พื้นฐานอาร์เรย์ 2 มิติและการวนซ้ำแบบซ้อนกัน

อาร์เรย์ 2 มิติคืออะไร

ประกาศและสร้าง

public class Main {
  public static void main(String[] args) {
    // Way 1: Create a 2D array with fixed size
    int[][] a = new int[2][3]; 
    // 2 rows, 3 columns → default values = 0
    // [
    //   [0, 0, 0],
    //   [0, 0, 0]
    // ]

    // Way 2: Create and initialize directly
    int[][] b = {
      {1, 2, 3},
      {4, 5, 6}
    };
    // [
    //   [1, 2, 3],
    //   [4, 5, 6]
    // ]

    // Print array a
    System.out.println("Array a:");
    for (int i = 0; i < a.length; i++) {
      for (int j = 0; j < a[i].length; j++) {
        System.out.print(a[i][j] + " ");
      }
      System.out.println();
    }

    // Print array b
    System.out.println("Array b:");
    for (int i = 0; i < b.length; i++) {
      for (int j = 0; j < b[i].length; j++) {
        System.out.print(b[i][j] + " ");
      }
      System.out.println();
    }
  }
}

แถวกับคอลัมน์

การวนซ้ำซ้อนกัน

public class Main {
  public static void main(String[] args) {
    // Example 2D array (matrix)
    int[][] m = {
      {1, 2, 3},
      {4, 5, 6}
    };

    // Loop over rows
    for (int r = 0; r < m.length; r = r + 1) {
      // Loop over columns in the current row
      for (int c = 0; c < m[r].length; c = c + 1) {
        int value = m[r][c]; // get element at row r, column c
        System.out.print(value + " "); // work: here we print
      }
      System.out.println(); // move to next row
    }
  }
}

ผลรวมแถวและคอลัมน์

public class Main {
  public static void main(String[] args) {
    // Example 2D array (matrix)
    int[][] m = {
      {1, 2, 3},
      {4, 5, 6},
      {7, 8, 9}
    };

    // Sum of row 1 (second row, index = 1)
    int rowSum = 0;
    for (int c = 0; c < m[1].length; c = c + 1) {
      rowSum = rowSum + m[1][c];
    }
    System.out.println("Sum of row 1 = " + rowSum);

    // Sum of column 2 (third column, index = 2)
    int colSum = 0;
    for (int r = 0; r < m.length; r = r + 1) {
      colSum = colSum + m[r][2];
    }
    System.out.println("Sum of column 2 = " + colSum);
  }
}

สาธิตอาร์เรย์ 2 มิติ

public class Main {
  static int total(int[][] m) {
    int s = 0;
    for (int r = 0; r < m.length; r = r + 1) {
      for (int c = 0; c < m[r].length; c = c + 1) {
        s = s + m[r][c];
      }
    }
    return s;
  }

  static int[] rowSums(int[][] m) {
    int[] sums = new int[m.length];
    for (int r = 0; r < m.length; r = r + 1) {
      int s = 0;
      for (int c = 0; c < m[r].length; c = c + 1) s = s + m[r][c];
      sums[r] = s;
    }
    return sums;
  }

  static String join(int[] a) {
    String s = "";
    for (int i = 0; i < a.length; i = i + 1) {
      s = s + a[i] + (i + 1 < a.length ? " " : "");
    }
    return s;
  }

  public static void main(String[] args) {
    int[][] m = {
      {1, 2, 3},
      {4, 5, 6}
    };

    System.out.println("rows = " + m.length);
    System.out.println("cols of row 0 = " + m[0].length);
    System.out.println("total = " + total(m));

    int[] rs = rowSums(m);
    System.out.println("row sums = " + join(rs));
  }
}

จำนวนคอลัมน์

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