by 0htoAi,写于2021.8.14
There are n×n cells on a grid, the top-left cell is at (1,1) while the bottom-right cell is at (n,n). You start at (1,1) and ......
9.1 plyr包简介
ddply {plyr}: Split data frame, apply function, and return results in a data frame.
ddply(.data, .variables, .fun = NULL, ..., .progres ......