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Some Operations on Matrix

A few days back someone asked me a question via email which I thought might be useful to others too. So I’m listing that question along with its answer below.

Q. I want to write a program such that users enter the value of matrix and each operation (listed below) is performed by functions. I want to use switch structure to call the functions.

1. Rotate the matrix around the diagonal.

Example:

   1 2 3 ---> 1 4 7
   4 5 6      2 5 8
   7 8 9      3 6 9

2. Rotate the matrix around the middle row.

Example:

   1 2 3 ---> 7 8 9
   4 5 6      4 5 6
   7 8 9      1 2 3

3. Rotate the matrix around the middle column.

Example:

   1 2 3 ---> 3 2 1
   4 5 6      6 5 4
   7 8 9      9 8 7

4. Set the upper triangle to zero.

Example:

   1 2 3 ---> 1 0 0
   4 5 6      4 5 0
   7 8 9      7 8 9

Ans. The following program does it. Please note that the matrix is declared as global so as to reduce complications in the program. Better way should have been to pass the matrix (local) to the functions from main().


  // Program that does some Rotations
  // about certain axes in two
  // dimensional arrays
  #include <iostream.h>

  // change this to hold more
  // values in the array
  #define MAX 3

  // it is declared to be global
  // so that every function can access it
  int ar[MAX][MAX];

  // function prototypes
  void EnterVal();
  void rDiagonal();
  void rMidRow();
  void rMidCol();
  void sUpperZero();
  void Show();

  void main()
  {
    int ch;

    // loop until 'quit' is not selected
    while(ch!=6)
    {
      cout<<"1> Enter Values\n";
      cout<<"2> Rotate around Diagonal\n";
      cout<<"3> Rotate around the Middle row\n";
      cout<<"4> Rotate around the Middle column\n";
      cout<<"5> Set the Upper triangle to zero\n";
      cout<<"6> Quit\n\n";

      cin>>ch;

      // do as per choice
      switch(ch)
      {
      case 1:
        EnterVal();
        break;

      case 2:
        rDiagonal();
        Show();
        break;

      case 3:
        rMidRow();
        Show();
        break;

      case 4:
        rMidCol();
        Show();
        break;

      case 5:
        sUpperZero();
        Show();
        break;
      }
    }
  }

  // --------------------
  // Function Definitions
  void EnterVal()
  {
    int i,j;

    cout<<"Enter Values\n";

    for(i=0;i<MAX;i++)
      for(j=0;j<MAX;j++)
        cin>>ar[i][j];
  }

  void rDiagonal()
  {
    int temp,i,j;

    for(i=0;i<MAX;i++)
      for(j=i;j<MAX;j++)
      {
        temp=ar[i][j];
        ar[i][j]=ar[j][i];
        ar[j][i]=temp;
      }
  }

  void rMidRow()
  {
    int temp,i,j;

    for(i=0;i<(MAX-1)/2;i++)
      for(j=0;j<MAX;j++)
      {
        temp=ar[(MAX-1)-i][j];
        ar[(MAX-1)-i][j]=ar[i][j];
        ar[i][j]=temp;
      }
  }

  void rMidCol()
  {
    int temp,i,j;

    for(i=0;i<MAX;i++)
      for(j=0;j<(MAX-1)/2;j++)
      {
        temp=ar[i][(MAX-1)-j];
        ar[i][(MAX-1)-j]=ar[i][j];
        ar[i][j]=temp;
      }
  }

  void sUpperZero()
  {
    int i,j;

    for(i=0;i<(MAX+1)/2;i++)
      for(j=i;j<MAX-i;j++)
        ar[i][j]=0;
  }

  void Show()
  {
    int i,j;

    for(i=0;i<MAX;i++)
    {
      for(j=0;j<MAX;j++)
        cout<<ar[i][j]<<"   ";

      cout<<endl;
    }
    cout<<"\n\n";
  }

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