Comparison               package:base               R Documentation

_R_e_l_a_t_i_o_n_a_l _O_p_e_r_a_t_o_r_s

_D_e_s_c_r_i_p_t_i_o_n:

     Binary operators which allow the comparison of values in atomic
     vectors.

_U_s_a_g_e:

     x < y
     x > y
     x <= y
     x >= y
     x == y
     x != y

_A_r_g_u_m_e_n_t_s:

    x, y: atomic vectors, symbols, calls, or other objects for which
          methods have been written.

_D_e_t_a_i_l_s:

     The binary comparison operators are generic functions: methods can
     be written for them individually or via the 'Ops') group generic
     function.  (See 'Ops' for how dispatch is computed.)

     Comparison of strings in character vectors is lexicographic within
     the strings using the collating sequence of the locale in use: see
     'locales'.  The collating sequence of locales such as 'en_US' is
     normally different from 'C' (which should use ASCII) and can be
     surprising.  Beware of making _any_ assumptions about the
     collation order: e.g. in Estonian 'Z' comes between 'S' and 'T',
     and collation is not necessarily character-by-character - in
     Danish 'aa' sorts as a single letter, after 'z'.  In Welsh 'ng'
     may or may not be a single sorting unit: if it is it follows 'g'. 
     Some platforms may not respect the locale and always sort in
     numerical order of the bytes in an 8-bit locale, or in Unicode
     point order for a UTF-8 locale (and may not sort in the same order
     for the same language in different character sets).  Collation of
     non-letters (spaces, punctuation signs, hyphens, fractions and so
     on) is even more problematic. 

     At least one of 'x' and 'y' must be an atomic vector, but if the
     other is a list R attempts to coerce it to the type of the atomic
     vector: this will succeed if the list is made up of elements of
     length one that can be coerced to the correct type.

     If the two arguments are atomic vectors of different types, one is
     coerced to the type of the other, the (decreasing) order of
     precedence being character, complex, numeric, integer, logical and
     raw.

     When comparisons are made between character strings, parts of the
     strings after embedded 'nul' characters are ignored.

     Missing values ('NA') and 'NaN' values are regarded as
     non-comparable even to themselves, so comparisons involving them
     will always result in 'NA'.  Missing values can also result when
     character strings are compared and one is not valid in the current
     collation locale. 

     Language objects such as symbols and calls are deparsed to
     character strings before comparison.

_V_a_l_u_e:

     A logical vector indicating the result of the element by element
     comparison.  The elements of shorter vectors are recycled as
     necessary.

     Objects such as arrays or time-series can be compared this way
     provided they are conformable.

_S_4 _m_e_t_h_o_d_s:

     These operators are members of the S4 'Compare' group generic, and
     so methods can be written for them individually as well as for the
     group generic (or the 'Ops' group generic), with arguments 'c(e1,
     e2)'.

_N_o_t_e:

     Do not use '==' and '!=' for tests, such as in 'if' expressions,
     where you must get a single 'TRUE' or 'FALSE'.  Unless you are
     absolutely sure that nothing unusual can happen, you should use
     the 'identical' function instead.

     For numerical and complex values, remember '==' and '!=' do not
     allow for the finite representation of fractions, nor for rounding
     error.  Using 'all.equal' with 'identical' is almost always
     preferable.  See the examples.

_R_e_f_e_r_e_n_c_e_s:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

     Collation of character strings is a complex topic.  For an
     introduction see <URL:
     http://en.wikipedia.org/wiki/Collating_sequence>.  The _Unicode
     Collation Algorithm_ (<URL: http://unicode.org/reports/tr10/>) is
     likely to be increasing influential.

_S_e_e _A_l_s_o:

     'factor' for the behaviour with factor arguments.

     'Syntax' for operator precedence.

_E_x_a_m_p_l_e_s:

     x <- stats::rnorm(20)
     x < 1
     x[x > 0]

     x1 <- 0.5 - 0.3
     x2 <- 0.3 - 0.1
     x1 == x2                           # FALSE on most machines
     identical(all.equal(x1, x2), TRUE) # TRUE everywhere

