Control Flow

Introduction

Every program you've written so far ran one line after another, top to bottom, with no choices along the way. This chapter starts decision-making — beginning with the three basic ways a program's execution order can go.

Concept

Control flow is the order in which a program's statements actually execute:

• Sequential — statements run one after another, top to bottom (everything so far).
• Selection — the program chooses between different paths based on a condition (this chapter).
• Iteration — a block of statements repeats (a later chapter).

As covered already, Python marks out which lines belong to the same block using consistent indentation, not braces.

Worked Example

marks = 75
if marks >= 40:
    print("You have passed.")
print("Result processing complete.")

The print() line directly under if marks >= 40: is indented, so it belongs to that if's block. The final print() is back at the original indentation level — outside the if block — so it runs regardless of whether marks >= 40 is True or False.

Warning: Indenting two lines of the same block by different amounts raises an IndentationError — Python treats indentation as part of the code's actual meaning.

Explanation

Selection is what lets a program respond differently depending on its data, rather than doing the exact same fixed sequence every single time it runs — the rest of this chapter builds up increasingly capable ways to express that choice.

Summary

Control flow is the order statements execute in: sequential (top to bottom), selection (choosing a path), and iteration (repeating a block) — Python marks out each block using consistent indentation.

Introduction

The simplest kind of selection is a single yes/no check: run a block only if a condition is true, otherwise skip it entirely.

Concept

if condition:
    statement(s)

condition is any expression evaluating to True or False. If True, the indented block runs. If False, the block is skipped completely, and the program continues after it.

Worked Example

temperature = 40

if temperature > 35:
    print("Heatwave warning issued.")

print("Weather check complete.")

temperature > 35 is True (40 > 35), so the warning prints, then "Weather check complete." prints regardless, since it's outside the if block. If temperature were 30, the warning would be skipped entirely.

Note: An if with no matching else simply does nothing when its condition is False — no error, no stopped program.

Explanation

An if block only ever runs when its condition is exactly True — which is why writing the condition correctly, using the right relational or logical operator, matters as much as writing the block itself.

Summary

if condition: runs its indented block only when condition is True; when it's False, the block is skipped and the program continues after it, with no error either way.

# Program to demonstrate the simple if statement
score = 94

if score >= 90:
    print("Excellent work!")
    print("You qualify for the KV Coders Scholarship.")
    print("Please collect your certificate.")

print("Program execution complete.")

Introduction

A plain if only handles one path — do something, or do nothing. Most real decisions need a second path for when the condition is false.

Concept

if condition:
    statement(s) run when condition is True
else:
    statement(s) run when condition is False

Exactly one of the two blocks runs, every time — never both, never neither.

Worked Example

marks = 32

if marks >= 40:
    print("Result: PASS")
else:
    print("Result: FAIL")

marks >= 40 is False (32 is less than 40), so the if block is skipped and the else block runs instead, printing Result: FAIL. If marks were 55, the if block would run and print Result: PASS.

Note: else never takes its own condition — it simply means "whenever the if above it was False." Writing else if is not valid Python syntax at all.

Explanation

Because exactly one branch always runs, if-else is the right structure whenever a decision genuinely has two outcomes and both must be handled — unlike a plain if, which is only appropriate when doing nothing is itself an acceptable outcome.

Summary

if-else runs exactly one of two blocks: the if block when its condition is True, or the else block when it's False — one of the two always executes.

# Program to check if a number is Even or Odd
num = 15

if num % 2 == 0:
    print(num, "is an Even number.")
    print("It is divisible by 2.")
else:
    print(num, "is an Odd number.")
    print("It leaves a remainder when divided by 2.")

print("Calculation finished.")

Introduction

if-else only chooses between two outcomes. Many real decisions — like assigning a grade from a mark — have more than two possible outcomes.

Concept

if condition1:
    statement(s)
elif condition2:
    statement(s)
else:
    statement(s)

Python checks each condition top to bottom and runs the block belonging to the first one that's True — every condition below it is skipped, even if it would also have been True. elif can repeat as many times as needed; else is optional and always goes last.

Worked Example

marks = 78

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
else:
    print("Grade: D")

Python checks marks >= 90 first — False. It then checks marks >= 75 — True — so Grade: B prints, and every condition below it is skipped, even though marks >= 60 would also have been True.

Warning: Ordering matters. Writing the conditions from lowest to highest instead would let a high mark match the wrong, lower boundary first. Write ladders from the strictest condition down to the loosest.

Explanation

"First match wins, then stop" is the one rule this structure depends on — exactly why a grading ladder's boundaries must be ordered strictest-first, top to bottom.

Summary

if-elif-else checks each condition in order and runs only the first one that's True, skipping every condition after it — order boundary-based ladders from strictest to loosest.

# Program to calculate Grade based on Marks
marks = 65

if marks >= 90:
    print("Grade: A+ (Outstanding)")
elif marks >= 80:
    print("Grade: A (Excellent)")
elif marks >= 70:
    print("Grade: B (Good)")
elif marks >= 33:
    print("Grade: C (Pass)")
else:
    print("Grade: F (Fail/Re-appear)")

print("Evaluation Complete.")

Introduction

Selection chooses between paths, but every path still only runs once. This topic covers iteration — repeating a block for as long as a condition stays true.

Concept

while condition:
    statement(s)

Python checks condition before each pass. As long as it's True, the block runs, then Python checks again. The moment it's False, the loop stops. Something inside the loop body must eventually make condition False — usually by updating a counter — or the loop never stops (an infinite loop).

Worked Example

count = 1

while count <= 5:
    print("Count is:", count)
    count += 1

print("Loop finished.")

count starts at 1. Each pass prints its value, then count += 1 increases it. Python checks count <= 5 before every pass: True for 1 through 5 — five passes — and False once count becomes 6, which is when the loop stops.

Warning: Removing count += 1 would leave condition True forever, since count never changes — the loop would never stop.

Explanation

A while loop is the right choice when you don't know in advance exactly how many times a loop needs to run — only the condition under which it should keep going.

Summary

while condition: repeats its block for as long as condition stays True, checked before every pass — the body must eventually make the condition False, or the loop runs forever.

# Program to simulate a Password Check

# 1. Initialization
correct_password = "KVCODERS"
user_attempt = "WRONG_PASS" # Simulating a wrong guess initially

print("System Locked.")

# 2. Condition: Keep running while the attempt is WRONG
while user_attempt != correct_password:
    print("Attempt:", user_attempt)
    print("Access Denied. Trying again...")
    
    # 3. Update: Simulate the user typing the correct password
    # In a real app, this would be: input("Enter Password: ")
    user_attempt = "KVCODERS"

print("Access Granted! Welcome User.")

Introduction

while repeats based on a condition you check yourself. When you already know how many times something should repeat, Python's for loop, paired with range(), is the more natural choice.

Concept

for variable in range(...):
    statement(s)

range() generates whole numbers, and for runs its block once per number, storing the current one in variable. Three forms: range(stop) (0 up to stop-1), range(start, stop), range(start, stop, step).

Worked Example

for i in range(5):
    print("i is:", i)

for num in range(2, 11, 2):
    print("Even number:", num)

range(5) generates 0, 1, 2, 3, 4 — five values — so the first loop prints five lines, i is: 0 through i is: 4. range(2, 11, 2) starts at 2, stops one before 11, counting up by 2: 2, 4, 6, 8, 10.

Note: range(stop) never includes stop itself — range(5) produces five values (0 through 4), not six, a common off-by-one mistake.

Explanation

This "stops one before" rule matches how valid index positions work: range(len(some_sequence)), covered once sequences like lists are introduced, produces exactly the sequence's valid positions, with no out-of-range value included.

Summary

for variable in range(...): repeats once per number range() generates. range(stop) counts from 0; range(start, stop) sets a custom start; range(start, stop, step) also sets the increment — stop itself is never included.

# 1. ONE ARGUMENT: range(stop)
# Python assumes start=0, step=1
print("--- range(5) ---")
for i in range(5):
    print(i, end=" ") 
print() # New line

# 2. TWO ARGUMENTS: range(start, stop)
# Python assumes step=1
print("\n--- range(10, 15) ---")
for j in range(10, 15):
    print(j, end=" ")
print()

# 3. THREE ARGUMENTS: range(start, stop, step)
# We define everything. Note the output stops before 11.
print("\n--- range(1, 11, 2) ---")
for k in range(1, 11, 2):
    print(k, end=" ")
print()

# 4. NEGATIVE STEP: Counting Down
print("\n--- range(5, 0, -1) ---")
for l in range(5, 0, -1):
    print(l, end=" ")