diff options
Diffstat (limited to 'pod/perllol.pod')
| -rw-r--r-- | pod/perllol.pod | 60 |
1 files changed, 30 insertions, 30 deletions
diff --git a/pod/perllol.pod b/pod/perllol.pod index f15b24384e..ac36364ae0 100644 --- a/pod/perllol.pod +++ b/pod/perllol.pod @@ -16,7 +16,7 @@ old array @LoL that you can get at with two subscripts, like C<$LoL[3][2]>. Her a declaration of the array: # assign to our array a list of list references - @LoL = ( + @LoL = ( [ "fred", "barney" ], [ "george", "jane", "elroy" ], [ "homer", "marge", "bart" ], @@ -39,10 +39,10 @@ but rather just a reference to it, you could do something more like this: print $ref_to_LoL->[2][2]; -Notice that the outer bracket type has changed, and so our access syntax +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. $ref_to_LoL is a reference to an -array, whereas @LoL is an array proper. Likewise, C<$LoL[2]> is not an +interchange arrays and references thereto. $ref_to_LoL is a reference to an +array, whereas @LoL is an array proper. Likewise, C<$LoL[2]> is not an array, but an array ref. So how come you can write these: $LoL[2][2] @@ -72,7 +72,7 @@ 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: @@ -81,7 +81,7 @@ You might also have loaded that from a function: } Or you might have had a temporary variable sitting around with the -list in it. +list in it. for $i ( 1 .. 10 ) { @tmp = somefunc($i); @@ -93,8 +93,8 @@ constructor. That's because this will be very wrong: $LoL[$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. +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 C<use strict>, you'll have to add some declarations to make it happy: @@ -104,13 +104,13 @@ declarations to make it happy: 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 push(). You could just make a direct assignment if you knew where you wanted to put it: @@ -119,30 +119,30 @@ if you knew where you wanted to put it: 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. +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 $ref_to_LoL variable as a reference to an array, you'd have to do something like this: while (<>) { push @$ref_to_LoL, [ split ]; - } + } Actually, if you were using strict, you'd have to declare not only $ref_to_LoL as you had to declare @LoL, but you'd I<also> having to @@ -152,7 +152,7 @@ perl version 5.001m that's been fixed for the 5.002 release.) my $ref_to_LoL = []; while (<>) { push @$ref_to_LoL, [ split ]; - } + } Ok, 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: @@ -165,9 +165,9 @@ dealing with just matrices, it's often easiest to use simple assignment: for $x ( 3, 7, 9 ) { $LoL[$x][20] += func2($x); - } + } -It doesn't matter whether those elements are already +It doesn't matter whether those elements are already there or not: it'll gladly create them for you, setting intervening elements to C<undef> as need be. @@ -186,7 +186,7 @@ to push() must be a real array, not just a reference to such. =head1 Access and Printing -Now it's time to print your data structure out. How +Now 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: @@ -198,10 +198,10 @@ say print @LoL; # WRONG because you'll get just references listed, and perl will never -automatically dereference things for you. Instead, you have to +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() construct to loop across the outer -set of subscripts. +set of subscripts. for $aref ( @LoL ) { print "\t [ @$aref ],\n"; @@ -221,7 +221,7 @@ or maybe even this. Notice the inner loop. } } -As you can see, it's getting a bit complicated. That's why +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 ) { @@ -254,10 +254,10 @@ Here's how to do one operation using a loop. We'll assume an @LoL variable as before. @part = (); - $x = 4; + $x = 4; for ($y = 7; $y < 13; $y++) { push @part, $LoL[$x][$y]; - } + } That same loop could be replaced with a slice operation: @@ -273,9 +273,9 @@ $x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way: for ($starty = $y = 7; $x <= 12; $y++) { $newLoL[$x - $startx][$y - $starty] = $LoL[$x][$y]; } - } + } -We can reduce some of the looping through slices +We can reduce some of the looping through slices for ($x = 4; $x <= 8; $x++) { push @newLoL, [ @{ $LoL[$x] } [ 7..12 ] ]; @@ -293,13 +293,13 @@ 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, $x_hi, + my ($x_lo, $x_hi, $y_lo, $y_hi) = @_; - return map { - [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ] + return map { + [ @{ $lrr->[$_] } [ $y_lo .. $y_hi ] ] } $x_lo .. $x_hi; - } + } =head1 SEE ALSO |