CBSE Class 12 Computer Science Important Questions with Solutions

Predict the output (mutable default argument):

def change(x, lst=[]):
    lst.append(x)
    return lst

print(change(1))
print(change(2))

Solution: The output is [1] then [1, 2]. The default list is created only once when the function is defined and is shared across every call, so the second call keeps the first call's value. This is a favourite 2-mark trap.

Predict the output (global vs local scope):

x = 10
def f():
    x = 20
    print(x)
f()
print(x)

Solution: The output is 20 then 10. Assigning x = 20 inside the function creates a new local variable; the global x is unchanged. To change the global inside the function you must write global x first.

Write a function to count the lines in "story.txt" that begin with the letter 'A'.

def countA():
    f = open("story.txt", "r")
    count = 0
    for line in f:
        if line[0] == 'A':
            count += 1
    f.close()
    print(count)

Solution: Open the file in read mode, loop through each line and test its first character. Text-file read/write functions are a guaranteed 3–4 mark question.

Write a function to search a binary file "stu.dat" (records stored as [rollno, name, marks] with pickle) and display the record for a given roll number.

import pickle
def search(r):
    f = open("stu.dat", "rb")
    try:
        while True:
            rec = pickle.load(f)
            if rec[0] == r:
                print(rec)
    except EOFError:
        f.close()

Solution: Use pickle.load() inside a loop and stop on EOFError. Binary-file search/update with pickle is a common 4–5 mark question.

Write a function to add a record [rollno, name] to a CSV file "data.csv".

import csv
def addRec(rollno, name):
    f = open("data.csv", "a", newline="")
    w = csv.writer(f)
    w.writerow([rollno, name])
    f.close()

Solution: Open the file in append mode with newline="", create a csv.writer and call writerow(). The newline="" argument prevents blank rows and is often worth a mark.

Write push() and pop() functions to implement a stack using a list.

def push(stk, item):
    stk.append(item)

def pop(stk):
    if stk == []:
        return "Underflow"
    return stk.pop()

Solution: A stack is Last-In-First-Out. append() pushes to the top and pop() removes from the top. Always check for underflow (empty stack) before popping.

Table EMP(Eno, Name, Dept, Salary). Show each department and its employee count, only for departments with more than 2 employees.

Solution: SELECT Dept, COUNT(*) FROM EMP GROUP BY Dept HAVING COUNT(*) > 2; — GROUP BY groups rows by department and HAVING filters the grouped result. WHERE cannot be used with an aggregate like COUNT; that is the trick being tested.

Display the names of all employees in descending order of salary.

Solution: SELECT Name FROM EMP ORDER BY Salary DESC; — ORDER BY sorts the result; DESC makes it high-to-low (ASC or nothing is low-to-high).

Write code to fetch and display all rows of the table "student" from the MySQL database "school".

import mysql.connector
con = mysql.connector.connect(host="localhost",
    user="root", passwd="pw", database="school")
cur = con.cursor()
cur.execute("SELECT * FROM student")
for row in cur.fetchall():
    print(row)
con.close()

Solution: Connect with mysql.connector.connect(), create a cursor, run the query with execute(), then loop over fetchall(). Connectivity is a reliable 4–5 mark question in the Database unit.

Difference between a hub and a switch: a hub broadcasts incoming data to every connected device, while a switch sends it only to the specific destination device, so a switch is faster and more secure.

Difference between XML and HTML: HTML is a fixed markup language for displaying data with predefined tags; XML is extensible, has user-defined tags and is used to store and carry data.

Common expansions to memorise: URL — Uniform Resource Locator, FTP — File Transfer Protocol, SMTP — Simple Mail Transfer Protocol, VoIP — Voice over Internet Protocol, HTTP — HyperText Transfer Protocol.