<Moved to perl6-language-data from PDL Porters> [EMAIL PROTECTED] wrote: > Jeremy Howard wrote: > > > <quote> > > C<;> is particularly useful for creating slices of multidimensional arrays: > > > > my int @array = ([1,2,3], > > [4,5,6], > > [7,8,9]); > > @col2 = @array[(0..2); 1]; # @array[[0,1],[1,1],[2,1]] == (2,5,8) > > > > Large matrices can be flexibly manipulated using infinite lists (from RFC > > 24) and list generation functions (from RFC 81): > > > > my int @matrix = get_big_file(); > > my @first_5_odd_cols = ( (0..) ; (1..9:2) ); # ([0,1],[0,3],[0,5],...) > > my @matrix_slice = @matrix[@first_5_odd_cols]; > > </quote> > > Nice quote. One itch remains for me: the need for the parentheses should > be done away with: > > @arr[0..2; 1] > > Should be fine if ';' implies list context for its args and since it > also is associative. Remember, we wan't to save extra key strokes ;) > I agree. I had planned to put this in the RFC but I got distracted by wondering under exactly what conditions the () are required to remove ambiguity. I never got around to answering this question and then forgot to mention it in the RFC at all! I'll add it in now. Can anyone think of situations in which the () would be required? BTW, it occurs to me that: @matrix[ 0.. ; 1..9:2 ] will be very hard to implement. The cartesian product expands to ([0,1],[0,3],[0,5],...). It will require some cleverness to recognise that the domain of the relevent indices is from non-contiguous parts of the list, with an infinite number of indices between each part. I haven't done an implementation section for this RFC yet, so any suggestions about how infinite lists with the ';' operator might work would be much appreciated. Here's the current draft of the RFC (we should be cleaning it up and submitting it in the next couple of days): ---- This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE New operator ';' for creating array slices =head1 VERSION Maintainer: Jeremy Howard <[EMAIL PROTECTED]> Created: 3 September 2000 Status: Developing Last modified: 3 September 2000 Version: 1 Mailing List: [EMAIL PROTECTED] Number: ? =head1 ABSTRACT <RFCC> described and extension of standard list indexing which allows indexing directly into a multidimensional array by using a list of lists of coordinates. This RFC describes a new operator C<;> that can create lists of coordinates corresponding to slices and blocks of multidimensional arrays. The C<;> operator creates the cartesian product of its operands as a list of lists. It can only operate within a list constructor. =head1 DESCRIPTION It is proposed that a new operator C<;> be introduced that operates within a list constructor to create the cartesian product of its operands: @lol = ( (1,2) ; (3,4) ); # ([1,3], [1,4], [2,3], [2,4]) The order of the resultant list is to generate pairs by iterating through the right-hand operand for each element of the left-hand operand in turn, going from left to right. If an operand is a list of lists, the C<;> operator creates the cartesian product of the component lists: @lol = ( ([1,3],[1,4]);(5,6) ) # ([1,3,5], [1,3,6], [1,4,5], [1,4,6]) The same logic applies if both operands are lists of lists. C<;> is particularly useful for creating slices of multidimensional arrays: my int @array = ([1,2,3], [4,5,6], [7,8,9]); @col2 = @array[(0..2); 1]; # @array[[0,1],[1,1],[2,1]] == (2,5,8) Large matrices can be flexibly manipulated using infinite lists (from RFC 24) and list generation functions (from RFC 81): my int @matrix = get_big_file(); my @first_5_odd_cols = ( (0..) ; (1..9:2) ); # ([0,1],[0,3],[0,5],...) my @matrix_slice = @matrix[@first_5_odd_cols]; @matrix_slice now contains the whole of columns 1,3,5,7,9 of @matrix. Furthermore, @first_5_odd_cols can be used to slice another matrix later, which may be of a different size. Because this whole-dimension slicing is so common, any argument to C<;> may be omitted. Omitted operands default to (0..): ( ;(1..9:2) ) == ( (0..) ; (1..9:2) ); Furthermore, in order to create generic slices that return 'all the nth elements' regardless of the number of dimensions of the array, the left-most of right-most operand to C<;> may be '*', which expands to (0..) for every missing dimension of the sliced array: my int @b :bounds(1,1,1) = get_some_matrix(); my @first_elems = @b[0;*]; # @b[[0,0,0],[0,0,1],[0,1,0],[0,1,1]] The '*' operand may only be used in an array slicing context. =head1 IMPLEMENTATION Lazy, naturally. =head1 REFERENCES <RFCA> RFC 81: Lazily evaluated list generation functions RFC 24: Semi-finite (lazy) lists
