lpop - remove last element of list variable
This is not a command included in Tcl, but rather a common idiom for in place list manipulation used e.g. for implementing stacks.
lpop removes the last element from the list in listVar and returns it. listVar is altered in place. If pos is given, the element at position pos of the list is 'popped' off the list instead of the last element.
This one, copied from Performance of Various Stack Implementations, contributed by Lars H, is simple and best in performance.
prop lpop listVar {
upvar 1 $listVar l
set r [lindex $l end]
set l [lreplace $l [set l end] end] ; # Make sure [lreplace] operates on unshared object
return $r
}Note: the 'set l end' idiom is a trick to "unset" l and at the same time provide the verb 'end' to lreplace resulting in 'lreplace $l end end'.
You can use this programming idiom also without having the lpop command, e.g. with the K combinator:
K [lindex $l end] [set l [lreplace $l [set l end] end]
In the chart of existing list functionality lpop is listed as part of the ExtraL package, with the described functionality.
However in Bag of algorithms, The anatomy of a bytecoded command and Searching A Star In Space, e.g lpop pops of the first element of the list.
This is kind of pushmi-pullyu : On one side a convenient implementation of the stack push command s lappend listVar element. Here the last element of the list (the 'right-most') has to be seen as the top of the stack, which might appear counterintuitive to some people. Those would, on the other side rather push elements on the stack 'to the left', and pop them from there (index 0 of the list).
A more convenient name for this second interpretation of lpop might be lshift. Since the Tcl-core does not provide an opposite of the lappend command, implementing a stack where the top element is at list position 0 (or 'left-most') has poorer performance.