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       PDL::NiceSlice - toward a nicer slicing syntax for PDL


         use PDL::NiceSlice;

         $a(1:4) .= 2;             # concise syntax for ranges
         print $b((0),1:$end);     # use variables in the slice expression
         $a->xchg(0,1)->(($pos-1)) .= 0; # default method syntax

         $idx = long 1, 7, 3, 0;   # a piddle of indices
         $a(-3:2:2,$idx) += 3;     # mix explicit indexing and ranges
         $a->clump(1,2)->(0:30);   # 'default method' syntax
         $a(myfunc(0,$var),1:4)++; # when using functions in slice expressions
                                   # use parentheses around args!

         $b = $a(*3);              # Add dummy dimension of order 3

         # modifiers are specified in a ;-separated trailing block
         $a($a!=3;?)++;            # short for $a->where($a!=3)++
         $a(0:1114;_) .= 0;        # short for $a->flat->(0:1114)
         $b = $a(0:-1:3;|);        # short for $a(0:-1:3)->sever
         $n = sequence 3,1,4,1;
         $b = $n(;-);              # drop all dimensions of size 1 (AKA squeeze)
         $b = $n(0,0;-|);          # squeeze *and* sever
         $c = $a(0,3,0;-);         # more compact way of saying $a((0),(3),(0))


       Slicing is a basic, extremely common operation, and PDL's slice method would be cumbersome
       to use in many cases.  "PDL::NiceSlice" rectifies that by incorporating new slicing syntax
       directly into the language via a perl source filter (see the perlfilter man page).
       NiceSlice adds no new functionality, only convenient syntax.

       NiceSlice is loaded automatically in the perldl or pdl2 shell, but (to avoid conflicts
       with other modules) must be loaded automatically in standalone perl/PDL scripts (see
       below).  If you prefer not to use a prefilter on your standalone scripts, you can use the
       slice method in those scripts, rather than the more compact NiceSlice constructs.

Use in scripts and "perldl" or "pdl2" shell

       The new slicing syntax can be switched on and off in scripts and perl modules by using or
       unloading "PDL::NiceSlice".

       But now back to scripts and modules.  Everything after "use PDL::NiceSlice" will be
       translated and you can use the new slicing syntax. Source filtering will continue until
       the end of the file is encountered.  You can stop sourcefiltering before the end of the
       file by issuing a "no PDL::NiceSlice" statement.

       Here is an example:

         use PDL::NiceSlice;

         # this code will be translated
         # and you can use the new slicing syntax

         no PDL::NiceSlice;

         # this code won't
         # and the new slicing syntax will raise errors!

       See also Filter::Simple and example in this distribution for further examples.

       NOTE: Unlike "normal" modules you need to include a "use PDL::NiceSlice" call in each and
       every file that contains code that uses the new slicing syntax. Imagine the following
       situation: a file

          # start
          use PDL;
          use PDL::NiceSlice;

          $a = sequence 10;
          print $a(0:4),"\n";

          require '';
          # end

       that "require"s a second file

          # begin
          $aa = sequence 11;
          print $aa(0:7),"\n";
          # end

       Following conventional perl wisdom everything should be alright since we "use"d "PDL" and
       "PDL::NiceSlice" already from within and by the time is "require"d
       things should be defined and imported, etc. A quick test run will, however, produce
       something like the following:

        [0 1 2 3 4]
        syntax error at line 3, near "0:"
        Compilation failed in require at line 7.

       This can be fixed by adding the line

         use PDL::NiceSlice;

       "before" the code in that uses the new slicing syntax (to play safe just include
       the line near the top of the file), e.g.

          # begin corrected
          use PDL::NiceSlice;
          $aa = sequence 11;
          print $aa(0:7),"\n";
          # end

       Now things proceed more smoothly

        [0 1 2 3 4]
        [0 1 2 3 4 5 6 7]

       Note that we don't need to issue "use PDL" again.  "PDL::NiceSlice" is a somewhat funny
       module in that respect. It is a consequence of the way source filtering works in Perl (see
       also the IMPLEMENTATION section below).

   evals and "PDL::NiceSlice"
       Due to "PDL::NiceSlice" being a source filter it won't work in the usual way within evals.
       The following will not do what you want:

         $a = sequence 10;
         eval << 'EOE';

         use PDL::NiceSlice;
         $b = $a(0:5);

         print $b;

       Instead say:

         use PDL::NiceSlice;
         $a = sequence 10;
         eval << 'EOE';

         $b = $a(0:5);

         print $b;

       Source filters must be executed at compile time to be effective. And "PDL::NiceFilter" is
       just a source filter (although it is not necessarily obvious for the casual user).

The new slicing syntax

       Using "PDL::NiceSlice" slicing piddles becomes so much easier since, first of all, you
       don't need to make explicit method calls. No


       calls, etc. Instead, "PDL::NiceSlice" introduces two ways in which to slice piddles
       without too much typing:

       · using parentheses directly following a scalar variable name, for example

            $c = $b(0:-3:4,(0));

       · using the so called default method invocation in which the piddle object is treated as
         if it were a reference to a subroutine (see also perlref). Take this example that slices
         a piddle that is part of a perl list @b:

           $c = $b[0]->(0:-3:4,(0));

       The format of the argument list is the same for both types of invocation and will be
       explained in more detail below.

   Parentheses following a scalar variable name
       An arglist in parentheses following directly after a scalar variable name that is not
       preceded by "&" will be resolved as a slicing command, e.g.

         $a(1:4) .= 2;         # only use this syntax on piddles
         $sum += $a(,(1));

       However, if the variable name is immediately preceded by a "&", for example


       it will not be interpreted as a slicing expression. Rather, to avoid interfering with the
       current subref syntax, it will be treated as an invocation of the code reference $a with
       argumentlist "(4,5)".

       The $a(ARGS) syntax collides in a minor way with the perl syntax.  In particular,
       ``foreach $var(LIST)'' appears like a PDL slicing call.  NiceSlice avoids translating the
       ``for $var(LIST)'' and ``foreach $var(LIST)'' constructs for this reason.  Since you can't
       use just any old lvalue expression in the 'foreach' 'for' constructs -- only a real perl
       scalar will do -- there's no functionality lost.  If later versions of perl accept
       ``foreach <lvalue-expr> (LIST)'', then you can use the code ref syntax, below, to get what
       you want.

   The default method syntax
       The second syntax that will be recognized is what I called the default method syntax. It
       is the method arrow "->" directly followed by an open parenthesis, e.g.

         $a->xchg(0,1)->(($pos)) .= 0;

       Note that this conflicts with the use of normal code references, since you can write in
       plain Perl

         $sub = sub { print join ',', @_ };

       NOTE: Once "use PDL::NiceSlice" is in effect (you can always switch it off with a line "no
       PDL::NiceSlice;" anywhere in the script) the source filter will incorrectly replace the
       above call to $sub with an invocation of the slicing method.  This is one of the pitfalls
       of using a source filter that doesn't know anything about the runtime type of a variable
       (cf. the Implementation section).

       This shouldn't be a major problem in practice; a simple workaround is to use the "&"-way
       of calling subrefs, e.g.:

         $sub = sub { print join ',', @_ };

   When to use which syntax?
       Why are there two different ways to invoke slicing?  The first syntax "$a(args)" doesn't
       work with chained method calls. E.g.


       won't work. It can only be used directly following a valid perl variable name. Instead,
       use the default method syntax in such cases:


       Similarly, if you have a list of piddles @pdls:

         $b = $pdls[5]->(0:-1);

   The argument list
       The argument list is a comma separated list. Each argument specifies how the corresponding
       dimension in the piddle is sliced. In contrast to usage of the slice method the arguments
       should not be quoted. Rather freely mix literals (1,3,etc), perl variables and function
       invocations, e.g.

         $a($pos-1:$end,myfunc(1,3)) .= 5;

       There can even be other slicing commands in the arglist:

         $a(0:-1:$pdl($step)) *= 2;

       NOTE: If you use function calls in the arglist make sure that you use parentheses around
       their argument lists. Otherwise the source filter will get confused since it splits the
       argument list on commas that are not protected by parentheses. Take the following example:

         sub myfunc { return 5*$_[0]+$_[1] }
         $a = sequence 10;
         $sl = $a(0:myfunc 1, 2);
         print $sl;
        PDL barfed: Error in slice:Too many dims in slice
        Caught at file /usr/local/bin/perldl, line 232, pkg main

       The simple fix is

         $sl = $a(0:myfunc(1, 2));
         print $sl;
        [0 1 2 3 4 5 6 7]

       Note that using prototypes in the definition of myfunc does not help.  At this stage the
       source filter is simply not intelligent enough to make use of this information. So beware
       of this subtlety.

       Another pitfall to be aware of: currently, you can't use the conditional operator in slice
       expressions (i.e., "?:", since the parser confuses them with ranges). For example, the
       following will cause an error:

         $a = sequence 10;
         $b = rand > 0.5 ? 0 : 1; # this one is ok
         print $a($b ? 1 : 2);    # error !
        syntax error at (eval 59) line 3, near "1,

       For the moment, just try to stay clear of the conditional operator in slice expressions
       (or provide us with a patch to the parser to resolve this issue ;).

       Following a suggestion originally put forward by Karl Glazebrook the latest versions of
       "PDL::NiceSlice" implement modifiers in slice expressions. Modifiers are convenient
       shorthands for common variations on PDL slicing. The general syntax is


       Four modifiers are currently implemented:

       ·   "_" : flatten the piddle before applying the slice expression. Here is an example

              $b = sequence 3, 3;
              print $b(0:-2;_); # same as $b->flat->(0:-2)
            [0 1 2 3 4 5 6 7]

           which is quite different from the same slice expression without the modifier

              print $b(0:-2);
             [0 1]
             [3 4]
             [6 7]

       ·   "|" : sever the link to the piddle, e.g.

              $a = sequence 10;
              $b = $a(0:2;|)++;  # same as $a(0:2)->sever++
              print $b;
            [1 2 3]
              print $a; # check if $a has been modified
            [0 1 2 3 4 5 6 7 8 9]

       ·   "?" : short hand to indicate that this is really a where expression

           As expressions like


           are used very often you can write that shorter as


           With the "?"-modifier the expression preceding the modifier is not really a slice
           expression (e.g. ranges are not allowed) but rather an expression as required by the
           where method.  For example, the following code will raise an error:

             $a = sequence 10;
             print $a(0:3;?);
            syntax error at (eval 70) line 3, near "0:"

           That's about all there is to know about this one.

       ·   "-" : squeeze out any singleton dimensions. In less technical terms: reduce the number
           of dimensions (potentially) by deleting all dims of size 1. It is equivalent to doing
           a reshape(-1).  That can be very handy if you want to simplify the results of slicing

             $a = ones 3, 4, 5;
             $b = $a(1,0;-); # easier to type than $a((1),(0))
             print $b->info;
            PDL: Double D [5]

           It also provides a unique opportunity to have smileys in your code!  Yes, PDL gives
           new meaning to smileys.

   Combining modifiers
       Several modifiers can be used in the same expression, e.g.

         $c = $a(0;-|); # squeeze and sever

       Other combinations are just as useful, e.g. ";_|" to flatten and sever. The sequence in
       which modifiers are specified is not important.

       A notable exception is the "where" modifier ("?") which must not be combined with other
       flags (let me know if you see a good reason to relax this rule).

       Repeating any modifier will raise an error:

         $c = $a(-1:1;|-|); # will cause error
        NiceSlice error: modifier | used twice or more

       Modifiers are still a new and experimental feature of "PDL::NiceSlice". I am not sure how
       many of you are actively using them. Please do so and experiment with the syntax. I think
       modifiers are very useful and make life a lot easier.  Feedback is welcome as usual. The
       modifier syntax will likely be further tuned in the future but we will attempt to ensure
       backwards compatibility whenever possible.

   Argument formats
       In slice expressions you can use ranges and secondly, piddles as 1D index lists (although
       compare the description of the "?"-modifier above for an exception).

       · ranges

         You can access ranges using the usual ":" separated format:

           $a($start:$stop:$step) *= 4;

         Note that you can omit the trailing step which then defaults to 1.  Double colons ("::")
         are not allowed to avoid clashes with Perl's namespace syntax. So if you want to use
         steps different from the default you have to also at least specify the stop position.

           $a(::2);   # this won't work (in the way you probably intended)
           $a(:-1:2); # this will select every 2nd element in the 1st dim

         Just as with slice negative indices count from the end of the dimension backwards with
         "-1" being the last element. If the start index is larger than the stop index the
         resulting piddle will have the elements in reverse order between these limits:

           print $a(-2:0:2);
          [8 6 4 2 0]

         A single index just selects the given index in the slice

           print $a(5);

         Note, however, that the corresponding dimension is not removed from the resulting piddle
         but rather reduced to size 1:

           print $a(5)->info
          PDL: Double D [1]

         If you want to get completely rid of that dimension enclose the index in parentheses
         (again similar to the slice syntax):

           print $a((5));

         In this particular example a 0D piddle results. Note that this syntax is only allowed
         with a single index. All these will be errors:

           print $a((0,4));  # will work but not in the intended way
           print $a((0:4));  # compile time error

         An empty argument selects the whole dimension, in this example all of the first

           print $a(,(0));

         Alternative ways to select a whole dimension are

           $a = sequence 5, 5;
           print $a(:,(0));
           print $a(0:-1,(0));
           print $a(:-1,(0));
           print $a(0:,(0));

         Arguments for trailing dimensions can be omitted. In that case these dimensions will be
         fully kept in the sliced piddle:

           $a = random 3,4,5;
           print $a->info;
          PDL: Double D [3,4,5]
           print $a((0))->info;
          PDL: Double D [4,5]
           print $a((0),:,:)->info;  # a more explicit way
          PDL: Double D [4,5]
           print $a((0),,)->info;    # similar
          PDL: Double D [4,5]

       · dummy dimensions

         As in slice, you can insert a dummy dimension by preceding a single index argument with
         '*'.  A lone '*' inserts a dummy dimension of order 1; a '*' followed by a number
         inserts a dummy dimension of that order.

       · piddle index lists

         The second way to select indices from a dimension is via 1D piddles of indices. A simple

           $a = random 10;
           $idx = long 3,4,7,0;
           $b = $a($idx);

         This way of selecting indices was previously only possible using dice ("PDL::NiceSlice"
         attempts to unify the "slice" and "dice" interfaces). Note that the indexing piddles
         must be 1D or 0D. Higher dimensional piddles as indices will raise an error:

           $a = sequence 5, 5;
           $idx2 = ones 2,2;
           $sum = $a($idx2)->sum;
          piddle must be <= 1D at /home/XXXX/.perldlrc line 93

         Note that using index piddles is not as efficient as using ranges.  If you can represent
         the indices you want to select using a range use that rather than an equivalent index
         piddle. In particular, memory requirements are increased with index piddles (and
         execution time may be longer). That said, if an index piddle is the way to go use it!

       As you might have expected ranges and index piddles can be freely mixed in slicing

         $a = random 5, 5;
         $b = $a(-1:2,pdl(3,0,1));

   piddles as indices in ranges
       You can use piddles to specify indices in ranges. No need to turn them into proper perl
       scalars with the new slicing syntax.  However, make sure they contain not more than one
       element! Otherwise a runtime error will be triggered. First a couple of examples that
       illustrate proper usage:

         $a = sequence 5, 5;
         $rg = pdl(1,-1,3);
         print $a($rg(0):$rg(1):$rg(2),2);
         [11 14]
         print $a($rg+1,:$rg(0));
         [2 0 4]
         [7 5 9]

       The next one raises an error

         print $a($rg+1,:$rg(0:1));
        multielement piddle where only one allowed at XXX/ line 1170.

       The problem is caused by using the 2-element piddle "$rg(0:1)" as the stop index in the
       second argument ":$rg(0:1)" that is interpreted as a range by "PDL::NiceSlice". You can
       use multielement piddles as index piddles as described above but not in ranges. And
       "PDL::NiceSlice" treats any expression with unprotected ":"'s as a range.  Unprotected
       means as usual "not occurring between matched parentheses".


       "PDL::NiceSlice" exploits the ability of Perl to use source filtering (see also
       perlfilter). A source filter basically filters (or rewrites) your perl code before it is
       seen by the compiler. "PDL::NiceSlice" searches through your Perl source code and when it
       finds the new slicing syntax it rewrites the argument list appropriately and splices a
       call to the "nslice" method using the modified arg list into your perl code. You can see
       how this works in the perldl or pdl2 shells by switching on reporting (see above how to do

       The "nslice" method is an extended version of mslice that knows how to deal with index
       piddles (and therefore combines slicing and dicing). Full documentation of "nslice" will
       be in the next PDL release.


   Conditional operator
       The conditional operator can't be used in slice expressions (see above).

   The "DATA" file handle
       Note: To avoid clobbering the "DATA" filehandle "PDL::NiceSlice" switches itself off when
       encountering the "__END__" or "__DATA__" tokens.  This should not be a problem for you
       unless you use "SelfLoader" to load PDL code including the new slicing from that section.
       It is even desirable when working with Inline::Pdlpp, see below.

   Possible interaction with Inline::Pdlpp
       There is currently an undesired interaction between "PDL::NiceSlice" and the new
       Inline::Pdlpp module (currently only in PDL CVS). Since PP code generally contains
       expressions of the type "$var()" (to access piddles, etc) "PDL::NiceSlice" recognizes
       those incorrectly as slice expressions and does its substitutions. This is not a problem
       if you use the "DATA" section for your Pdlpp code -- the recommended place for Inline code
       anyway. In that case "PDL::NiceSlice" will have switched itself off before encountering
       any Pdlpp code (see above):

           # use with Inline modules
         use PDL;
         use PDL::NiceSlice;
         use Inline Pdlpp;

         $a = sequence(10);
         print $a(0:5);



         ... inline stuff

       Otherwise switch "PDL::NiceSlice" explicitly off around the Inline::Pdlpp code:

         use PDL::NiceSlice;

         $a = sequence 10;

         no PDL::NiceSlice; # switch off before Pdlpp code
         use Inline Pdlpp => "Pdlpp source code";

       The cleaner solution is to always stick with the "DATA" way of including your "Inline"
       code as in the first example. That way you keep your nice Perl code at the top and all the
       ugly Pdlpp stuff etc at the bottom.

   Bug reports
       Feedback and bug reports are welcome. Please include an example that demonstrates the
       problem. Log bug reports in the PDL bug database at

       or send them to the pdl-porters mailing list <>.


       Copyright (c) 2001, 2002 Christian Soeller. All Rights Reserved.  This module is free
       software. It may be used, redistributed and/or modified under the same terms as PDL itself
       (see <>).