Q3: 参见"Software for data analysis"第5章
1)x[1:2] <- c(1,2)这种形式叫Replacement expression: "an assignment where the left side is not a name but an expression, identifying some aspect of the object we want to change."
2) dim(x)<-c(1,2)叫replacement function,等价于"dim<-"
参见http://stat.ethz.ch/R-manual/R-devel/library/base/html/dim.html
3)另见http://cran.r-project.org/doc/manuals/R-lang.html#Subset-assignment