A filter is a pipeline that produces data which represents an edited version of its input.
and stdout makes it fairly easy to write these types of programs
RS 2005-03-08: filter is also a classic operator in functional programming, and it makes nice simple code:
proc filter {list script} {
set res {}
foreach e $list {if {[uplevel 1 $script $e]} {lappend res $e}}
set res
}Testing, with a simple way to do intersection:
proc in {list e} {expr {[lsearch -exact $list $e]>=0}}
% filter {a b c} {in {b c d}}
b clpz 2013-06-09: The elements I use have spaces, so I changed the filter routine to:
proc filter {list script} {
set res {}
foreach e $list {if {[uplevel 1 $script [list $e]]} {lappend res $e}}
set res
}Testing
% filter { {s pace} x d} {in {{s pace} a b c d e}}
{s pace} dsmh2 2006-08-26: Here is a routine based on this to find the intersection of many lists:
proc intersection { items } {
set res [lindex $items 0] ; # start with the first sub-list
foreach item [lrange $items 1 end] { ; # loop through the rest
set res [filter $res {in $item}] ; # and filter each new subset against the next
}
set res
}AMG: Implementation using lcomp:
proc filter {list test_expression} {
lcomp {$x} for x in $list if $test_expression
}test_expression is a boolean if expression that should be a function of $x, the list element being tested.