Richard Suchenwirth 2002-11-27 - A visitor in the Tcl chatroom asked for a modified foreach to cover all combinations of the input lists. In database technical terms, this is called a "nested-loop join", at least one functional programming tutorial tells me so.
Well, and reason enough to do some Tcl'ing (where metaprogramming is as trivial as virtual destructors in other languages ;-) - from the input, build up a nested command of foreaches, insert the specified body in the middle, and presto:
proc nljoin args {
set body [lindex $args end]
set cmd ""
foreach {vars list} [lrange $args 0 end-1] {
append cmd "[list foreach $vars $list] \{\n"
append body "\}"
}
uplevel 1 $cmd$body
}#------------------------- Testing:
% nljoin x {a b} y {c d} {puts x:$x,y:$y}
x:a,y:c
x:a,y:d
x:b,y:c
x:b,y:dNo error-checking yet - the length of args must be odd and >2. Caveat user.
Filtering the lists in advance reduces the runtime needed - for instance, after an example in [?], to get the list of all professors who have published since 1990, using the fancy all list comprehension:
nljoin pub [all x from $publications where {$date($x)>=1990}] \
prof [all x from $staff where {$status($x)=="professor"}] {
if {$empID($pub)==$empID($prof)} {
lappend res $prof
}
}Come to think, what this does is just iterate over the Cartesian product of a list of lists...
A join is not the same as a cartesian product. Joining a table of 3 records with one of 5 does not necessarily produce a table of 15 records. -jcw
RS: Thanks for elucidation. So what's called "join" is the matching of IDs in the body, right?