Clear questions and runnable code get the best and fastest answer |
|
PerlMonks |
perlman:perllolby gods (Initiate) |
on Aug 25, 1999 at 06:22 UTC ( [id://386]=perlman: print w/replies, xml ) | Need Help?? |
perllolCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlLoL - Manipulating Lists of Lists in Perl
DESCRIPTION
Declaration and Access of Lists of ListsThe simplest thing to build is a list of lists (sometimes called an array of arrays). It's reasonably easy to understand, and almost everything that applies here will also be applicable later on with the fancier data structures.
A list of lists, or an array of an array if you would,
is just a regular old array
# assign to our array a list of list references @LoL = ( [ "fred", "barney" ], [ "george", "jane", "elroy" ], [ "homer", "marge", "bart" ], );
print $LoL[2][2]; bart Now you should be very careful that the outer bracket type is a round one, that is, a parenthesis. That's because you're assigning to an @list, so you need parentheses. If you wanted there not to be an @LoL, but rather just a reference to it, you could do something more like this:
# assign a reference to list of list references $ref_to_LoL = [ [ "fred", "barney", "pebbles", "bambam", "dino", ], [ "homer", "bart", "marge", "maggie", ], [ "george", "jane", "alroy", "judy", ], ];
print $ref_to_LoL->[2][2];
Notice that the outer bracket type has changed, and so our access syntax has also changed. That's because unlike
C, in perl you can't freely interchange arrays and references thereto.
$LoL[2][2] $ref_to_LoL->[2][2] instead of having to write these:
$LoL[2]->[2] $ref_to_LoL->[2]->[2]
Well, that's because the rule is that on adjacent brackets only (whether
square or curly), you are free to omit the pointer dereferencing arrow. But
you cannot do so for the very first one if it's a scalar containing a
reference, which means that
Growing Your OwnThat's all well and good for declaration of a fixed data structure, but what if you wanted to add new elements on the fly, or build it up entirely from scratch?
First, let's look at reading it in from a file. This is something like
adding a row at a time. We'll assume that there's a flat file in which each
line is a row and each word an element. If you're trying to develop an
while (<>) { @tmp = split; push @LoL, [ @tmp ]; } You might also have loaded that from a function:
for $i ( 1 .. 10 ) { $LoL[$i] = [ somefunc($i) ]; } Or you might have had a temporary variable sitting around with the list in it.
for $i ( 1 .. 10 ) { @tmp = somefunc($i); $LoL[$i] = [ @tmp ]; }
It's very important that you make sure to use the
$LoL[$i] = @tmp; You see, assigning a named list like that to a scalar just counts the number of elements in @tmp, which probably isn't what you want.
If you are running under
use strict; my(@LoL, @tmp); while (<>) { @tmp = split; push @LoL, [ @tmp ]; } Of course, you don't need the temporary array to have a name at all:
while (<>) { push @LoL, [ split ]; }
You also don't have to use
my (@LoL, $i, $line); for $i ( 0 .. 10 ) { $line = <>; $LoL[$i] = [ split ' ', $line ]; } or even just
my (@LoL, $i); for $i ( 0 .. 10 ) { $LoL[$i] = [ split ' ', <> ]; } You should in general be leery of using potential list functions in a scalar context without explicitly stating such. This would be clearer to the casual reader:
my (@LoL, $i); for $i ( 0 .. 10 ) { $LoL[$i] = [ split ' ', scalar(<>) ]; }
If you wanted to have a
while (<>) { push @$ref_to_LoL, [ split ]; } Now you can add new rows. What about adding new columns? If you're dealing with just matrices, it's often easiest to use simple assignment:
for $x (1 .. 10) { for $y (1 .. 10) { $LoL[$x][$y] = func($x, $y); } }
for $x ( 3, 7, 9 ) { $LoL[$x][20] += func2($x); } It doesn't matter whether those elements are already there or not: it'll gladly create them for you, setting intervening elements to undef as need be. If you wanted just to append to a row, you'd have to do something a bit funnier looking:
# add new columns to an existing row push @{ $LoL[0] }, "wilma", "betty"; Notice that I couldn't say just:
push $LoL[0], "wilma", "betty"; # WRONG!
In fact, that wouldn't even compile. How come? Because the argument to
Access and PrintingNow it's time to print your data structure out. How are you going to do that? Well, if you want only one of the elements, it's trivial:
print $LoL[0][0]; If you want to print the whole thing, though, you can't say
print @LoL; # WRONG
because you'll get just references listed, and perl will never automatically dereference things for you. Instead, you have to roll yourself a loop or two. This prints the whole structure, using the shell-style
for $aref ( @LoL ) { print "\t [ @$aref ],\n"; } If you wanted to keep track of subscripts, you might do this:
for $i ( 0 .. $#LoL ) { print "\t elt $i is [ @{$LoL[$i]} ],\n"; } or maybe even this. Notice the inner loop.
for $i ( 0 .. $#LoL ) { for $j ( 0 .. $#{$LoL[$i]} ) { print "elt $i $j is $LoL[$i][$j]\n"; } } As you can see, it's getting a bit complicated. That's why sometimes is easier to take a temporary on your way through:
for $i ( 0 .. $#LoL ) { $aref = $LoL[$i]; for $j ( 0 .. $#{$aref} ) { print "elt $i $j is $LoL[$i][$j]\n"; } } Hmm... that's still a bit ugly. How about this:
for $i ( 0 .. $#LoL ) { $aref = $LoL[$i]; $n = @$aref - 1; for $j ( 0 .. $n ) { print "elt $i $j is $LoL[$i][$j]\n"; } }
SlicesIf you want to get at a slice (part of a row) in a multidimensional array, you're going to have to do some fancy subscripting. That's because while we have a nice synonym for single elements via the pointer arrow for dereferencing, no such convenience exists for slices. (Remember, of course, that you can always write a loop to do a slice operation.)
Here's how to do one operation using a loop. We'll assume an
@part = (); $x = 4; for ($y = 7; $y < 13; $y++) { push @part, $LoL[$x][$y]; } That same loop could be replaced with a slice operation:
@part = @{ $LoL[4] } [ 7..12 ]; but as you might well imagine, this is pretty rough on the reader.
Ah, but what if you wanted a two-dimensional slice, such as having
@newLoL = (); for ($startx = $x = 4; $x <= 8; $x++) { for ($starty = $y = 7; $y <= 12; $y++) { $newLoL[$x - $startx][$y - $starty] = $LoL[$x][$y]; } } We can reduce some of the looping through slices
for ($x = 4; $x <= 8; $x++) { push @newLoL, [ @{ $LoL[$x] } [ 7..12 ] ]; } If you were into Schwartzian Transforms, you would probably have selected map for that
@newLoL = map { [ @{ $LoL[$_] } [ 7..12 ] ] } 4 .. 8; Although if your manager accused of seeking job security (or rapid insecurity) through inscrutable code, it would be hard to argue. :-) If I were you, I'd put that in a function:
@newLoL = splice_2D( \@LoL, 4 => 8, 7 => 12 ); sub splice_2D { my $lrr = shift; # ref to list of list refs! my ($x_lo, $x_hi, $y_lo, $y_hi) = @_;
return map { [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ] } $x_lo .. $x_hi; }
SEE ALSO
|
|