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

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.

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

This program demonstrates the Banker's Algorithm, a classic deadlock avoidance strategy used in operating systems to manage resource allocation safely.

C Source Code

#include <stdio.h>

int n, m;
int alloc[10][10], max[10][10], need[10][10];
int avail[10], work[10], finish[10];

// Safety Algorithm
int checkSafety() {
    int seq[10], count = 0;

    for (int i = 0; i < n; i++) finish[i] = 0;
    for (int i = 0; i < m; i++) work[i] = avail[i];

    while (count < n) {
        int found = 0;
        for (int i = 0; i < n; i++) {
            if (!finish[i]) {
                int j;
                for (j = 0; j < m; j++) {
                    if (need[i][j] > work[j]) break;
                }
                if (j == m)

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

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 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

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" »

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

C TCP Sliding Window Client Example

Source code with timeout and acknowledgment handling

Note: This example demonstrates socket programming in C using a sliding window and a receive timeout. It sends numbered frames, waits for acknowledgments, and resends frames on timeout.

Key points

• Uses setsockopt to set a receive timeout (SO_RCVTIMEO)
• Sends sequential frames and handles acknowledgments
• Resends a window of frames when acknowledgments are not received

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <arpa/inet.h>
#define MAX 80
#define PORT 8080
#define SA struct sockaddr