CBSE Class 11 Computer Science Question Paper Set 12 with Solutions

Answer: (c) Identifier. Names given by the programmer to variables, functions, classes and so on are identifiers.

Teacher's Note:
1. Identifiers are user-chosen names, while literals are fixed values such as 5 or 'a'.
2. A token is the general term for any smallest unit of a program, so it is too broad an answer.

Answer: (d) dictionary. A dictionary can be changed after creation (items added, modified or deleted), whereas int, tuple and string cannot.

Teacher's Note:
1. Remember the immutable group: int, float, bool, string and tuple.
2. Lists, dictionaries and sets are the mutable types.

Answer: (d) Error. int() can convert a string only if it holds digits; 'ABC' raises ValueError: invalid literal for int().

Teacher's Note:
1. int('25') works, but int('ABC') does not because the text is not a number.
2. Always check what the string contains before judging a type conversion.

Answer: (c) (2+0j). complex(2) creates a complex number with real part 2 and imaginary part 0, and Python prints it as (2+0j).

Teacher's Note:
1. When only one argument is given, the imaginary part defaults to 0.
2. Python writes the imaginary unit as j and prints complex values in brackets.

Answer: (c) print(x%y). 5 % 3 is the remainder 2. For the others: 5/3 = 1.666..., 5//3 = 1, 5**3 = 125.

Teacher's Note:
1. Use / for true division, // for floor division and % for the remainder.
2. Python is case sensitive, so the variables must be written the same way everywhere.

Answer: (a) False, True.
1. Precedence is not, then and, then or. First line: (not a) or ((not b) and c) = False or (True and False) = False or False = False.
2. Second line: (not a) or b or ((not b) and a) = False or False or (True and True) = True.

Teacher's Note:
1. Evaluate not first, then and, and finally or.
2. Putting brackets around each and-part makes such expressions easy to check.

Answer: (b) False. Python treats a chained comparison as (1 > 2) and (2 < 3) = False and True = False.

Teacher's Note:
1. Chained comparisons are joined with an implicit and between neighbouring pairs.
2. Here the first pair fails, so the whole result is False.

Answer: (c) IndexError. The list has 6 elements with valid indexes 0 to 5, so index 6 is out of range.

Teacher's Note:
1. The last valid index is always len(list) - 1.
2. Slicing never raises this error, but a single out-of-range index does.

Answer: (a) ['Green', 'Blue']. A slice [1:3] takes indexes 1 and 2; the end index 3 is excluded.

Teacher's Note:
1. In a slice start:stop the stop index is never included.
2. Counting indexes from 0 avoids the usual off-by-one slip.

Answer: (b) Cyber trolling. A troll posts provocative or offensive comments to upset people or start arguments.

Teacher's Note:
1. Trolling aims to provoke a reaction, while bullying targets and harasses a particular person.
2. Phishing is a different crime, in which fake messages steal personal data.

Answer: (a) Worm. A worm is standalone and copies itself across networks without needing a host file.

Teacher's Note:
1. The key difference is that a virus needs a host program, while a worm does not.
2. A Trojan hides inside software that looks useful but does not copy itself.

Answer: (c) Spam. Unsolicited bulk messages sent automatically are called spam.

Teacher's Note:
1. Think of spam as junk mail sent in bulk to many people.
2. Adware shows advertisements on your device, and is not an email message.

Answer: (d). Options (a), (b) and (c) all describe encryption: converting data into code that only key holders can read. Option (d) describes the broader idea of privacy or data protection, not what encryption itself is.

Teacher's Note:
1. Encryption is a technique, while privacy protection is a goal that encryption helps to achieve.
2. In an 'is not true' question, test each option against the definition.

Answer: (a) The items are accessed via the keys. A dictionary stores key:value pairs and a value is retrieved with its key, for example D['what'].

Teacher's Note:
1. Lists and tuples use positions, but dictionaries use keys.
2. Keys must be unique and immutable, while values can be of any type.

Answer: (a) Overwriting the value for a particular key. For example D['what'] = 5 replaces the old value of the key 'what'.

Teacher's Note:
1. A dictionary is mutable, but its keys cannot be changed in place.
2. To rename a key, you must add a new key and delete the old one.

Answer: (d) 'abc' + 3. Concatenation with + needs two strings, and adding a string and an int raises TypeError. The others are valid: 'abcabc', 'abcabcabc' and 'abcabcabc'.

Teacher's Note:
1. The + operator joins two strings and * repeats a string by an integer.
2. Use str(3) first if you really want to join a number to text.

Answer: 1. Write LHS = AB + BC + ĀC.
2. Multiply BC by (A + Ā), which equals 1: BC = BC(A + Ā) = ABC + ĀBC.
3. LHS = AB + ABC + ĀBC + ĀC.
4. Group terms: AB(1 + C) + ĀC(B + 1).
5. Since 1 + C = 1 and B + 1 = 1, LHS = AB + ĀC = RHS. Hence proved. (This is the consensus theorem.)

Teacher's Note:
1. Insert the missing variable using A + Ā = 1 to expand the redundant term BC.
2. Then apply the absorption law X + XY = X to remove the extra terms.

Answer: 1. The ALU (Arithmetic Logic Unit) is the part of the CPU that performs all arithmetic and logical operations.
2. The control unit fetches the data and instruction and sends the operands (from registers or memory) to the ALU along with the operation to perform.
3. The ALU does arithmetic operations (addition, subtraction, multiplication, division) and logical operations (comparisons such as greater than, equal to, and AND, OR, NOT).
4. The result is sent back to a register or memory, and flags are set to show conditions such as zero or negative.

Teacher's Note:
1. Remember that the ALU only calculates, and the control unit decides what happens next.
2. Mention both arithmetic and logical operations to earn full marks.

Answer: Replace each hex digit with its 4-bit binary code: 4 = 0100, A = 1010, A = 1010, D = 1101.
So (4AAD)₁₆ = (0100 1010 1010 1101)₂ = (100101010101101)₂.

Teacher's Note:
1. Each hexadecimal digit corresponds to exactly four bits.
2. Remember A = 10, B = 11, C = 12, D = 13, E = 14 and F = 15.

Answer: 1. Make both numbers 5 bits: 10000 and 01111.
2. 1's complement of 01111 is 10000; add 1 to get the 2's complement: 10001.
3. Add: 10000 + 10001 = 1 00001.
4. Discard the carry (the leftmost 1), so the result is 00001 = 1.
Check: 16 − 15 = 1.

Teacher's Note:
1. Always write both numbers with the same number of bits before taking the complement.
2. A carry out of the leftmost bit is dropped, and it shows that the answer is positive.

Answer: 1. A token is the smallest individual unit of a program that has a meaning for the interpreter. It is also called a lexical unit.
2. Python tokens are of five types: keywords, identifiers, literals, operators and punctuators.
3. Example: in the statement age = 15 + 2, the tokens are age (identifier), = and + (operators), and 15, 2 (literals).

Teacher's Note:
1. Always give a complete example that names each kind of token it contains.
2. Do not confuse a token with an identifier, because an identifier is just one kind of token.

Answer: 1. Protects: copyright protects original creative works such as books, music, software code and art. A patent protects a new invention or process.
2. Registration: copyright arises automatically when the work is created, while a patent must be applied for and granted.
3. What is covered: copyright covers the expression of an idea, not the idea itself. A patent covers the idea or technical working of the invention.
4. Duration: copyright generally lasts for the creator's lifetime plus 60 years in India, while a patent lasts for 20 years from filing.

Teacher's Note:
1. Learn the contrast as 'creative expression' for copyright against 'inventions' for a patent.
2. Give at least two clear points of difference for 2 marks.

Answer: 1. X = 10; then X = 20; then X = 15, so the first print gives 15.
2. In X, Y = X - 2, 22, the right side is evaluated first with the old X: 15 − 2 = 13 and 22. So X = 13 and Y = 22.
3. The second print gives 13 22.
Output:

15
13 22

Teacher's Note:
1. In a multiple assignment, the whole right side is evaluated before any name is changed.
2. Trace the value of X line by line to avoid slips.

Answer: import math
math.sqrt(x) + (x + y) / z
(Equivalent: x ** 0.5 + (x + y) / z.)

Teacher's Note:
1. Brackets around x + y are essential, otherwise only y would be divided by z.
2. Import the math module if you use math.sqrt().

Answer: x ** (1/4) (or x ** 0.25).

Teacher's Note:
1. A fourth root is the same as raising to the power 1/4.
2. Brackets are needed around 1/4, because ** has higher precedence than /.

Answer: 1. if num<0 has no colon at the end; it should be if num<0:.
2. AND must be written in lower case: and. The line also needs a colon at the end.
3. elif : num>=10 OR ... has the colon in the wrong place and uses OR; it should be elif num>=10 or num<=99: (logically it should be and).
4. elif : num<=999 must be elif num<=999:.
5. The else is wrongly indented, so it must align with the if and elif keywords.
Corrected code:

num = int(input('Enter an integer: '))
if num < 0:
    num = -num
if num >= 0 and num <= 9:
    print('Single digit number')
elif num >= 10 and num <= 99:
    print('Two digit number')
elif num <= 999:
    print('Three digit number')
else:
    print('Number contains more than 3 digits')

Teacher's Note:
1. The condition always comes before the colon, and the colon ends every if, elif and else header.
2. Python's logical operators are in lower case: and, or, not.

Answer: 1. Hacking means gaining access to a computer system, network or data by finding and exploiting weaknesses in it.
2. Ethical hacker: works with the owner's permission to find security weaknesses so that they can be fixed. The work is legal and is done for protection.
3. Non-ethical hacker (cracker or black-hat hacker): breaks into systems without permission, to steal, damage or misuse data. The work is illegal and done for personal gain or harm.

Teacher's Note:
1. Permission and intention are the two points that separate ethical from non-ethical hackers.
2. Add a one-line definition of hacking, since it is asked separately.

Answer: 1. They keep online communication respectful and polite, which avoids conflict, trolling and bullying.
2. They protect your privacy and that of others, by not sharing personal details or posting someone's photo without consent.
3. They build a positive digital reputation, because what is posted stays on the internet and can affect studies and jobs.
4. They stop the spread of fake news, hate speech and unverified content, and give credit to the owners of content (avoiding plagiarism).

Teacher's Note:
1. Link each point to safety, respect or reputation, and support it with an example.
2. Two or more well explained points are enough for full marks.

Answer: 1. break terminates the loop completely and control moves to the first statement after the loop.
2. continue skips the rest of the current iteration only, and the loop goes on with the next iteration.
Example:

for i in range(1, 6):
    if i == 3:
        break
    print(i)      # prints 1, 2

for i in range(1, 6):
    if i == 3:
        continue
    print(i)      # prints 1, 2, 4, 5

Teacher's Note:
1. Break exits the loop, but continue only skips the current round.
2. Show the output of both examples so the difference is clear.

Answer: 1. range(0, 10) produces 0, 1, 2, ..., 9, which is 10 values, so the loop body runs 10 times.
2. The loop variable keeps the last value it received, so the final value of i is 9.

Teacher's Note:
1. The stop value of range() is never included.
2. The variable i keeps its last value even after the loop ends.

Answer: All four actions (a, b, c and d) may have created a digital footprint.
1. Each search or website visit is stored in the browser history, cookies and the search engine's records.
2. Websites also log your IP address, the pages you visited and the time. This is a passive (unintentional) digital footprint.
3. If you log in, book tickets or hotels, or share details such as your name and card number, an active (intentional) footprint is left as well.

Teacher's Note:
1. A digital footprint is the trail of data you leave while using the internet, whether you intend it or not.
2. Name both active and passive footprints to show full understanding.

Answer:

for i in range(100, 0, -3):
    print(i)

Teacher's Note:
1. The three arguments of range() are start, stop and step, and a negative step counts downwards.
2. Check the first and last printed values against the original loop.

Answer: 1. A single-element tuple is created by writing a comma after the element: T = (3,) (or T = 3,).
2. In X = (3) there is no comma, so the brackets are only for grouping. X is just the integer 3, and type(X) gives <class 'int'>, not a tuple.

Teacher's Note:
1. It is the comma that makes a tuple, and not the brackets.
2. Check with type() if you are not sure what an expression has produced.

Answer:

for i in range(4):
    for j in range(i, -1, -1):
        print(chr(65 + j), end=' ')
    print()

Teacher's Note:
1. Row i holds i + 1 letters, written in reverse order starting from the row's own letter.
2. Use end=' ' inside the inner loop, and an empty print() after it.

Answer:

x = float(input('Enter x: '))
n = int(input('Enter n: '))
s = 0
for i in range(1, n + 1):
    s = s + x ** i / i
print('Sum of the series =', s)

Teacher's Note:
1. Start the sum at 0 and use range(1, n + 1) so that n is included.
2. Check the program once with small values such as x = 2 and n = 4.

Answer: (i) L[1] is ['are', 'a', 'few', 'words']; taking every second element from index 0 gives ['are', 'few'].
(ii) L[:1] is ['These'] and L[1] is the inner list, so joining them gives ['These', 'are', 'a', 'few', 'words'].
(iii) Starting from index 2 with step 2 picks indexes 2 and 4, giving ['that', 'will'].

Teacher's Note:
1. Work out the inner expression first, then apply the slice to its result.
2. In a slice a:b:c, c is the step, and an empty b means up to the end.

Answer: Errors are caused by (ii), (iv) and (v).
1. (ii) and (iv): L1 and L2 are lists, and a list has no upper() method (AttributeError); upper() works only on strings.
2. (v): L2[1] is the list ['is', 'another'], so again upper() cannot be applied (AttributeError).
3. (i) gives False, because the lists differ; comparing is allowed. (iii) gives 'LIST'. (vi) gives 'ANOTHER'; these are strings.

Teacher's Note:
1. Always find out the type of the object before calling a method on it.
2. upper() belongs to strings and not to lists, even if the list holds strings.

Answer: (a) The value of 'what' becomes 11: {'what': 11, 'are': 11, 'you': 14, 'doing': 5, 'next': 9, 'Saturday?': 4}
(b) 'what' is updated to 5 and the new key 'Sunday' is added at the end: {'what': 5, 'are': 11, 'you': 14, 'doing': 5, 'next': 9, 'Saturday?': 4, 'Sunday': 25}
(c) The key 'you' is removed: {'what': 22, 'are': 11, 'doing': 5, 'next': 9, 'Saturday?': 4}
(d) A new key 'which' is added: {'what': 22, 'are': 11, 'you': 14, 'doing': 5, 'next': 9, 'Saturday?': 4, 'which': 19}

Teacher's Note:
1. Assigning to an existing key changes its value, while a new key is added at the end.
2. Restart each part from the original dictionary, since the question says so.

Answer: Blanks: (i) all_freq   (ii) all_freq[i] + 1   (iii) 1
If the character is already a key, its count goes up by 1; otherwise it is stored for the first time with count 1.
Output:

Count of all characters is :
{'A': 1, 'R': 1, 'M': 1, 'Y': 1, ' ': 3, 'P': 1, 'U': 1, 'B': 1, 'L': 4, 'I': 2, 'C': 2, 'S': 2, 'H': 2, 'O': 3, 'N': 1, 'G': 1}

Teacher's Note:
1. The in operator checks the keys of a dictionary, so use it before reading a count.
2. Do not forget that spaces are characters too and are counted.