Control Statements

Control statements are special instructions that alter the normal sequential flow of execution. They let a program decide which statements to execute, when to skip them, and how many times to repeat them. They are needed because real situations require choices and repetition, not just a straight sequence.

• Decision making (selection) statements: execute selectively based on conditions.
• Iteration (looping) statements: execute a set of instructions repeatedly until a condition is satisfied.

The sequence in which statements run is the flow of control. Conditions (comparisons) are processed by the Arithmetic and Logic Unit (ALU), which returns True or False.

The rhombus. It has one entry point and two exit points: one for True/Yes and one for False/No.

if, if...else, if...elif...else ladder, match...case, and nested if...else.

if condition:
    block of code

The block runs only when the condition is True; otherwise Python skips the block and continues with the next statement. A colon must follow the condition and the block must be indented.

Indentation defines the scope of a code block under statements like if. Without proper indentation, Python raises an IndentationError.

score = int(input("Enter the score: "))
if score >= 30:
    print("Congratulations... You scored above pass mark.")
print("Your score is ", score)

The last line is outside the if block, so the score is always displayed.

if condition:
    Statement block 1
else:
    Statement block 2

Block 1 runs if the condition is True, otherwise block 2 runs. Only one block executes.

attendance = int(input("Enter your attendance percentage: "))
if attendance >= 75:
    print("You are eligible to attend the exam.")
else:
    print("You have to apply for condonation.")

h1 = int(input("Enter the height of first student: "))
h2 = int(input("Enter the height of second student: "))
if h1 > h2:
    print("Larger height is: ", h1)
else:
    print("Larger height is: ", h2)

if condition1:
    Statement Block 1
elif condition2:
    Statements Block 2
else:
    Block when all conditions are False

Conditions are checked from top to bottom. As soon as one is True, its block runs and the remaining conditions are skipped. If none is True, the else block runs. The else part is optional. Use it to select one action from several.

n = int(input("Enter a number: "))
if n > 0:
    print(n, "is positive")
elif n < 0:
    print(n, "is negative")
else:
    print(n, "is zero")

Introduced in Python 3.10. It compares a value against several patterns and executes the block of the first matching pattern, somewhat like a switch statement in other languages. It is useful when a decision depends on the value of a single variable (menu options, days of week, fixed codes, grades).

match expression:
    case value1:
        Statements Block 1
    case value2:
        Statements Block 2
    case _:
        Default Statements Block

The underscore _ case is the default action when no case matches. Case values can be integers, strings, tuples or other literal patterns.

day = int(input("Enter a number (1-7): "))
match day:
    case 1:
        print("Sunday")
    case 2:
        print("Monday")
    case 3:
        print("Tuesday")
    case 4:
        print("Wednesday")
    case 5:
        print("Thursday")
    case 6:
        print("Friday")
    case 7:
        print("Saturday")
    case _:
        print("Invalid day number")

Grades: 90+ A+, 80–89 A, 70–79 B+, 60–69 B, 50–59 C+, 40–49 C, 30–39 D+, 20–29 D, below 20 E. Using match–case with a pattern variable s and if guards:

score = int(input("Enter the score (out of 100): "))
match score:
    case s if 90 <= s <= 100:
        print("Grade: A+")
    case s if 80 <= s <= 89:
        print("Grade: A")
    case s if 70 <= s <= 79:
        print("Grade: B+")
    case s if 60 <= s <= 69:
        print("Grade: B")
    case s if 50 <= s <= 59:
        print("Grade: C+")
    case s if 40 <= s <= 49:
        print("Grade: C")
    case s if 30 <= s <= 39:
        print("Grade: D+")
    case s if 20 <= s <= 29:
        print("Grade: D")
    case s if 0 <= s < 20:
        print("Grade: E")
    case _:
        print("Invalid Score")

| (pipe) is an OR pattern that lets one case match several values:

score = 10
match score:
    case 10 | 9:
        print("Excellent")

It works only inside a case pattern. case 10 or 9: is invalid because or returns a Boolean result, not a pattern.

Writing one decision-making statement inside another is nesting. A nested if...else places an if...else inside the if or the else block of another, so a second-level decision is made only after the first condition is evaluated.

# inner decision inside the if block
if condition1:
    if condition2:
        ...both True
    else:
        ...condition1 True, condition2 False
else:
    ...condition1 False

# inner decision inside the else block
if condition1:
    ...condition1 True
else:
    if condition2:
        ...condition1 False, condition2 True
    else:
        ...both False

year = int(input("Enter a year: "))
if year % 4 == 0:
    if year % 100 == 0:
        if year % 400 == 0:
            print("Leap year")
        else:
            print("Not a leap year")
    else:
        print("Leap year")
else:
    print("Not a leap year")

Algorithm: Start → Input N → If N % 2 == 0 print "Even" else print "Odd" → Stop.

n = int(input("Enter a number: "))
if n % 2 == 0:
    print(n, "is even")
else:
    print(n, "is odd")

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))
if a >= b and a >= c:
    print("Largest is", a)
elif b >= a and b >= c:
    print("Largest is", b)
else:
    print("Largest is", c)

age = int(input("Enter age: "))
if age >= 60:
    print("Eligible for senior citizen discount")
else:
    print("Not eligible for senior citizen discount")

bill = float(input("Enter the total bill amount: "))
if bill > 1000:
    final = bill - bill * 10 / 100
else:
    final = bill - bill * 5 / 100
print("Final amount =", final)

Example: 1200 → 1080.0; 800 → 760.0.

year = int(input("Enter a year: "))
if year % 100 == 0:
    print("Century year")
else:
    print("Not a century year")

marks = int(input("Enter marks: "))
if marks >= 30:
    print("Pass")
else:
    print("Fail")

ch = input("Enter a character: ").lower()
match ch:
    case 'a' | 'e' | 'i' | 'o' | 'u':
        print("Vowel")
    case _:
        print("Consonant")

The OR pattern | groups the five vowels in one case. (A fuller program could first check that the input is a letter.)

units = int(input("Enter units consumed: "))
if units <= 100:
    print("Low usage")
elif units <= 300:
    print("Moderate usage")
else:
    print("High usage")

(1) Greater than 5 (the first condition is False, the second is True).

(2) A (85 // 10 = 8, which matches case 8).

(3) B+ (72 fails ≥ 90 and ≥ 80, but satisfies ≥ 70).

Iteration is executing a set of statements repeatedly as long as a specified condition remains true; it ends when the condition becomes false. Loops automate repetitive tasks without rewriting instructions, making programs efficient, organised and easy to maintain.

• Initialisation: setting the starting values (e.g. i = 1, total = 0).
• Condition checking: deciding before each cycle whether to continue (e.g. i <= 6).
• Body: the statements repeated (e.g. add the mark to the total).
• Updation: changing the counter so the loop moves toward termination (e.g. i = i + 1).

In Python these parts may not always appear explicitly or in the same place, but conceptually they exist in every loop.

1 to 10: Start; i = 1; Repeat steps 4 and 5 while i ≤ 10; Print i; i = i + 1; Stop.

Sum and average: Start; i = 1, sum = 0; Repeat steps 4 to 6 while i ≤ 6; Input score; sum = sum + score; i = i + 1; average = sum / 6; Print sum and average; Stop.

In the flowchart, the rhombus tests the condition and a flow line returns from the update step to the rhombus, forming a cycle.

while condition:
    statements

The condition is evaluated before every iteration. While it is True the body executes; when it becomes False, control moves to the statement after the loop. It is called an indefinite loop because it is used when the number of repetitions is not known in advance and depends on a condition. Initialisation and updation are not part of the syntax; the programmer writes them separately.

n = int(input("Enter a number: "))
count = 0
while n > 0:
    count = count + 1
    n = n // 10
print("Number of digits:", count)

For 321: n becomes 32, 3, 0 and count becomes 3.

A while loop repeats as long as choice != 5. Each time it displays the menu (1 Add, 2 Subtract, 3 Multiply, 4 Divide, 5 Exit), reads the choice, and for options 1–4 reads two numbers and prints the result. For division it checks that the divisor is not zero. Choosing 5 prints "Program ended" and the condition becomes False. The number of repetitions depends on the user, so it is an indefinite loop.

choice = 0
while choice != 5:
    print("\nArithmetic Menu")
    print("1. Add\n2. Subtract\n3. Multiply\n4. Divide\n5. Exit")
    choice = int(input("Enter your choice: "))
    if choice in (1, 2, 3, 4):
        a = float(input("Enter first number: "))
        b = float(input("Enter second number: "))
        if choice == 1:
            print("Result =", a + b)
        elif choice == 2:
            print("Result =", a - b)
        elif choice == 3:
            print("Result =", a * b)
        elif b != 0:
            print("Result =", a / b)
        else:
            print("Cannot divide by zero")
    elif choice == 5:
        print("Program ended")
    else:
        print("Invalid choice")

An infinite loop occurs when the loop's condition never becomes false, often because the updation step is missing or incorrect. It can freeze or slow down the system. Avoid it by designing loops carefully, ensuring correct updation inside the body, and confirming the condition will eventually become false.

i = 1
while i <= 20:
    print(i)
    i = i + 1

i = 2
while i <= 50:
    print(i, end=" ")
    i = i + 2

i = 1
total = 0
while i <= 10:
    total = total + i
    i = i + 1
print("Sum =", total)

Output: Sum = 55.

n = int(input("Enter a number: "))
i = 1
while i <= 10:
    print(n, "x", i, "=", n * i)
    i = i + 1

A perfect number equals the sum of its proper divisors (e.g. 6 = 1 + 2 + 3).

n = int(input("Enter a number: "))
i = 1
total = 0
while i < n:
    if n % i == 0:
        total = total + i
    i = i + 1
if total == n and n > 0:
    print(n, "is a perfect number")
else:
    print(n, "is not a perfect number")

n = int(input("Enter a number: "))
rev = 0
total = 0
while n > 0:
    d = n % 10
    rev = rev * 10 + d
    total = total + d
    n = n // 10
print("Reversed number =", rev)
print("Sum of digits =", total)

For 123: reverse 321, sum of digits 6.

n = int(input("Enter a number: "))
fact = 1
i = 1
while i <= n:
    fact = fact * i
    i = i + 1
print("Factorial =", fact)

For 5: 120.

n = int(input("Enter a number: "))
original = n
rev = 0
while n > 0:
    rev = rev * 10 + n % 10
    n = n // 10
if rev == original:
    print("Palindrome")
else:
    print("Not a palindrome")

total = 0
n = int(input("Enter a number (0 to stop): "))
while n != 0:
    total = total + n
    n = int(input("Enter a number (0 to stop): "))
print("Total =", total)

A for loop iterates over a range of values or the elements of a sequence (string, list, tuple), processing each item one by one. It is definite because the number of repetitions is known beforehand.

The stop value is always excluded. If start > stop with a positive step, the sequence is empty and the loop does not run; use a negative step to count down.

for i in range(5):
    print(i)

for roll in range(1, 11):
    print("Number:", roll)

for num in range(2, 21, 2):
    print(num)

sum = 0
for i in range(1, 7):
    score = float(input("Enter score of subject " + str(i) + ": "))
    sum = sum + score
average = sum / 6
print("Total Score:", sum)
print("Average Score:", average)

range(1, 7) gives 1 to 6 and initialises the loop variable; sum is initialised to 0 before the loop.

The loop variable takes one character of the string in each iteration.

text = input("Enter a string: ")
for ch in text:
    print(ch, end=" ")

Input "Python is interesting" prints P y t h o n i s .... end=" " keeps the output on one line.

The underscore is a placeholder used when the loop variable is not needed; its value is intentionally ignored. The code for _ in range(5): print("Hello") prints Hello five times.

for i in range(1, 16):
    print(i, end=" ")

for i in range(1, 11):
    print(i, "square =", i * i, " cube =", i ** 3)

total = 0
for i in range(2, 21, 2):
    total = total + i
print("Sum =", total)

Output: Sum = 110.

count = 0
for i in range(1, 51):
    if i % 3 == 0 or i % 5 == 0:
        count = count + 1
print("Count =", count)

Output: 23 (16 multiples of 3 + 10 multiples of 5 – 3 multiples of 15).

nums = list(map(int, input("Enter numbers separated by spaces: ").split()))
product = 1
for x in nums:
    product = product * x
print("Product =", product)

for i in range(100, 201):
    if i % 7 == 0:
        print(i, end=" ")

Output: 105, 112, 119, …, 196. A shortcut is range(105, 201, 7).

n = int(input("Enter a number: "))
fact = 1
for i in range(1, n + 1):
    fact = fact * i
print("Factorial =", fact)

nums = list(map(int, input("Enter numbers separated by spaces: ").split()))
largest = nums[0]
for x in nums:
    if x > largest:
        largest = x
print("Largest =", largest)

k = int(input("Enter a number to compare: "))
count = 0
for x in nums:
    if x > k:
        count = count + 1
print("Numbers greater than", k, ":", count)

(1) 0 1 2 3 4, each on a new line.

(2) 1 2 3 4 5 on one line.

(3) 10, 8, 6, each on a new line.

break ends the loop at once and control passes to the first statement after the loop.

for roll in range(1, 11):
    print("Roll Number:", roll)
    if roll == 5:
        print("Stopping attendance check.")
        break

roll = 1
while roll <= 10:
    print("Roll Number:", roll)
    if roll == 5:
        print("Stopping attendance check.")
        break
    roll = roll + 1

Both print roll numbers 1 to 5 and then the message.

for roll in range(1, 11):
    if roll == 5:
        continue
    print("Roll Number:", roll)

When roll is 5, continue skips the print() and moves to the next number. The loop is not terminated.

pass is a null statement that does nothing. It is used where a statement is syntactically required but no action is needed, e.g. to keep the structure of a block during development. Unlike continue, it does not skip the remaining statements of the iteration.

count = 0
for i in range(1, 6):
    subject = input("Enter subject name " + str(i) + ": ")
    if subject == "":
        pass              # nothing to do for empty input
    else:
        count = count + 1
    print("Number of iterations completed:", i)
print("Number of subjects entered:", count)

The else block of a for or while loop runs only if the loop finishes all its iterations without a break.

n = int(input("Enter a number: "))
for i in range(2, n):
    if n % i == 0:
        print("Not a prime number")
        break
else:
    print("Prime number")

If a divisor is found, break skips the else. If none is found, the loop completes and "Prime number" is printed. For 49 → Not a prime; for 59 → Prime. (For n = 0 or 1 the loop does not run, so this version would report prime; add a check n > 1 for complete correctness.)

n = int(input("Enter a number: "))
while n >= 0:
    print("You entered", n)
    n = int(input("Enter a number: "))
print("Negative number entered. Stopped.")

# or using break
while True:
    n = int(input("Enter a number: "))
    if n < 0:
        break

for i in range(1, 101):
    print(i, end=" ")
    if i == 42:
        break

for i in range(1, 31):
    if i % 3 == 0:
        continue
    print(i, end=" ")

absent = list(map(int, input("Enter absent roll numbers: ").split()))
for roll in range(1, 21):
    if roll in absent:
        continue
    print(roll, end=" ")

valid = []
while True:
    mark = int(input("Enter mark (999 to stop): "))
    if mark == 999:
        break
    if mark > 100:
        print("Invalid mark ignored")
        continue
    valid.append(mark)
print("Valid marks entered:", valid)

correct_pin = 4321
for attempt in range(5):
    pin = int(input("Enter 4-digit PIN: "))
    if pin == correct_pin:
        print("Access Granted")
        break
else:
    print("Account Locked")

The else runs only if all five attempts fail (no break).

A nested loop is a loop written inside another loop. The containing loop is the outer loop and the one inside is the inner loop. For every single iteration of the outer loop, the inner loop completes all its iterations. Nested loops handle two-level repetition such as tables, patterns, and rows and columns.

for variable1 in sequence1:        # Outer loop
    for variable2 in sequence2:    # Inner loop
        statements

while condition1:                  # Outer loop
    while condition2:              # Inner loop
        statements

Any combination (while inside for, for inside while) is allowed.

for roll in range(1, 6):
    print("\nEntering marks of Student", roll)
    total = 0
    for subject in range(1, 7):
        mark = float(input("Enter mark for Subject " + str(subject) + ": "))
        total += mark
    print("Total marks for Student", roll, ":", total)

total is reset to 0 for each student; the inner loop runs 6 times per student.

for i in range(1, 6):
    for j in range(1, 6):
        print(i, '*', j, '=', i * j)
    print()

i = 1
while i <= 5:
    j = 1
    while j <= i:
        print("*", end=" ")
        j = j + 1
    print()
    i = i + 1

The inner print("*", end=" ") keeps the row on one line; the outer print() moves to the next line. Printing j instead of "*" gives the number triangle 1 / 1 2 / 1 2 3 …

They affect only the loop in which they appear. A break in the inner loop stops only the inner loop and control passes to the next statement of the outer loop. A continue in the inner loop skips the rest of that inner iteration and moves to the inner loop's next cycle.

n = int(input("Enter number of lines: "))

# Pattern 1: 1 / 1 2 / 1 2 3 ...
for i in range(1, n + 1):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

# Pattern 2: 1 2 3 4 5 / 1 2 3 4 / ... / 1
for i in range(n, 0, -1):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

# Pattern 3: * * * * * / * * * * / ... / *
for i in range(n, 0, -1):
    for j in range(i):
        print("*", end=" ")
    print()

# Pattern 4: right-aligned 1 / 1 2 / 1 2 3 ...
for i in range(1, n + 1):
    print("  " * (n - i), end="")
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

i = 1
while i <= 10:
    j = 1
    while j <= 10:
        print(i, "x", j, "=", i * j)
        j = j + 1
    print()
    i = i + 1

for student in range(1, 4):
    total = 0
    for subject in range(1, 5):
        mark = float(input("Student " + str(student) + ", subject " + str(subject) + ": "))
        total = total + mark
    print("Total marks of student", student, "=", total)

i = 1
while i <= 4:
    j = 1
    while j <= 4:
        print("(", i, ",", j, ")", end=" ")
        j = j + 1
    print()
    i = i + 1

This prints 16 pairs, from (1, 1) to (4, 4).