(i)

This gives the output:

0
1 
2 
3 
4

continue

continue skips the rest of the current iteration.

for i in range(10):
    if i == 5:
        continue  # Skip when i equals 5
    print(i)

This gives the output:

0
1
2
3
4
6
7
8
9

Common Patterns

Data collection pattern {data-collection}

# Collect user input into a list
scores = []
while True:
    score = input("Enter score (or 'done'): ")
    if score == "done":
        break
    scores.append(int(score))

Data lookup pattern

# Quick lookups with dictionaries
phone_book = {"Alice": "123-4567", "Bob": "987-6543"}
name = input("Enter name: ")
if name in phone_book:
    print(f"Phone: {phone_book[name]}")
else:
    print("Name not found")

Data processing pattern

# Process all items in a collection
numbers = [1, 2, 3, 4, 5]
doubled = []
for num in numbers:
    doubled.append(num * 2)
# Result: [2, 4, 6, 8, 10]

Sentinel loop

while True:
    command = input("Enter command (or 'exit' to quit): ")
    if command == "exit":
        break
    process_command(command)

This keeps asking for a user’s input after processing their command unless they type “exit”. Here, “exit” is the sentinel, the specific value that breaks the loop.

User input loop

password = ""
while password != "secret":
    password = input("Enter password: ")
    if password != "secret":
        print("Wrong password!")

print("Access granted!")

The while password != "secret" part means that the user is repeatedly asked for input until their input is "secret".

Input validation

while True:
    age = input("Enter your age: ")
    if age.isdigit():
        age = int(age)
        break
    else:
        print("Please enter a valid number!")

print(f"Your age is {age}")

The while True part means that the user is repeatedly asked for input until the condition age.isdigit() is met.

Menu system

while True:
    print("1. Say Hello")
    print("2. Say Goodbye")
    print("3. Exit")
    
    choice = input("Choose an option: ")
    
    if choice == "1":
        print("Hello there!")
    elif choice == "2":
        print("Goodbye!")
    elif choice == "3":
        print("Exiting...")
        break
    else:
        print("Invalid choice!")

The while True part means the commands below are repeated until the user enters a viable choice (in this case, 1, 2 or 3).

Accumulator pattern

# Calculate total score
scores = [85, 92, 78, 95, 88]
total = 0  # Initialize accumulator

for score in scores:
    total = total + score  # Update accumulator

print(f"Total score: {total}")

This gives the output:

Total score: 438

which is the sum of the scores 85, 92, 78, 95, 88.

The accumulator pattern is used to build up a result over multiple iterations of a loop. In this case, 0 is the initial value you set the accumulator to. This value is then updated by adding an element from the scores list each iteration through the loop.

This can also be combined with conditional if statements.

answers = ["correct", "wrong", "correct", "correct", "wrong"]
correct_count = 0  # Initialize accumulator

for answer in answers:
    if answer == "correct":
        correct_count = correct_count + 1  # Update accumulator

print(f"You got {correct_count} correct!")

This gives the output:

You got 3 correct!

Practice Exercises

Solidify your understanding of control flows with these exercises.

Q1: if-else

Write a program that asks the user for the temperature and prints whether they should wear a coat.

Expected output:

What's the temperature? 12
Wear a coat!

temperature = int(input("What's the temperature? "))

if temperature < 15:
    print("Wear a coat!")
else:
    print("No coat needed!")

Q2: if-elif-else

Write a program that converts a numerical score to a letter grade.

Grading scale:

• 90 or above: A
• 80-89: B
• 70-79: C
• 60-69: D
• Below 60: F

Expected output:

Enter your score: 85
Your grade is: B

score = int(input("Enter your score: "))

if score >= 90:
    print("Your grade is: A")
elif score >= 80:
    print("Your grade is: B")
elif score >= 70:
    print("Your grade is: C")
elif score >= 60:
    print("Your grade is: D")
else:
    print("Your grade is: F")

Q3: Loop for counting

Write a program that asks the user for a number and counts down from that number to 1, then prints “Blast off!”

Expected output:

Enter countdown start: 5
5
4
3
2
1
Blast off!

countdown = int(input("Enter countdown start: "))

for i in range(countdown, 0, -1):
    print(i)

print("Blast off!")

countdown = int(input("Enter countdown start: "))

while countdown > 0:
    print(countdown)
    countdown = countdown - 1

print("Blast off!")

Q3: while loop with variable tracking

Write a program that keeps asking the user for a password until they enter the correct one (“python123”). After 3 wrong attempts, the program should lock them out.

Expected output:

With wrong attempts

Enter password: hello
Wrong password! 2 attempts remaining.
Enter password: world
Wrong password! 1 attempt remaining.
Enter password: python123
Access granted!

When locked out

Enter password: hello
Wrong password! 2 attempts remaining.
Enter password: world
Wrong password! 1 attempt remaining.
Enter password: test
Wrong password! 0 attempts remaining.
Account locked! Too many failed attempts.

correct_password = "python123"
attempts = 3

while attempts > 0:
    password = input("Enter password: ")
    
    if password == correct_password:
        print("Access granted!")
        break
    else:
        attempts = attempts - 1
        if attempts > 0:
            print(f"Wrong password! {attempts} attempts remaining.")
        else:
            print("Wrong password! 0 attempts remaining.")
            print("Account locked! Too many failed attempts.")

correct_password = "python123"
attempts = 3

while attempts > 0:
    password = input("Enter password: ")
    
    if password == correct_password:
        print("Access granted!")
        break
    else:
        attempts = attempts - 1
        if attempts > 0:
            # "attempts" if more than 1 attempt, otherwise "attempt"
            print(f"Wrong password! {attempts} attempt{'s' if attempts != 1 else ''} remaining.")
        else:
            print("Wrong password! 0 attempts remaining.")
            print("Account locked! Too many failed attempts.")