SS 29Oct2004: filter is a very useful function in functional programming, that given a list and a predicate (a function with arity of one, returing true or false) creates a new list which contains only the members of the original list meeting the predicate (in the same order they appeared).
I plan to submit a TIP for the addition of this function together with map in functional programming.
The following is an implementation for Tcl, with this command structure:
filter var list expr
Some usage example:
% filter x {1 2 3 4 5 6 7 8 9} {$x > 4}
5 6 7 8 9
% filter x {1 2 3 4 5 6 7 8 9} {$x % 3}
1 2 4 5 7 8
% filter x {1 2 3 4 5 6 7 8 9} {($x % 3) == 0}
3 6 9
% filter x [list a b {} c d e {} {} f g] {[llength $x]}
a b c d e f gDo you need filter? If you can see the following pattern in your code, you need it:
set res {}
foreach x $list {
if {[foobar $x]} {
lappend res $x
}
}
use $resFinally the (very simple) code:
proc filter {fvar flist fexpr} {
upvar 1 $fvar var
set res {}
foreach var $flist {
set varCopy $var
if {[uplevel 1 [list expr $fexpr]]} {
lappend res $varCopy
}
}
return $res
}RS would prefer an argument sequence with the list in the end:
filter x {$x > 4} $listso the second and third argument, which form a lambda, are left together.
SS Indeed, it's not clear what's the best. I like more what you proposed from an aesthetic point of view, but to have the same arguments order as map and foreach can be a good point.
There is another option too:
filter $list x {$x > 4}that sounds to me slightly better.