/*****************************************************************************
*                                                                            *
*   TRIL.SPL     Copyright (C) 2002 DSP Development Corporation              *
*                               All Rights Reserved                          *
*                                                                            *
*   Author:      Randy Race                                                  *
*                                                                            *
*   Synopsis:    Returns lower triangle                                      *
*                                                                            *
*   Revisions:   13 Aug 2002  RRR  Creation                                  *
*                                                                            *
*****************************************************************************/

#include 

#if @HELP_TRIL

    TRIL

    Purpose: Returns the lower triangle of a matrix

    Syntax:  TRIL(m, d)

                  m -  An array.

                  d -  Optional An integer, the diagonal on and below which
                       to include matrix elements. Defaults to 0, the main
                       diagonal.


    Returns: An array consisting of the lower triangle of m where the other
             elements are zero.

    Example:
             W1: {{1, 2, 3},
                  {4, 5, 6},
                  {7, 8, 9}}

             W2: tril(w1)

             W2 == {{1, 0, 0},
                    {4, 5, 0},
                    {7, 8, 9}}


             W3: tril(W1, 1)

             W3 == {{1, 2, 0},
                    {4, 5, 6},
                    {7, 8, 9}}


             W4: tril(W1, -1)

             W4 == {{0, 0, 0},
                    {4, 0, 0},
                    {7, 8, 0}}


    Remarks:
             TRIL(m, -1) is equivalent to LOTRIX(m).

    See Also:
             LOTRI
             LOTRIX
             TRIU
#endif


/* lower triangle */
tril(M, d)
{
        if (argc < 2)
        {
                if (argc < 1) error("tril - input array required");
                
                d = 0;
        }
        
        if (not(isarray(M))) M = castseries(M);

        return((M)*(rownos(M) >= (colnos(M) - d)));
}