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  • Starting #100DaysOfCode

    What exactly I will do during the 100 Days?

    • DSA – this will be my main focus during this 100 days for my interview preparation and will be doing LeetCode questions side-by-side. [Main resource: Coding Minutes DSA Course] and (Blind’s 75 List Qs/ Grokking Coding Interviews)
    • Web – One of my long term goal is to make useful things on the internet by building them in public (this is an attempt to that) for the people. So, I want to build meaningful things will indie-hacking.

    Some Techniques

    • Will take care of my health – daily morning walks (atleast 20min brisk), I guess I have to consider being an early riser 🙂
    • Have felt I am more productive when I use Pomodoro technique (25mins – then 5mins break) – will use that more often. (writing this blog on a Pomodoro :P) – helps well with procrastination!
    • Plan well my tasks and schedule!
    • For Stress and Anxiety – try Box Breathing, (inhale: 4s, hold: 4s, exhale: 4s, hold: 4s), listen to some music 🙂
    • HAVE FUN!

  • Behavioural Patterns

    Memento Pattern

    • Undo in text editor

    Observer Pattern

    • WeatherStation broadcasting temperature to DisplayDevice

    Strategy Pattern

    • PaymentService processing Payments with PaymentStrategy

    Command Pattern

    • Bold, Italic Command Buttons – Generic button

    Template Pattern

    • DataParser – JSONParser, CSVParser

    Iterator Pattern

    • Java Iterable – .hasNext(), .next()

    State Pattern

    • TransportationService – ETA/Directions based on TransportationMode

    Mediator Pattern

    • ChatUser sending messages to other ChatUsers in a ChatRoom

  • UML Relationships

    Association

    Are related but can exist independently.
    Eg: Professor teaches Students.

    Aggregation

    Weak ‘has-a’ relationship.
    Eg: Department has Professors, but Professors exists independently.

    Composition

    Strong ‘has-a’ relationship.
    Eg: House has Rooms, Rooms can’t exist independently.

    Inheritance

    ‘is-a’ relationship.
    Eg: Dog is Animal.

    Dependency

    A class depends on another for its functionality.
    Eg: Printer depends on Document.

    Realization

    A class implementing an interface.

  • Year 2025!

    Well, this year I am planning big again haha – heads down and execute.
    Want to read a lot, get a new job!
    Excited for this year 🙂

  • 2D Arrays

    Multi-dimensional Arrays

    • 1D: Marks of Students in a subject
    • 2D: Marks of Students in multiple subjects
    • 3D: Image – Rows x Columns in B/W image (2D) but in colour images we have 3D arrays will R, G, B values.
    • 4D: Videos – Images with time
    • ND

    Input/ Output

    #include <iostream>
using namespace std;

void print(int arr[][100], int n, int m) // pass by ref, passing arr size of row is optional
{

  for (int i = 0; i < n; i++)
  {
    for (int j = 0; j < m; j++)
    {
      cout << arr[i][j] << '\t';
    }
    cout << endl;
  }
}

int main()
{
  int arr[100][100];

  int n, m; // n: rows / m: cols
  cin >> n >> m;

  for (int i = 0; i < n; i++)
  {
    for (int j = 0; j < m; j++)
    {
      cin >> arr[i][j];
    }
  }

  print(arr, n, m);
}
    • In Memory: Row Major (mostly used), Column Major

    2D Character Arrays

    #include <iostream>

using namespace std;

int main()
{

  char nums[][10] = {"one", "two", "three", "eighteen", "seventy", "hundred"};

  cout << nums[3] << endl; // nums[3] returns address but cout prints as string unlike int
  cout << nums[3][2] << endl;

  for (int i = 0; i < 6; i++)
  {
    cout << nums[i] << " ";
  }
  cout << endl;

  return 0;
}
    , , ,

  • Character Arrays: Strings

    • End of the string is marked by NULL character ‘\0’ – so cout can print the string and not start printing garbage values
    • cin can take in only first word and will not take words after the whitespace
    #include <iostream>
#include <cstring>

using namespace std;

int main()
{
  char a[10];
  char b[] = {'a', 'b', 'c', 'd'};       // wrong - gets garbage value too
  char c[] = {'a', 'b', 'c', 'd', '\0'}; // correct
  char d[10] = "abc";

  char e[100];
  // cout << c << endl;

  cin >> e; // issue is you can't input strings like "hello world"
  cout << e << endl;

  cout << "c: "
 << "strlen: " << strlen(c) << " sizeof: " << sizeof(c) << endl; // sizeof gives +1 because of NULL char

  return 0;
}

    cin.get()

    • accepts all characters but does so – one at a time from the input buffer
    #include <iostream>

using namespace std;

int main()
{

  char sentence[100];

  char temp = cin.get(); // takes one character at a time from the input buffer
  int len = 0;
  // while (temp != '#') // takes input until # is encountered

  while (temp != '\n') // ALSO, add a check len < 100 ! or seg fault
  {
    // cout << temp;
    sentence[len++] = temp;
    // len++;
    temp = cin.get();
  }
  sentence[len] = '\0';

  cout << sentence << endl;
  cout << "Length: " << len << endl;

  return 0;
}

    Count Digits, Spaces, Letters

    #include <iostream>

using namespace std;

int main()
{

  char ch = cin.get();

  // counts
  int alphabets = 0;
  int spaces = 0;
  int digits = 0;

  while (ch != '\n')
  {
    if (ch >= '0' && ch <= '9') // '0' - '9' has ascii values
    {
      digits++;
    }
    else if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'))
    {
      alphabets++;
    }
    else if (ch == ' ' || ch == '\t')
    {
      spaces++;
    }
    ch = cin.get();
  }
  cout << "Alphabets: " << alphabets << " Spaces: " << spaces << " Digits: " << digits << endl;
  return 0;
}

    cin.getline()

    #include <iostream>
using namespace std;

int main()
{
  // char sentence[1000];
  // cin.getline(sentence, 1000); // by default stops at '\n'

  char paragraph[1000];
  cin.getline(paragraph, 1000, '#'); // at '#'

  cout << paragraph << endl;
  return 0;
}

    Shortest Path

    #include <iostream>
using namespace std;

int main()
{
  char route[1000];
  cin.getline(route, 1000);

  int x = 0, y = 0;

  for (int i = 0; route[i] != '\0'; i++)
  {
    switch (route[i])
    {
    case 'N':
      y++;
      break;
    case 'S':
      y--;
      break;
    case 'E':
      x++;
      break;
    case 'W':
      x--;
      break;
    }
  }

  cout << "x: " << x << " y: " << y << endl;
  // I
  if (x >= 0 and y >= 0)
  {
    while (y--)
    {
      cout << "N";
    }
    while (x--)
    {
      cout << "E";
    }
  }
  // II
  else if (x >= 0 && y < 0)
  {
    while (y++)
    {
      cout << "S";
    }
    while (x--)
    {
      cout << "E";
    }
  }
  // III
  else if (x <= 0 && y <= 0)
  {
    while (y++)
    {
      cout << "S";
    }
    while (x++)
    {
      cout << "W";
    }
  }
  // IV
  else if (x <= 0 && y >= 0)
  {
    while (y--)
    {
      cout << "N";
    }
    while (x++)
    {
      cout << "W";
    }
  }

  cout << endl;
  return 0;
}

    Copy, Concat, Compare

    #include <iostream>
#include <cstring>
using namespace std;

int main()
{

  char a[100] = "apple";
  char b[100];

  cout << strlen(a) << endl;

  strcpy(b, a); // copy a to b
  cout << b << endl;

  cout << strcmp(a, b) << endl; // returns 0 if same

  char web[] = "www.";
  char domain[] = "aritra.xyz";

  cout << strcat(web, domain) << endl; // concatenates two strings

  cout << strcmp(web, a) << endl; // returns a +ve or -ve value on false

  return 0;
}

    Largest String

    #include <iostream>
#include <cstring>

using namespace std;

int main()
{

  char sentence[1000];
  char largest[1000];

  int n;
  int largest_len = 0;
  cout << "Enter no. of lines: ";
  cin >> n;
  cin.get(); // to consume \n

  while (n--)
  {
    cin.getline(sentence, 1000);
    int len = strlen(sentence);
    if (len > largest_len)
    {
      strcpy(largest, sentence);
      largest_len = len;
    }
  }

  cout << largest_len << " - " << largest << endl;
  return 0;
}

    String class & getline()

    #include <iostream>
#include <string>
#include <vector>

using namespace std;

int main()
{
  string s; // dynamic vector
  string s1 = "hello world";
  string s2("hello world");

  // cout << s1 << endl;
  // cout << s2 << endl;

  getline(cin, s);
  // getline(cin, s, '#');
  for (char ch : s)
  {
    cout << ch << ", ";
  }
  cout << endl;
  //
  int n;
  cin >> n;
  cin.get();
  string temp;
  vector<string> sarr;
  while (n--)
  {
    getline(cin, temp);
    sarr.push_back(temp);
  }
  for (string s : sarr)
  {
    cout << s << ", ";
  }
  cout << endl;
  return 0;
}

    Run Length Encoding

    #include <iostream>
using namespace std;

string compressString(string s)
{
  int n = s.length();
  string res;
  for (int i = 0; i < n; i++)
  {
    int count = 1;
    while (i < n - 1 && s[i] == s[i + 1])
    {
      count++;
      i++;
    }
    res += s[i];
    res += to_string(count);
  }

  if (res.length() >= s.length())
  {
    return s;
  }
  return res;
}

int main()
{
  cout << compressString("aaaabbbbccccccccc") << endl;
  cout << compressString("abcd") << endl;

  return 0;
}
    , , ,

  • Sorting

    Bubble Sort

    We take the largest element to the end by repeated swapping.

    #include <iostream>
using namespace std;

void bubble_sort(int arr[], int n)
{

  for (int i = 0; i < n - 1; i++) // or 1 to n-1
  {
    for (int j = 0; j < n - i - 1; j++)
    {

      if (arr[j] > arr[j + 1])
      {
        swap(arr[j], arr[j + 1]);
      }
    }
  }
}

int main()
{
  int arr[] = {-2, 3, 4, -1, 5, -12, 6, 1, 3};
  int n = sizeof(arr) / sizeof(int);

  bubble_sort(arr, n);

  for (int x : arr)
  {
    cout << x << " ";
  }
  cout << endl;
}

    Insertion Sort

    • Like cards – say you have cards numbered 1, 3, 4 and you get a card numbered 2 – so you make space for 2 by moving 4, then 3 – then you insert the card numbered 2.
    #include <iostream>
using namespace std;

void insertion_sort(int arr[], int n)
{
  for (int i = 1; i < n; i++)
  {
    int current = arr[i];
    int prev = i - 1;

    while (prev >= 0 && current < arr[prev])
    {
      arr[prev + 1] = arr[prev];
      prev--;
    }

    arr[prev + 1] = current;
  }
}

int main()
{
  int arr[] = {-2, 3, 4, -1, 5, -12, 6, 1, 3};
  int n = sizeof(arr) / sizeof(int);

  insertion_sort(arr, n);

  for (int x : arr)
  {
    cout << x << " ";
  }
  cout << endl;

  return 0;
}

    Selection Sort

    • Find the minimum element in the unsorted part and swap it with the beginning element of the unsorted array. 
    #include <bits/stdc++.h>
using namespace std;

void selection_sort(int arr[], int n)
{
  for (int pos = 0; pos < n - 1; pos++) // 0 to n - 2, coz last element will become the largest
  {
    int min_pos = pos;
    for (int curr = pos; curr < n; curr++)
    {

      if (arr[curr] < arr[min_pos])
      {
        min_pos = curr;
      }
    }
    swap(arr[pos], arr[min_pos]);
  }
}

int main()
{
  int arr[] = {-2, 3, 4, -1, 5, -12, 6, 1, 3};
  int n = sizeof(arr) / sizeof(int);

  selection_sort(arr, n);

  for (int x : arr)
  {
    cout << x << " ";
  }
  cout << endl;

  return 0;
}

    Inbuilt Sort and Comparators

    • O(n * logn)
    #include <iostream>
#include <algorithm> // contains sort()
using namespace std;

// comparator
bool compare(int a, int b)
{
  return a > b;
}

int main()
{
  int arr[] = {-2, 3, 4, -1, 5, -12, 6, 1, 3};
  int n = sizeof(arr) / sizeof(int);

  // sort(arr, arr + n, greater<int>()); // greater - inbuilt comparator
  // reverse(arr, arr + n);

  sort(arr, arr + n, compare); // compare func is passed

  //
  for (int x : arr)
  {
    cout << x << " ";
  }
  cout << endl;

  return 0;
}

    Couting Sort

    • O(N + Range)
    #include <iostream>
#include <vector>

using namespace std;

void counting_sort(int arr[], int n)
{
  // find largest element to find the range (size of freq/ count arr)
  int largest = -1;
  for (int i = 0; i < n; i++)
  {
    largest = max(largest, arr[i]);
  }

  // create count/ freq arr
  vector<int> freq(largest + 1, 0);
  for (int i = 0; i < n; i++)
  {
    freq[arr[i]]++;
  }

  // put back elements in order
  int j = 0;
  for (int i = 0; i <= largest; i++) // O(n + range)
  {
    while (freq[i] > 0)
    {
      arr[j] = i;
      j++;
      freq[i]--;
    }
  }
}

int main()
{

  int arr[] = {88, 97, 10, 12, 15, 1, 5, 6, 12, 5, 8};
  int n = sizeof(arr) / sizeof(int);

  counting_sort(arr, n);

  for (int x : arr)
  {
    cout << x << " ";
  }

  cout << endl;

  return 0;
}
    , ,

  • Arrays

    Arrays are Linear Data Structures with elements of same type stored in contiguous memory locations

    Why we even need arrays?

    • To store multiple related elements – say marks of 100 students in a class, it’s hectic to declare 100 different variables – instead we can have a “marks” array with the size of 100 to store the marks.
    • Arrays can be accessed with indexes. marks[0] gives the mark of the first student
    • Indexes in arrays start with 0 and ends with n-1 (where n = size of array)
    • Arrays store elements of same datatype – an array can’t have both int and string (For example: {10, “orange”} but this is possible: {“10”, “orange”} as both “10” and “orange” are strings)
    • strings are character arrays – NULL character marks the end of the char array.
    • Static Allocation – size of an array can’t be changed once declared, compiler determines the size of the array.

    Input/Output – Update Arrays

    • Sometimes, you may want to use array from 1 to n instead of 0 to n-1 (but for that you need to declare the size to n + 1)

    Passing Arrays to Functions

    • You need to pass array’s size when passing it to a function because array’s are passed by reference.
    You have to pass the size in the function
    • Pass the size from main()
    • If the array is changed inside the function – the main array will be changed as the function references the same array in memory.

    Linear Search

    • O(n) – time complexity
    • Traverses through each element one by one to find the key.
    key = 5

    Binary Search

    • Works with Sorted Arrays
    • O(log n) time complexity

    Reversing an array

    • one way is iterating the array in reverse, copy the elements to a new array *extra space :(*, then copy the elements from new array to the main array
    • a better way is having two pointers (start & end) and keep on swapping the elements. (code below and diagram below!)

    Print Pairs

    Print Subarrays

    • elements should be contiguous
    • 3 for loops: k for looping through i to j

    Largest Subarray Sum

    Brute Force

    • O(n ^ 3)

    Prefix Sum

    • prefix array takes extra space
    • O(n ^ 2) time

    Kadane’s Algorithm

    • O(n)

    Vectors

    • Dynamic array
    • Vectors on getting full creates a new vector with double the size and copies the elements from old vector to the new one.
    , ,

  • Hello World!

    I have been planning to start writing a blog to document my learnings and life. I wanted to start writing to clear my thoughts. So, here it is.

    Well, Why WordPress.com then? Answering that, I have tried using other blogging platforms like Hugo, Jekyll. You can host them for free on Netlify or GitHub Pages, I know. But, I always had a hard time working with them, they are really good and maybe I again start using them but for now I am thinking of using WordPress.com as it has been here for a while, it was what first got me started into exploring Web Development and I feel it is not going away anytime soon too. I don’t want to self-host a WordPress blog now as:

    1. I don’t have the money to pay for a server. 😦
    2. I don’t want to manage my site because I won’t have the time.
    3. I want to focus more on writing. I want to practice writing here first, if I ever switch back to Hugo and figure it out 🙂
    4. I don’t want to lose my work in the future just because I couldn’t pay the server bill, and my stuff stays there.