Python Control Flow

中文版：Python 控制流

Control flow structures determine the order in which statements are executed, including conditionals and loops.

Definition

Control flow refers to the order in which individual statements, instructions, or function calls are executed or evaluated. Python provides if statements for conditional branching and for / while loops for repetition.

Info

Python uses indentation (4 spaces or a tab) to define code blocks. Colons : are used to start blocks.

If Statements

num = float(input("Enter a number: "))
if num > 0:
    print("Positive!")
elif num < 0:
    print("Negative!")
else:
    print("Zero!")

Combining Conditions

Comparison operators (==, !=, >, <, >=, <=) and logical operators (and, or, not) can be combined:

num = 15
if 10 <= num <= 20:
    print("In range!")
else:
    print("Out of range!")
 
username = input("Username: ")
password = input("Password: ")
if username == "admin" and password == "1234":
    print("Login successful!")
else:
    print("Invalid credentials.")

For Loops

# Print numbers from 1 to 5
for i in range(1, 6):
    print(i)
 
# Sum all numbers in a list
numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
print("Sum:", total)  # 15

While Loops

# Countdown from 5 to 1
count = 5
while count > 0:
    print(count)
    count -= 1

Continue and Break

• continue: Skips the rest of the current iteration and moves to the next.
• break: Exits the loop immediately.

# Skip even numbers using continue
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)  # Prints 1, 3, 5, 7, 9
 
# Stop loop when a condition is met using break
for num in range(1, 100):
    if num % 7 == 0:
        print("First multiple of 7:", num)  # 7
        break

Nested Loops

# Multiplication table (1 to 3)
for i in range(1, 2):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")
    print("------")

Common Misconceptions

• Forgetting colons: Every if, elif, else, for, while, def, and class statement must end with a colon.
• Inconsistent indentation: Mixing tabs and spaces can cause IndentationError.

Related Concepts

• Python Variables — Variables used in conditions and loops
• Python Data Types — Collections iterated over in loops
• Introduction to Python — The lecture covering these topics

Sources

• 1-Introduction-to-python