C                    package:base                    R Documentation

Sets Contrasts for a Factor

Description:

     Sets the `"contrasts"' attribute for the factor.

Usage:

     C(object, contr, how.many, ...)

Arguments:

  object: a factor or ordered factor

   contr: which contrasts to use. Can be a matrix with one row for each
          level of the factor or a suitable function like `contr.poly'
          or a character string giving the name of the function

how.many: the number of contrasts to set, by default one less than
          `nlevels(object)'.

     ...: Additional arguments for the function `contr'.

Details:

     For compatibility with S, `contr' can be `treatment', `helmert',
     `sum' or `poly' (without quotes) as shorthand for
     `contr.treatment' and so on.

Value:

     The factor `object' with the `"contrasts"' attribute set.

Author(s):

     B.D. Ripley

See Also:

     `contrasts', `contr.sum', etc.

Examples:

     ## reset contrasts to defaults
     options(contrasts=c("contr.treatment", "contr.poly"))
     data(warpbreaks)
     attach(warpbreaks)
     tens <- C(tension, poly, 1)
     attributes(tens)
     detach()
     ## tension SHOULD be an ordered factor, but as it is not we can use
     aov(breaks ~ wool + tens + tension, data=warpbreaks)

     ## show the use of ...  The default contrast is contr.treatment here
     summary(lm(breaks ~ wool + C(tension, base=2), data=warpbreaks))

     data(esoph) # following on from help(esoph)
     model3 <- glm(cbind(ncases, ncontrols) ~ agegp + C(tobgp, , 1) +
          C(alcgp, , 1), data = esoph, family = binomial())
     summary(model3)

