Multi-dimensional arrays

1
Multi-dimensional arrays

• Allows you to work with matrices
• int array new int2030
• First dimension is number of rows
• Second dimension is number of columns
• Can have more than two dimensions
• Allocated row-pointer in memory
• A row can be stored anywhere in memory
• Elements in a row are stored contiguously
• Can visualize as array of arrays

2
Transpose

• class transpose
• public static void main(String args)
• int i, j
• int table new int34
• int ttable new int43
• for(i0 i lt 3 i)
• for(j0 j lt 4 j)
• tableij (i4)j1
• System.out.print(tableij " ")
• System.out.println()
• System.out.println()
• for(j0 j lt 4 j)
• for(i0 i lt 3 i)
• ttableji tableij
• System.out.print(ttableji " ")

3
Matrix multiplication

• class MatrixMultiply
• public static void main (String arg)
• int m 20, n 30, p 40
• double A new doublemn
• double B new doublenp
• double C
• InitializeArray (A, m, n)
• InitializeArray (B, n, p)
• C Multiply (A, B, m, n, p)
• // continued in next slide

4
Matrix multiplication

• public static void InitializeArray (double
  x, int m, int n)
• int i, j
• for (i0iltmi)
• for (j0jltnj)
• xij ij

5
Matrix multiplication

• public static double Multiply (double
  x, double y, int p, int q, int r)
• double z new doublepr
• int i, j, k
• for (i0iltpi)
• for (j0jltrj)
• zij 0
• for (k0kltqk)
• zij (xikykj)
• return z
• // end class

6
Allocating a triangular matrix

• May save memory for symmetric matrices
• Allocate space to store the starting addresses of
  m rows
• Then allocate the m rows themselves
• int m 20
• double triangle new doublem
• int i
• for (i0iltmi)
• // allocate lower triangle
• trianglei new doublei1

7
arraylength

• class arraylength
• public static void main(String args)
• int list new int10
• int nums 1, 2, 3
• int table // a variable-length table
  
• 1, 2, 3,
• 4, 5,
• 6, 7, 8, 9

8

• System.out.println("length of list is "
  list.length)
• System.out.println("length of nums is "
  nums.length)
• System.out.println("length of table is "
  table.length)
• System.out.println("length of table0 is "
  table0.length)
• System.out.println("length of table1 is "
  table1.length)
• System.out.println("length of table2 is "
  table2.length)
• System.out.println()

9

• // use length to initialize list
• for(int i0 i lt list.length i)
• listi i i
• System.out.print("Here is list ")
• // now use length to display list
• for(int i0 i lt list.length i)
• System.out.print(listi " ")
• System.out.println()