c     ..................................................................
c
c        subroutine loc
c
c        purpose
c           compute a vector subscript for an element in a matrix of
c           specified storage mode
c
c        usage
c           call loc (i,j,ir,n,m,ms)
c
c        description of parameters
c           i   - row number of element
c           j   - column number  of element
c           ir  - resultant vector subscript
c           n   - number of rows in matrix
c           m   - number of columns in matrix
c           ms  - one digit number for storage mode of matrix
c                  0 - general
c                  1 - symmetric
c                  2 - diagonal
c
c        remarks
c           none
c
c        subroutines and function subprograms required
c           none
c
c        method
c           ms=0   subscript is computed for a matrix with n*m elements
c                  in storage (general matrix)
c           ms=1   subscript is computed for a matrix with n*(n+1)/2 in
c                  storage (upper triangle of symmetric matrix). if
c                  element is in lower triangular portion, subscript is
c                  corresponding element in upper triangle.
c           ms=2   subscript is computed for a matrix with n elements
c                  in storage (diagonal elements of diagonal matrix).
c                  if element is not on diagonal (and therefore not in
c                  storage), ir is set to zero.
c
c     ..................................................................
c
      subroutine loc (i,j,ir,n,m,ms)
c
      ix=i
      jx=j
      if (ms-1) 10,20,50
   10 irx=n*(jx-1)+ix
      go to 70
   20 if (ix-jx) 30,40,40
   30 irx=ix+(jx*jx-jx)/2
      go to 70
   40 irx=jx+(ix*ix-ix)/2
      go to 70
   50 irx=0
      if (ix-jx) 70,60,70
   60 irx=ix
   70 ir=irx
      return
      end