Klaus Saalfeld A function lselect is proposed that selects a number of items from a list and returns them as a new list. In contrast to known lindex with multiple indices lselect doesn't operate on nested lists but on flat lists. The returned list contains the items in the same order as specified with the indices argument.
# Returns from the specified list one or more elements identified by given indices.
# The first element in list is given by index 0, the last element of list is given by "end".
# An optional negative offset (e.g. "end-1") can be used to specify elements relative to the end of list.
# The list to operate on is passed by value.
proc lselect {value indices} {
set result {}
if {0 != [llength $value]} {
foreach index $indices {
if {0 == [string compare -length 3 $index end]} {
set index [expr ([llength $value] - 1) [string range $index 3 end]]
}
if {($index >= 0) && ($index < [llength $value])} {
lappend result [lindex $value $index]
}
}
}
return $result
}Some examples:
% lselect {car house tiger penguin} {end-1 1 end 0}
tiger house penguin car
% lselect {banana pineapple orange} 1
pineapple
% lselect {tcl is great} {-1 17 end-8}
<empty>
% lselect {more awesome stuff} {1 1 1}
awesome awesome awesomeMany list operations are a special case of lselect. For example:
lselect is moreover useful to implement stack or buffer like behavior with lists or to shuffle lists.