Purpose: Definition of set functionality for an official extension.
This is a part of the Chart of proposed list functionality.
- AK (1999, Mar 17/18)
- Had to revamp all the Set pages due to the removal of clutter from various scripts and the introduction of a new way of doing things, which had to be included into the timings, of course. This new way of operation uses the local namespace of a procedure as an implicit array, fast to retrieve, and fast to set. This principle was fastest among all currently tested functions and implementation variants. Its disadvantage is however that no set must contain the names of the internal variables required in the computation itself, or incorrect results will occur. This should be avoidable, if documented properly, but is nevertheless an incitement to switch to a C implementation in the future.
- Determine membership of an element in a list.
- Compute the union of 2 sets represented by 2 lists.
- Compute the intersection of 2 sets represented by 2 lists.
- Compute the symmetric difference of 2 sets represented by 2 lists (symmetric difference being the set containing those elements which are in one but not the other, the inverse of intersection.).
- Compute the onesided difference of 2 sets represented by 2 lists (onesided difference being the set containing those elements which are in the first but not the second.).
- Check for an empty list, eh, set.
Sometimes intersection and both onesided differences are
required. A procedure computing them in parallel should be more
efficient than doing it separately.
- A procedure to compute intersection and both onesided differences.
[AK] (Mar 20, 1999): The timing of the possible implementations
proved me wrong, this procedure is not necessary.
- The first implementation should use lists to represent sets, as they can be transfered by value. A C implementation OTOH should define a new Tcl_Obj-Type.
- Name of the extension
- Sets are represented on the tcl-level by lists. A C implementation does not exist yet, but may employ hash-tables to gain performance.
LV: Are there other list oriented languages from whom we can 'borrow' set functionality ? Perl's idea of adopting concepts found in other languages, done in the spirit of perl, is one that could really work in Tcl as well. Mar 20, 1999
Edrx: Icon has a built-in "set" type, with at least these built-in operators and functions (see [L2 ] and [L3 ]):
S1 ++ S2 set union; returns a set
S1 ** S2 set intersection
S1 -- S2 set difference
*S1 the number of elements in S1
?S returns a random element from S
set(L) makes a set from a list
member(S,x) fails if x is not a member of S, else returns x
insert(S,x,...) insert x,... in S; return the new set
delete(S,x,...) delete x,... from S; return the new set
There's also "!S", that "generates" each element of S, in no predictable order, but the concept of "generators" is too Icon-specific. If it's a matter of borrowing syntax, I would suggest using something like S++, ..., Smember... or like Set::++, ... . Nov 19, 1999
DKF: Membership test should be a boolean operator, and not something with a failure semantics (given correctly typed input.)
AK: The contains command as defined above returns a boolean. Wrappers could be provided to make porting Icon apps easier.
DKF: Hmm. Reading comprehension is obviously not my strong point!