R中的错误：没有适用于“预测”的适用方法应用于“ regsubsets”类的对象

Question

我试图从ISLR课本中运行代码。关于最佳子集选择：

library(ISLR) #contain data Hitters
library(leaps) #use regsubsets
regfit.full=regsubsets(Salary~.,Hitters)
predict(regfit.full)

然后R显示错误：

Error in UseMethod("predict") :no applicable method for 'predict' applied to an object of class "regsubsets" 

我正在使用Windows RStudio 0.99.893版本，R-3.2.4修订版-win。我真的不知道为什么“预测”不能应用于“ regsubsets”。

这是我第一次在这里提出问题，请让我知道我可以做些什么来帮助解决这个问题。提前谢谢了！

--------------------更新----------------------------- ----------

感谢李哲远，我弄清楚了。regsubsets没有预测方法。我们需要自己写一个。以下是来自互联网的一种：

predict.regsubsets = function(object, newdata, id, ...) {
    form = as.formula(object$call[[2]])
    mat = model.matrix(form, newdata)
    coefi = coef(object, id = id)
    mat[, names(coefi)] %*% coefi
}

其他信息：

> sessionInfo()
R version 3.2.4 Revised (2016-03-16 r70336)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] ISLR_1.0  leaps_2.9

loaded via a namespace (and not attached):
[1] tools_3.2.4

> methods(predict)
     [1] predict.ar*                predict.Arima*             predict.arima0*            predict.glm               
     [5] predict.HoltWinters*       predict.lm                 predict.loess*             predict.mlm*              
     [9] predict.nls*               predict.poly*              predict.ppr*               predict.prcomp*           
    [13] predict.princomp*          predict.smooth.spline*     predict.smooth.spline.fit* predict.StructTS*         
    see '?methods' for accessing help and source code

> regfit.full
Subset selection object
Call: regsubsets.formula(Salary ~ ., Hitters)
19 Variables  (and intercept)
           Forced in Forced out
AtBat          FALSE      FALSE
Hits           FALSE      FALSE
HmRun          FALSE      FALSE
Runs           FALSE      FALSE
RBI            FALSE      FALSE
Walks          FALSE      FALSE
Years          FALSE      FALSE
CAtBat         FALSE      FALSE
CHits          FALSE      FALSE
CHmRun         FALSE      FALSE
CRuns          FALSE      FALSE
CRBI           FALSE      FALSE
CWalks         FALSE      FALSE
LeagueN        FALSE      FALSE
DivisionW      FALSE      FALSE
PutOuts        FALSE      FALSE
Assists        FALSE      FALSE
Errors         FALSE      FALSE
NewLeagueN     FALSE      FALSE
1 subsets of each size up to 8
Selection Algorithm: exhaustive