2D Arrays | Java Complete Placement Course | Lecture 11

Main ideas / lessons conveyed

• Why 2D arrays matter for placements

  • The lecturer connects 2D arrays to more advanced algorithms—specifically dynamic programming.
  • They emphasize that understanding tabulation and grid-based thinking with 2D arrays is foundational for harder problems later.

• What a 2D array is (memory + math perspective)

  • A 2D array is visualized like a matrix:
    • multiple rows
    • multiple columns
    • each element addressed by a (row, column) coordinate.
  • Elements are stored in a rectangular block of memory (described as “boxes in a rectangle”).
  • Terminology:
    • A specific element has a row index and column index.
    • The array’s layout is organized in rows and columns (often indexed in a row-major-like manner in common languages).

• Indexing concept

  • Indexing starts from 0 for both dimensions (typical Java behavior).
  • To access an element, use:
    • the row number
    • the column number
  • Example locations mentioned: (row 0, col 1), (row 1, col 3), etc.

• Memory size / storage calculation

  • Cell size depends on the data type (e.g., int vs boolean).
  • Key takeaway formula:
    • Total memory ≈ (number of rows × number of columns) × size_of_each_element
  • “Single cell” size varies:
    • boolean → 1 byte (as stated by the lecturer)
    • int → 4 bytes (as stated)

• How to declare a 2D array in Java

  • General pattern:
    • Use two dimension specifiers with two pairs of square brackets.
    • Use new to allocate memory:
      • first dimension = number of rows
      • second dimension = number of columns
  • Conceptually:
    • type[][] name = new type[rows][cols];

• How to take input into a 2D array

  • Input requires nested loops:
    • outer loop iterates over rows
    • inner loop iterates over columns
  • Total inputs:
    • rows × columns
  • Each value is stored at:
    • arr[i][j]

• How to output/print a 2D array

  • Use nested loops again:
    • print each element arr[i][j] with spacing
    • after finishing columns for a row, print a newline

• Searching for a value in a 2D array

  • Use nested loops to traverse every cell.
  • For each arr[i][j]:
    • compare it with the target x
    • if found, print where it was found (row/column)
  • If not found, print a message indicating it wasn’t present (lecturer mentions printing at the end).
  • Output formatting is also highlighted to keep results clear.

---

Methodology / instruction lists (as presented)

1) Declaring a 2D array in Java (approach described)

• Use a 2D declaration:
  • dataType[][] arrayName
• Allocate with new:
  • arrayName = new dataType[rows][cols]
• Meaning:
  • First bracket dimension → rows
  • Second bracket dimension → columns
• Result:
  • arrayName[i][j] refers to:
    • row i, column j

2) Taking input for all elements of a 2D array

• Let rows and cols be given by the user.
• Create the array (example):
  • arr = new int[rows][cols] (or appropriate type)
• Use nested loops:
  • Outer loop: i from 0 to rows - 1
  • Inner loop: j from 0 to cols - 1
  • For each (i, j):
    • read one value from the user
    • store it in arr[i][j]

3) Printing a 2D array

• Use nested loops:
  • For each row i:
    • For each column j:
      • print arr[i][j] followed by a space
    • after finishing the inner loop:
      • print a newline

4) Searching an element in a 2D array

• Input a target number x.
• Traverse all elements using nested loops:
  • For each cell (i, j):
    • if arr[i][j] == x:
      • print a “found” message with row and column
• If the loops finish without finding it:
  • print that it was not found (lecturer indicates the “print at the end” behavior)

---

Speakers / sources featured (as inferred from subtitles)

• A single main lecturer (no explicit name given in the subtitles; a “Proctor Dr.” is mentioned at the end, likely not the lecturer’s name).
• No other speakers or external sources are clearly identified in the subtitles.