1. You shouldn't use `=` for assignments in R but `<-`. `=` does late binding. 2. You shouldn't use `class()` here but `mode()` to check the actual underlying

1. You shouldn't use `=` for assignments in R but `<-`. `=` does late binding. 2. You shouldn't use `class()` here but `mode()` to check the actual underlying data structure. > mode(d) [1] "list" > mode(e) [1] "list" 3. The reason `[` works differently is because it a S3 method which invokes different functions for lists and data.frames -- that's why class(d) doesn't return "list". See `methods("[")`. See https://cran.r-project.org/doc/manuals/r-release/R-lang.html#Data-frame-objects https://cran.r-project.org/doc/manuals/r-release/R-lang.html... for details.