Getting Started with Python — NCERT Solutions

Rules for valid Python identifiers:

• Must begin with a letter (A–Z or a–z) or an underscore (_).
• Can contain letters, digits (0–9), and underscores only.
• Cannot be a Python keyword.
• No spaces or special characters allowed.

i. Serial_no. — Invalid. Contains a dot (.), which is not allowed in an identifier.

ii. 1st_Room — Invalid. Starts with a digit (1), which is not permitted.

**iii. Hundred$** — **Invalid**. Contains a special character ($), which is not allowed.

iv. Total Marks — Invalid. Contains a space, which is not allowed.

v. Total_Marks — Valid. Starts with a letter and contains only letters and an underscore.

vi. total-Marks — Invalid. Contains a hyphen (-), which is not allowed (it is interpreted as the minus operator).

vii. Percentage — Valid. Starts with a letter and contains only letters.

viii. True — Invalid. True is a reserved keyword in Python and cannot be used as an identifier.

a) Assign 10 to length and 20 to breadth:
$$\text{length, breadth} = 10, 20$$

length, breadth = 10, 20

b) Assign the average of length and breadth to sum:
$$\text{sum} = (\text{length} + \text{breadth}) / 2$$

sum = (length + breadth) / 2

c) Assign a list to stationery:

stationery = ['Paper', 'Gel Pen', 'Eraser']

d) Assign strings to first, middle, last:

first, middle, last = 'Mohandas', 'Karamchand', 'Gandhi'

e) Concatenate with spaces to form fullname:

fullname = first + ' ' + middle + ' ' + last

This gives fullname = 'Mohandas Karamchand Gandhi'.

a) The sum of 20 and -10 is less than 12:

(20 + (-10)) < 12

Evaluation: $20 + (-10) = 10$, and $10 < 12$ is True.

b) num3 is not more than 24:

num3 <= 24

("Not more than" means less than or equal to.)

c) 6.75 is between the values of num1 and num2 (assuming num1 < num2):

num1 < 6.75 < num2

or equivalently:

(num1 < 6.75) and (6.75 < num2)

d) The string middle is larger than first and smaller than last (comparing variable values, not the literal strings):

(middle > first) and (middle < last)

e) List stationery is empty:

stationery == []

or equivalently:

len(stationery) == 0

a) Original: $0 == 1 == 2$ (evaluates to False)

Add parentheses:

0 == (1 == 2)

Evaluation: $(1 == 2)$ is False, which equals 0 in Python. So $0 == \text{False}$ is $0 == 0$ which is True. ✓

b) Original: $2 + 3 == 4 + 5 == 7$ (evaluates to False)

Add parentheses:

2 + 3 == (4 + 5 == 7)

Evaluation: $(4 + 5 == 7)$ → $(9 == 7)$ → False → $0$. Then $2 + 3 == 0$ → $5 == 0$ → False.

Alternative:

(2 + 3 == 4) + 5 == 7

Evaluation: $(2+3==4)$ → False → $0$. Then $0 + 5 == 7$ → $5 == 7$ → False.

Best valid answer:

2 + (3 == 4 + 5) == 2

Evaluation: $(3 == 4+5)$ → $(3==9)$ → False → $0$. Then $2 + 0 == 2$ → $2 == 2$ → True. ✓

c) Original: $1 < -1 == 3 > 4$ (evaluates to False)

Add parentheses:

1 < (-1 == 3) > 4

Evaluation: $(-1 == 3)$ → False → $0$. Then $1 < 0$ → False. Not True.

Alternative:

(1 < -1) == (3 > 4)

Evaluation: $(1 < -1)$ → False; $(3 > 4)$ → False. So False == False → True. ✓

a)

num1 = 4
num2 = num1 + 1   # num2 = 4 + 1 = 5
num1 = 2          # num1 is reassigned to 2
print(num1, num2)

Output: 2 5

b)

num1, num2 = 2, 6
num1, num2 = num2, num1 + 2
# Right-hand side is evaluated first:
# num2 = 6, num1 + 2 = 2 + 2 = 4
# So num1 = 6, num2 = 4
print(num1, num2)

Output: 6 4

c)

num1, num2 = 2, 3
num3, num2 = num1, num3 + 1

Here, num3 is used on the right-hand side (num3 + 1) before it has been assigned any value. This will raise a NameError: name 'num3' is not defined.

Output: NameError: name 'num3' is not defined

a) Number of months in a year:
Data type: int (Integer)
Reason: The number of months is a whole number (12). It has no fractional part.

b) Resident of Delhi or not:
Data type: bool (Boolean)
Reason: This is a yes/no (True/False) condition — either the person is a resident or not.

c) Mobile number:
Data type: str (String)
Reason: Although a mobile number consists of digits, no arithmetic operations are performed on it. It may also contain a leading zero or a '+' prefix (e.g., +91...), so it is best stored as a string.

d) Pocket money:
Data type: float (Floating point)
Reason: Pocket money can include paise (decimal values), e.g., ₹150.50, so a float is appropriate.

e) Volume of a sphere:
Data type: float (Floating point)
Reason: Volume involves $\pi$ and fractional values, e.g., $\frac{4}{3}\pi r^3$, which results in a decimal number.

f) Perimeter of a square:
Data type: float (Floating point)
Reason: The side length may be a decimal value, making the perimeter ($4 \times \text{side}$) a float. Even if integer, float is safer for general use.

g) Name of the student:
Data type: str (String)
Reason: A name is a sequence of alphabetic characters.

h) Address of the student:
Data type: str (String)
Reason: An address is a combination of letters, digits, and special characters, best represented as a string.

Given: num1 = 4, num2 = 3, num3 = 2

a)

num1 += num2 + num3
# num1 = 4 + (3 + 2) = 4 + 5 = 9
print(num1)

Output: 9

b)

num1 = num1 ** (num2 + num3)
# num1 = 4 ** (3 + 2) = 4 ** 5 = 1024
print(num1)

Output: 1024

c)

num1 **= num2 + num3
# num1 = 4 ** (3 + 2) = 4 ** 5 = 1024

(No print statement, so no output. num1 becomes 1024.)

d)

num1 = '5' + '5'
print(num1)

String concatenation: '5' + '5' = '55'
Output: 55

e)

print(4.00 / (2.0 + 2.0))
# = 4.00 / 4.0 = 1.0

Output: 1.0

f)

num1 = 2 + 9 * ((3 * 12) - 8) / 10
# Step 1: 3 * 12 = 36
# Step 2: 36 - 8 = 28
# Step 3: 9 * 28 = 252
# Step 4: 252 / 10 = 25.2
# Step 5: 2 + 25.2 = 27.2
print(num1)

Output: 27.2

g)

num1 = 24 // 4 // 2
# Left to right: 24 // 4 = 6, then 6 // 2 = 3
print(num1)

Output: 3

h)

num1 = float(10)
print(num1)

Converts integer 10 to float.
Output: 10.0

i)

num1 = int('3.14')
print(num1)

int() cannot directly convert a string containing a decimal point. This raises a ValueError: invalid literal for int() with base 10: '3.14'.
Output: ValueError

j)

print('Bye' == 'BYE')

Python string comparison is case-sensitive. 'Bye' ≠ 'BYE'.
Output: False

k)

print(10 != 9 and 20 >= 20)
# 10 != 9 → True
# 20 >= 20 → True
# True and True → True

Output: True

l)

print(10 + 6 * 2 ** 2 != 9 // 4 - 3 and 29 >= 29 / 9)
# Step 1: 2 ** 2 = 4
# Step 2: 6 * 4 = 24
# Step 3: 10 + 24 = 34
# Step 4: 9 // 4 = 2
# Step 5: 2 - 3 = -1
# Step 6: 34 != -1 → True
# Step 7: 29 / 9 ≈ 3.222
# Step 8: 29 >= 3.222 → True
# Step 9: True and True → True

Output: True

m)

print(5 % 10 + 10 < 50 and 29 <= 29)
# Step 1: 5 % 10 = 5
# Step 2: 5 + 10 = 15
# Step 3: 15 < 50 → True
# Step 4: 29 <= 29 → True
# Step 5: True and True → True

Output: True

n)

print((0 < 6) or (not(10 == 6) and (10 < 0)))
# Step 1: 0 < 6 → True
# Since first operand of 'or' is True, result is True (short-circuit)
# (The second part: not(10==6) → not False → True; 10<0 → False; True and False → False)
# True or False → True

Output: True

a) 25 / 0

Category: Runtime Error

Reason: The expression is syntactically correct (Python can parse it), but when Python attempts to execute the division, it encounters division by zero, which is mathematically undefined. Python raises a ZeroDivisionError at runtime.

b) num1 = 25; num2 = 0; num1/num2

Category: Runtime Error

Reason: The syntax is correct and the variables are properly assigned. However, when the expression num1/num2 is evaluated at runtime, num2 is 0, causing a ZeroDivisionError. The error occurs during execution, not during parsing.

Concept: A point $(x, y)$ lies within or on a circle of radius $r$ centred at the origin if:
$$x^2 + y^2 \leq r^2$$
Here, $r = 10$, so the condition is $x^2 + y^2 \leq 100$.

Python Expression:

(x**2 + y**2) <= 100

Evaluation:

a) (0, 0):
$$0^2 + 0^2 = 0 \leq 100 \Rightarrow \textbf{True}$$
The dart hits the centre of the board.

b) (10, 10):
$$10^2 + 10^2 = 100 + 100 = 200 \leq 100 \Rightarrow \textbf{False}$$
The dart misses the board.

c) (6, 6):
$$6^2 + 6^2 = 36 + 36 = 72 \leq 100 \Rightarrow \textbf{True}$$
The dart hits the board.

d) (7, 8):
$$7^2 + 8^2 = 49 + 64 = 113 \leq 100 \Rightarrow \textbf{False}$$
The dart misses the board.

Formula: $T(°F) = T(°C) \times \dfrac{9}{5} + 32$

Program:

# Program to convert Celsius to Fahrenheit
celsius = float(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9/5) + 32
print("Temperature in Fahrenheit =", fahrenheit)

Verification:

Boiling point (100°C):
$$F = 100 \times \frac{9}{5} + 32 = 180 + 32 = 212°F$$

Freezing point (0°C):
$$F = 0 \times \frac{9}{5} + 32 = 0 + 32 = 32°F$$

Output:

• Boiling point of water = 212.0 °F
• Freezing point of water = 32.0 °F

Formula:
$$SI = \frac{P \times R \times T}{100}$$
$$\text{Amount} = P + SI$$

Program:

# Program to calculate Simple Interest and Amount Payable
P = float(input("Enter Principal (P): "))
R = float(input("Enter Rate of Interest (R) in % per annum: "))
T = float(input("Enter Time (T) in years: "))

SI = (P * R * T) / 100
Amount = P + SI

print("Simple Interest =", SI)
print("Amount Payable =", Amount)

Sample Run:

Enter Principal (P): 1000
Enter Rate of Interest (R) in % per annum: 5
Enter Time (T) in years: 2
Simple Interest = 100.0
Amount Payable = 1100.0