Python Control Flow - If Statements, Loops, and More

Control flow lets you make decisions and repeat actions in your Python programs. It's how you make your code smart and efficient!

If Statements

If statements let you make decisions based on conditions.

Basic If

age = 18
 
if age >= 18:
    print("You are an adult")

If-Else

age = 16
 
if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

If-Elif-Else

score = 85
 
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

Nested If

age = 20
has_license = True
 
if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license")
else:
    print("You're too young to drive")

Comparison Operators

a = 10
b = 5
 
print(a == b)   # False (equal)
print(a != b)   # True (not equal)
print(a > b)    # True (greater than)
print(a < b)    # False (less than)
print(a >= b)   # True (greater than or equal)
print(a <= b)   # False (less than or equal)

Logical Operators

age = 25
has_license = True
 
# AND - both must be true
if age >= 18 and has_license:
    print("Can drive")
 
# OR - at least one must be true
if age < 18 or not has_license:
    print("Cannot drive")
 
# NOT - reverses the condition
if not has_license:
    print("Need license")

For Loops

For loops iterate over sequences (lists, strings, etc.).

Basic For Loop

fruits = ["apple", "banana", "orange"]
 
for fruit in fruits:
    print(fruit)
# Output:
# apple
# banana
# orange

For Loop with Range

# Range from 0 to 4
for i in range(5):
    print(i)
# Output: 0, 1, 2, 3, 4
 
# Range from 1 to 5
for i in range(1, 6):
    print(i)
# Output: 1, 2, 3, 4, 5
 
# Range with step
for i in range(0, 10, 2):
    print(i)
# Output: 0, 2, 4, 6, 8

For Loop with Index

fruits = ["apple", "banana", "orange"]
 
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
# Output:
# 0: apple
# 1: banana
# 2: orange

Nested For Loops

for i in range(3):
    for j in range(3):
        print(f"({i}, {j})")

While Loops

While loops repeat as long as a condition is true.

Basic While Loop

count = 0
 
while count < 5:
    print(count)
    count += 1
# Output: 0, 1, 2, 3, 4

While with User Input

name = ""
 
while name == "":
    name = input("Enter your name: ")
 
print(f"Hello, {name}!")

Infinite Loop (with break)

while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

Break and Continue

Break - Exit Loop

for i in range(10):
    if i == 5:
        break  # Exit loop
    print(i)
# Output: 0, 1, 2, 3, 4

Continue - Skip Iteration

for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)
# Output: 1, 3, 5, 7, 9

Else with Loops

Else runs when loop completes normally (not broken):

for i in range(5):
    print(i)
else:
    print("Loop completed!")
# Output: 0, 1, 2, 3, 4, Loop completed!
 
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("Loop completed!")  # Won't print (loop was broken)

Pass Statement

Pass does nothing - useful as placeholder:

if age >= 18:
    pass  # TODO: Add code later
else:
    print("Too young")

Match-Case (Python 3.10+)

Like switch statements in other languages:

day = "Monday"
 
match day:
    case "Monday":
        print("Start of week")
    case "Friday":
        print("Weekend coming!")
    case "Saturday" | "Sunday":
        print("Weekend!")
    case _:
        print("Regular day")

Visual Explanation: Control Flow

Here's how control flow works:

If Statement Flow:
┌──────────────┐
│ Check        │
│ Condition    │
└──────┬───────┘
       │
   ┌───┴───┐
   │       │
True?    False?
   │       │
   ▼       ▼
Execute  Skip
Code     Code
 
Loop Flow:
┌──────────────┐
│ Start Loop   │
└──────┬───────┘
       │
       ▼
┌──────────────┐
│ Check        │
│ Condition    │
└──────┬───────┘
       │
   ┌───┴───┐
   │       │
True?    False?
   │       │
   ▼       ▼
Execute  Exit
Code     Loop
   │
   └───► Repeat

Frequently Asked Questions (FAQ)

Q1: What's the difference between == and =?

= - Assignment (assigns value)
== - Comparison (checks if equal)

x = 5      # Assign 5 to x
x == 5     # Check if x equals 5 (returns True)

Q2: Can I use else with if?

A: Yes! Else runs when if condition is false:

if condition:
    # Do this
else:
    # Do that

Q3: What's the difference between for and while?

for - Iterate over sequence (know how many times)
while - Repeat while condition true (might not know how many times)

Q4: How do I loop backwards?

A: Use reversed() or negative step:

for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1
 
for item in reversed([1, 2, 3]):
    print(item)  # 3, 2, 1

Q5: What's the difference between break and continue?

break - Exit loop completely
continue - Skip current iteration, continue loop

Q6: Can I have multiple conditions in if?

A: Yes! Use and, or, not:

if age >= 18 and has_license:
    # Both must be true

Q7: What's a nested loop?

A: A loop inside another loop:

for i in range(3):
    for j in range(3):
        print(i, j)

Q8: How do I exit a loop early?

A: Use break:

for i in range(10):
    if i == 5:
        break  # Exit at 5

Q9: What's pass used for?

A: Placeholder when you need syntax but no action:

if condition:
    pass  # Will add code later

Q10: Can I use else with for/while?

A: Yes! Else runs when loop completes normally (not broken):

for i in range(5):
    print(i)
else:
    print("Done!")  # Runs after loop

Next Steps

Now that you understand control flow, here's what to learn next:

Next: Learn about Python Functions - Use control flow in functions
Explore Python Data Structures - Loop through lists and dictionaries
Understand Python Classes - Object-oriented programming
Master Python File Handling - Read and write files

Control flow is essential in Python. Practice writing if statements and loops to get comfortable!