Notes, summaries, assignments, exams, and problems for Computers

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Sistema de Gestión de Usuarios con Java Web

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Archivos HTML del Frontend

INDEX.HTML: Menú Principal del Sistema

<!DOCTYPE html>
<html>
<head>
    <title>Menú Principal del Sistema</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>
        h2 { color: #789; font-size: 45px; }
        select { color: brown; font-size: 30px; }
        h3 { color: #8b4513; font-size: 30px; }
    </style>
</head>
<body>
    <h2>Menú de Opciones del Sistema</h2>
    <h3>Seleccione una opción:</h3>
    <ol>
        <li><a href="Registro.html">Registrar Usuario</a></li>
        <li><a href="Acceso.html">Acceso
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PHP Programming Essentials and Core Features

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What is PHP?

PHP (Hypertext Preprocessor) is a widely-used open-source scripting language primarily designed for web development. It runs on the server-side, meaning the code is executed on the web server before the output is sent to the user's browser. PHP is embedded within HTML and supports various databases, making it a powerful tool for creating dynamic and interactive web applications.

Key Features of PHP

PHP has several features that make it a popular choice for web development:

  • Open-Source – PHP is free to use, with a large community of developers providing support and updates.
  • Cross-Platform Compatibility – PHP works on multiple operating systems like Windows, Linux, and macOS.
  • Easy to Learn and Use – Its syntax is simple and similar
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C++ Code Examples: Arithmetic Mean, Sum, Product, Square

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Arithmetic Mean

The following C++ code calculates the arithmetic mean of numbers from 1 to n:

int main() {
    int n;
    double suma = 0;
    cout << "Vnesi broj: ";
    cin >> n;
    if (n <= 0) {
        cout << "Brojot na elementi mora da bide pogolem od 0!" << endl;
        return 1;  
    }
    for (int i = 1; i <= n; i++) {
        suma += i;  
    }
    double sredina = suma / n;
    cout << "Aritmetichkata sredina na broevite od 1 do " << n << " e: " << sredina << endl;
    return 0;
}

Sum

The following C++ code calculates the sum of numbers from 1 to n:

int main() {
    int n, sum = 0;
    cout << "Vnesi broj n: ";
    cin >> n;
    for (int i = 1; i <= n; i++)
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Network Security & Configuration: Routing, VLANs, DHCP, and Attack Mitigation

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Router-on-a-Stick Inter-VLAN Routing

The router's port connecting to the LAN has multiple sub-interfaces, each the default gateway for a specific VLAN. For example, VLAN 10 traffic destined for VLAN 20 is first forwarded to VLAN 10's default gateway (the router sub-interface). The router then routes this traffic to VLAN 20's gateway (its corresponding sub-interface) and finally to the user in VLAN 20.

Why STP Is Needed for Redundant Ethernet LANs

  • Preventing Broadcast Storms: In redundant networks, frames can loop endlessly, exponentially increasing traffic. STP prevents this by disabling redundant paths, ensuring one active path between devices.
  • Ensuring MAC Address Table Consistency: Loops cause switches to receive the same frame on different
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Python Programming Essentials: Core Concepts & Techniques

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Understanding Variables & Data Types

Variables: Storing Data

Variables are containers used to store data. They are assigned values using the = operator (e.g., x = 5).

Essential Data Types

Python supports several fundamental data types:

  • int: Represents integer numbers (e.g., 10, -5).
  • float: Represents floating-point numbers (e.g., 3.14, -0.01).
  • str: Represents strings of characters (e.g., "hello", "123").
  • bool: Represents Boolean values, either True or False.

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Type Casting: Converting Data Types

Type casting allows you to convert data from one type to another using functions like int(), float(), and str().

Example: x = int("5") converts the string "5" to an integer. y = float("3.14") converts the string "3.14" to a float.

Input and Output Operations

Getting

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Classic Algorithm Solutions: Step-by-Step Examples

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1) DIJKSTRA (SSSP) — STEP TABLE (PAST-PAPER INSTANCE A)

Given edges: a→b(2), a→c(7), a→e(10), b→d(2), c→d(1), c→g(2), d→f(2), e→g(1), f→g(2). Source = a.
Rule: Pick unsettled vertex with minimum dist; relax outgoing edges.

Vertices:   a   b   c   d   e   f   g

Init:
Dist      = 0   2   7   ∞  10   ∞   ∞
S = ∅

Step 1: Pick a (min=0), S={a}
Relax from a: already set b=2, c=7, e=10
Dist      = 0   2   7   ∞  10   ∞   ∞

Step 2: Pick b (min=2), S={a,b}
Relax b→d: dist(d) = dist(b)+2 = 2+2 = 4
Dist      = 0   2   7   4  10   ∞   ∞

Step 3: Pick d (min=4), S={a,b,d}
Relax d→f: dist(f) = 4+2 = 6
Dist      = 0   2   7   4  10   6   ∞

Step 4: Pick f (min=6), S={a,b,d,f}
Relax f→g: dist(g) = 6+2 = 8
Dist
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8051 Microcontroller Architecture and Flip-Flop Types

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SR Flip-Flop (Set-Reset Flip-Flop)

The SR flip-flop is one of the simplest sequential circuits and serves as a fundamental building block for more complex flip-flops. It is a 1-bit memory device with two inputs: Set (S) and Reset (R). It has two outputs, Q and its complement Q̅ (Q with overline).

Logic Diagram and Working

An SR flip-flop can be constructed using two cross-coupled NAND gates (or NOR gates). Below is the working principle for the NAND-gate implementation:

  1. Set Condition (S = 0, R = 1):

    • When S = 0, the output of the top NAND gate (Q) is forced to 1 (since NAND is the inverse of AND).

    • This Q = 1 is fed into the bottom NAND gate with R = 1; the bottom gate's inputs become 1 and 1, so its output (Q̅) becomes 0.

    • The circuit is now in the

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C Language Fundamentals: Output, Control Flow, Strings, and Sorting

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C printf Function: Format Specifiers Explained

The printf function in C is used to display formatted output to the standard output (usually the console). It allows programmers to control how data is presented by using **format specifiers**. Format specifiers are placeholders that define the type of data being printed and how it should be formatted. They begin with a % symbol, followed by a character that specifies the data type.

Role of Format Specifiers

Format specifiers serve two main purposes:

  1. Data Type Identification: They inform the printf function about the type of data being passed as an argument. For example, %d is used for integers, while %f is used for floating-point numbers.
  2. Formatting Control: They allow customization of how the data
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Core Concepts in Network Layer Protocols and Routing

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1)A virtual-circuit network (VCN) is a hybrid network model that combines features of both circuit-switched and datagram networks. It provides a balance between connection-oriented and connectionless transmission methods :-Three Phases in a Virtual-Circuit Network In a virtual-circuit network, the communication between a source and destination involves three phases: setup, data transfer, and teardown. These phases ensure that a reliable path is established and maintained for the communication session. 1. Setup Phase: o The source and destination use their global addresses to establish a connection. During this phase, switches along the path create table entries to store information about the virtual circuit. This phase ensures that each switch... Continue reading "Core Concepts in Network Layer Protocols and Routing" »

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Essential Linux Commands & File System Structure

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Linux File System Structure: An archive of Linux is associated with 3 parts: superblock, inode table, and data blocks.

Network Ports: To see the ports assigned to services.

Display Active TCP/IP Connections: netstat -a

User Management:

  • Create password: passwd (user)
  • Add user to group: usermod -g group_name
  • Disable: 60001
  • Enable: 60002

Practical Commands:

Add User: adduser

  1. Change folder privileges: chmod
  2. Check privileges: ls -de (see if you changed privileges)
  1. Create a user: useradd newuser
    passwd newuser
  2. Create a directory: The command mkdir is used to create directories:
    mkdir mydirectory
  3. Create a report: ps -aux >> reporte.txt
  4. Directories associated with the user: -d dirname
  5. Changing permission: chmod 744 file.txt /file.txt
  6. Change owner: chown
    Entering
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