interaction               package:base               R Documentation

_C_o_m_p_u_t_e _F_a_c_t_o_r _I_n_t_e_r_a_c_t_i_o_n_s

_D_e_s_c_r_i_p_t_i_o_n:

     'interaction' computes a factor which represents the interaction
     of the given factors.  The result of 'interaction' is always
     unordered.

_U_s_a_g_e:

     interaction(..., drop = FALSE, sep = ".", lex.order = FALSE)

_A_r_g_u_m_e_n_t_s:

     ...: the factors for which interaction is to be computed, or a
          single list giving those factors.

    drop: if 'drop' is 'TRUE', unused factor levels are dropped from
          the result.  The default is to retain all factor levels.

     sep: string to construct the new level labels by joining the
          constituent ones.

lex.order: logical indicating if the order of factor concatenation
          should be lexically ordered.

_V_a_l_u_e:

     A factor which represents the interaction of the given factors.
     The levels are labelled as the levels of the individual factors
     joined by 'sep' which is '.' by default.

     By default, when 'lex.order = FALSE', the levels are ordered so
     the level of the first factor varies fastest, then the second and
     so on.  This is the reverse of lexicographic ordering (which you
     can get by  'lex.order = TRUE'), and differs from ':'.  (It is
     done this way for compatibility with S.)

_R_e_f_e_r_e_n_c_e_s:

     Chambers, J. M. and Hastie, T. J. (1992) _Statistical Models in
     S_. Wadsworth & Brooks/Cole.

_S_e_e _A_l_s_o:

     'factor'; ':' where 'f:g' is similar to 'interaction(f, g,
     sep=":")' when 'f' and  'g' are factors.

_E_x_a_m_p_l_e_s:

     a <- gl(2, 4, 8)
     b <- gl(2, 2, 8, labels = c("ctrl", "treat"))
     s <- gl(2, 1, 8, labels = c("M", "F"))
     interaction(a, b)
     interaction(a, b, s, sep = ":")
     stopifnot(identical(a:s,
                         interaction(a, s, sep = ":", lex.order = TRUE)),
               identical(a:s:b,
                         interaction(a, s, b, sep = ":", lex.order = TRUE)))