Important CS Programs

def greet():
    print("Hello, Student!")
    print("Welcome to Python Functions.")

# Function Call
greet()

def add_numbers(a, b):
    sum_val = a + b
    print("The sum is:", sum_val)

# Passing arguments 10 and 20
add_numbers(10, 20)

def square(num):
    return num * num

result = square(5)
print("Square of 5 is:", result)

def arithmetic(a, b):
    add = a + b
    sub = a - b
    return add, sub

s, d = arithmetic(10, 5)
print("Sum:", s)
print("Difference:", d)

def introduce(name, age):
    print("Name:", name)
    print("Age:", age)

# Order matters here
introduce("Rahul", 17)

def introduce(name, age):
    print("Name:", name)
    print("Age:", age)

# Order does not matter
introduce(age=18, name="Simran")

def greet(name, msg="Good Morning"):
    print("Hello", name, msg)

greet("Amit")           # Uses default msg
greet("Sita", "Good Night") # Overrides default

def my_func():
    x = 10 # Local variable
    print("Inside:", x)

my_func()
# print(x) # This would cause an Error: name 'x' is not defined

x = 50 # Global variable

def my_func():
    print("Inside function:", x) # Accessing global

my_func()
print("Outside function:", x)

count = 0

def increment():
    global count # Declare we are using the global variable
    count = count + 1

increment()
print("Count is now:", count)

# abs(), len(), max() are built-in
print(abs(-10))
print(max(5, 10, 2))
print(len("Python"))

import random

# Generates random integer between 1 and 6
dice = random.randint(1, 6) 
print("You rolled:", dice)

print("1. Program Start")

def func():
    print("3. Inside Function")

print("2. Before Call")
func()
print("4. After Call")

def display():
    print("I return nothing")

result = display()
print("Result is:", result) # Output: None

def append_item(my_list):
    my_list.append(100)

data = [1, 2, 3]
append_item(data)
print("List outside:", data) # [1, 2, 3, 100]

def change_val(x):
    x = 100 # Creates a new local x

num = 5
change_val(num)
print("Num outside:", num) # Still 5

try:
    a = 10
    b = 0
    print(a / b)
except ZeroDivisionError:
    print("Error: Cannot divide by zero!")

try:
    num = int(input("Enter a number: "))
    print("You entered:", num)
except ValueError:
    print("Invalid! Please enter digits only.")

try:
    a = int(input("Enter A: "))
    b = int(input("Enter B: "))
    print(a / b)
except ValueError:
    print("Please enter integers.")
except ZeroDivisionError:
    print("Divisor cannot be zero.")

try:
    # Some risky code
    x = 1 / 0
except Exception as e:
    print("Something went wrong:", e)

try:
    print("Trying code...")
    # x = 1/0
except:
    print("Error occurred")
finally:
    print("This runs always (Cleanup).")

try:
    val = 10
except:
    print("Error")
else:
    print("No error occurred, success!")

# 'w' mode creates/overwrites file
f = open("sample.txt", "w")
f.write("Hello World")
f.close() # Always close the file!

with open("sample.txt", "w") as f:
    f.write("Python is great.")
# File is closed automatically here

with open("sample.txt", "r") as f:
    content = f.read()
    print(content)

with open("sample.txt", "r") as f:
    data = f.read(5) # Read first 5 chars
    print(data)

with open("sample.txt", "r") as f:
    line1 = f.readline()
    line2 = f.readline()
    print("Line 1:", line1, end='')
    print("Line 2:", line2, end='')

with open("sample.txt", "r") as f:
    lines = f.readlines()
    print(lines) # Output: ['line1\n', 'line2\n']

with open("sample.txt", "a") as f:
    f.write("\nThis is a new line.")

lines = ["Apple\n", "Banana\n", "Cherry\n"]
with open("fruits.txt", "w") as f:
    f.writelines(lines)

with open("fruits.txt", "r") as f:
    for line in f:
        print(line.strip()) # strip() removes newline char

with open("sample.txt", "r") as f:
    print("Start Pos:", f.tell())
    f.read(5)
    print("Current Pos:", f.tell())
    f.seek(0) # Go back to start
    print("Reset Pos:", f.tell())

count = 0
with open("sample.txt", "r") as f:
    data = f.read()
    words = data.split()
    count = len(words)
print("Total words:", count)

count = 0
with open("sample.txt", "r") as f:
    for line in f:
        count += 1
print("Total lines:", count)

with open("source.txt", "r") as f1:
    with open("dest.txt", "w") as f2:
        f2.write(f1.read())
print("File copied.")

target = "the"
count = 0
with open("story.txt", "r") as f:
    words = f.read().split()
    for w in words:
        if w.lower() == target:
            count += 1
print("'the' appears:", count, "times")

with open("story.txt", "r") as f:
    for line in f:
        if line.startswith("A"):
            print(line)

with open("data.txt", "r") as f:
    lines = [line.rstrip() for line in f]
print(lines)

f = open("log.txt", "w")
f.write("Critical Data")
f.flush() # Data is written even if file isn't closed yet
f.close()

with open("data.txt", "r+") as f:
    content = f.read()
    f.write("New Data") # Appends to end if read() finished

with open("data.txt", "w+") as f:
    f.write("Fresh Start")
    f.seek(0) # Go back to read what we wrote
    print(f.read())

import pickle
print("Pickle module imported.")

import pickle
data = [10, 20, 30, 40]
# Use 'wb' for write binary
with open("list.dat", "wb") as f:
    pickle.dump(data, f)
print("List dumped.")

import pickle
# Use 'rb' for read binary
with open("list.dat", "rb") as f:
    data = pickle.load(f)
    print("Loaded data:", data)

import pickle
f = open("students.dat", "wb")
while True:
    r = int(input("Roll: "))
    n = input("Name: ")
    stu = {"Roll": r, "Name": n}
    pickle.dump(stu, f)
    ans = input("More? (y/n): ")
    if ans == 'n': break
f.close()

import pickle
f = open("students.dat", "rb")
try:
    while True:
        stu = pickle.load(f)
        print(stu)
except EOFError:
    print("End of file reached.")
f.close()

import pickle
# 'ab' stands for Append Binary
with open("students.dat", "ab") as f:
    stu = {"Roll": 99, "Name": "New Student"}
    pickle.dump(stu, f)

import pickle
search_roll = 2
found = False

f = open("students.dat", "rb")
try:
    while True:
        s = pickle.load(f)
        if s["Roll"] == search_roll:
            print("Found:", s)
            found = True
except EOFError:
    pass
f.close()

import pickle
all_data = []
# 1. Read all
with open("students.dat", "rb") as f:
    try:
        while True:
            all_data.append(pickle.load(f))
    except EOFError: pass

# 2. Modify in memory
for s in all_data:
    if s["Roll"] == 1:
        s["Name"] = "Updated Name"

# 3. Write back
with open("students.dat", "wb") as f:
    for s in all_data:
        pickle.dump(s, f)

import pickle
count = 0
with open("students.dat", "rb") as f:
    try:
        while True:
            pickle.load(f)
            count += 1
    except EOFError: pass
print("Total Records:", count)

import csv
data = ["Roll", "Name", "Marks"]
with open("marks.csv", "w", newline="") as f:
    w = csv.writer(f)
    w.writerow(data) # Write header
    w.writerow([1, "Amit", 90])

import csv
rows = [
    [2, "Sumit", 85],
    [3, "Tina", 95]
]
with open("marks.csv", "a", newline="") as f:
    w = csv.writer(f)
    w.writerows(rows)

import csv
with open("marks.csv", "r") as f:
    r = csv.reader(f)
    for row in r:
        print(row) # row is a list of strings

import csv
with open("data.csv", "r") as f:
    r = csv.reader(f, delimiter='|')
    for row in r:
        print(row)

import csv
target_roll = "2" # CSV data is string
with open("marks.csv", "r") as f:
    r = csv.reader(f)
    for row in r:
        if row[0] == target_roll:
            print("Found:", row)

stack = []
def push(stk, item):
    stk.append(item)
    print(item, "pushed.")

push(stack, 10)
push(stack, 20)
print(stack) # [10, 20]

def pop(stk):
    if len(stk) == 0:
        return "Underflow! Stack is empty."
    else:
        return stk.pop()

s = [10, 20]
print(pop(s)) # 20
print(pop(s)) # 10
print(pop(s)) # Underflow

def peek(stk):
    if not stk:
        return "Empty"
    else:
        return stk[-1] # Last element

s = [5, 10, 15]
print("Top element:", peek(s))

def display(stk):
    if not stk:
        print("Empty Stack")
    else:
        # Loop backwards
        for i in range(len(stk)-1, -1, -1):
            print(stk[i])

s = [1, 2, 3]
display(s)

stack = []
while True:
    print("\n1. Push 2. Pop 3. Display 4. Exit")
    ch = int(input("Choice: "))
    if ch == 1:
        v = int(input("Val: "))
        stack.append(v)
    elif ch == 2:
        if not stack: print("Underflow")
        else: print("Popped:", stack.pop())
    elif ch == 3:
        print(stack[::-1])
    elif ch == 4:
        break

with open("sample.txt", "r") as f:
    f.seek(5) # Skip first 5 bytes
    data = f.read(3) # Read next 3 bytes
    print(data)

f = open("image.jpg", "rb")
f.seek(0, 2) # Move to end (2 means end relative to 0)
size = f.tell()
print("Size in bytes:", size)
f.close()

string = "Hello"
stack = []
for char in string:
    stack.append(char)

rev = ""
while stack:
    rev += stack.pop()

print("Reversed:", rev)

try:
    print("Outer Try")
    try:
        val = int("abc")
    except ValueError:
        print("Inner Catch: Value Error")
except:
    print("Outer Catch")

square = lambda x: x * x
print(square(5))

import csv
data = {"Name": "Ali", "Age": 17}
with open("names.csv", "w", newline="") as f:
    fields = ["Name", "Age"]
    w = csv.DictWriter(f, fieldnames=fields)
    w.writeheader()
    w.writerow(data)

with open("img.png", "rb") as f1:
    with open("copy.png", "wb") as f2:
        f2.write(f1.read())

def shout(text):
    return text.upper()

yell = shout
print(yell("hello"))

def divide(a, b):
    assert b != 0, "Divisor cannot be zero"
    return a / b

print(divide(10, 2))
# print(divide(10, 0)) # Raises AssertionError

text_var = ""
with open("poem.txt", "r") as f:
    text_var = f.read()
print(text_var)

count = 0
with open("data.txt", "r") as f:
    text = f.read()
    for char in text:
        if char.isupper():
            count += 1
print("Uppercase count:", count)

def area(r):
    """Calculates area of circle."""
    return 3.14 * r * r

print(area.__doc__)

def future_feature():
    pass # To be implemented later

future_feature()

# Windows example path
path = "C:\\Users\\Student\\Desktop\\file.txt"
# f = open(path, "r")
print("Use double backslash in paths.")

import os
if os.path.exists("sample.txt"):
    print("File exists")
else:
    print("File not found")

import os
# os.remove("useless.txt")
print("Code commented for safety")

import os
# os.rename("old.txt", "new.txt")
print("Renaming logic")

import pickle
x = 999
with open("int.dat", "wb") as f:
    pickle.dump(x, f)

def add_all(*args):
    return sum(args)

print(add_all(1, 2, 3, 4))

s = []
if len(s) == 0:
    print("Stack Empty")