Control Flow in Programming

Control Flow – Making Decisions and Repeating Actions with Conditionals and Loops

Your first programs ran in a straight line. But real programs need to make decisions ("if the user's password is correct, log them in") and repeat actions ("print numbers 1 to 1000"). This is called control flow – the ability to change the order in which instructions execute.

1. Conditionals – If This, Then That

# Basic if statement

temperature = 30

if temperature > 25:

    print("It is hot outside.")

 

# if-else statement

password = input("Enter password: ")

if password == "secret123":

    print("Access granted.")

else:

    print("Access denied.")

 

# if-elif-else – multiple paths

score = 85

if score >= 90:

    grade = "A"

elif score >= 80:

    grade = "B"

elif score >= 70:

    grade = "C"

elif score >= 60:

    grade = "D"

else:

    grade = "F"

print("Your grade is", grade)

Conditions use comparison operators: == (equal), != (not equal), >, <, >=, <=. Logical operators combine conditions: and, or, not.

Indentation matters in Python: Indentation (4 spaces) defines which code belongs to the if block. Other languages use braces {}.

2. The for Loop – Iterating Over a Range or Collection

# Print numbers 1 to 5

for i in range(1, 6):

    print(i)

 

# Iterate over a list

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

for fruit in fruits:

    print("I like", fruit)

 

# Sum of first 100 numbers

total = 0

for num in range(1, 101):

    total = total + num

print("Sum of 1 to 100:", total)   # 5050

3. The while Loop – Repeat Until a Condition Changes

# Countdown from 5 to 1

count = 5

while count > 0:

    print(count)

    count = count - 1

print("Blast off!")

 

# Keep asking until user enters a valid number

user_input = input("Enter a positive number: ")

while int(user_input) <= 0:

    print("That is not positive.")

    user_input = input("Try again: ")

Risk: If the condition never becomes false, the loop runs forever (infinite loop). Always ensure the loop variable changes toward the stopping condition.

4. Controlling Loops – break and continue

# break – exit loop immediately

for i in range(1, 10):

    if i == 5:

        break

    print(i)   # prints 1 2 3 4 only

 

# continue – skip to next iteration

for i in range(1, 6):

    if i == 3:

        continue

    print(i)   # prints 1 2 4 5 (skips 3)

5. Real-World Example – Login System with Retries

max_attempts = 3

attempt = 0

logged_in = False

 

while attempt < max_attempts and not logged_in:

    password = input("Enter password: ")

    if password == "secret":

        logged_in = True

        print("Welcome!")

    else:

        attempt = attempt + 1

        print(f"Wrong password. Attempts left: {max_attempts - attempt}")

 

if not logged_in:

    print("Account locked.")

6. Common Mistakes

Mistake	Why it fails	Fix
Using = instead of == in if	Assignment instead of comparison.	Use == for equality checks.
Forgetting colon after if/for/while (Python)	Syntax error.	Add : at end of the line.
Infinite loop (condition never false)	Program hangs.	Ensure loop variable changes.
Off-by-one errors in range	range(1, 5) gives 1,2,3,4 (not 5).	Remember range stops before the end value.

Summary

Term	Definition
Conditional	Code that executes only if a condition is true (if, else, elif).
for loop	Repeats a block a known number of times (over a sequence).
while loop	Repeats a block as long as a condition remains true.
break	Exits the current loop immediately.
continue	Skips the rest of the current iteration and goes to the next.
Infinite loop	A loop whose termination condition is never met.

Review Questions

1. Write an if-elif-else statement that prints "Child" for age < 13, "Teenager" for age 13–19, and "Adult" for age 20+.
2. What will the following loop print? for i in range(3, 10, 2): print(i)
3. Explain the difference between break and continue using an example.