For files of moderate size – or if you really don't care how long it takes – the sort-compare subroutine can also be programmed to split the key string into its component parts every time. The components are then compared by the subroutine as has already been shown. The difference is the classic trade-off between "speed versus space." Splitting all the strings ahead of time saves time but costs memory and vice-versa. (Because of the number of times a sort-compare subroutine might be executed, the difference in speed can be substantial.) Also note that many external sort-commands are quite capable of handling delimited-string keys efficiently, such that you might not actually have to "write a program" to get the job done.