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Diffstat (limited to 'pod/perllol.pod')
| -rw-r--r-- | pod/perllol.pod | 104 |
1 files changed, 52 insertions, 52 deletions
diff --git a/pod/perllol.pod b/pod/perllol.pod index 11632e0c97..ac36364ae0 100644 --- a/pod/perllol.pod +++ b/pod/perllol.pod @@ -12,11 +12,11 @@ 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 @LoL that you can get at with two subscripts, like $LoL[3][2]. Here's +old array @LoL that you can get at with two subscripts, like C<$LoL[3][2]>. Here's a declaration of the array: # assign to our array a list of list references - @LoL = ( + @LoL = ( [ "fred", "barney" ], [ "george", "jane", "elroy" ], [ "homer", "marge", "bart" ], @@ -27,7 +27,7 @@ a declaration of the array: Now you should be very careful that the outer bracket type is a round one, that is, parentheses. That's because you're assigning to -an @list, so you need parens. If you wanted there I<not> to be an @LoL, +an @list, so you need parentheses. If you wanted there I<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 @@ -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, $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] @@ -54,8 +54,8 @@ instead of having to write these: $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 array. -But you need not do so for the very first one if it's a scalar containing +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 $ref_to_LoL always needs it. =head1 Growing Your Own @@ -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: @@ -104,58 +104,58 @@ 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: my (@LoL, $i, $line); - for $i ( 0 .. 10 ) + for $i ( 0 .. 10 ) { $line = <>; $LoL[$i] = [ split ' ', $line ]; - } + } or even just my (@LoL, $i); - for $i ( 0 .. 10 ) + for $i ( 0 .. 10 ) { $LoL[$i] = [ split ' ', <> ]; - } + } -You should in general be leary of using potential list functions -in a scalar context without explicitly stating such. +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 ) + 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 not only have to declare $ref_to_LoL as -you had to declare @LoL, but you'd I<also> having to initialize it to a -reference to an empty list. (This was a bug in 5.001m that's been fixed -for the 5.002 release.) +Actually, if you were using strict, you'd have to declare not only +$ref_to_LoL as you had to declare @LoL, but you'd I<also> having to +initialize it to a reference to an empty list. (This was a bug in +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 -just dealing with matrices, it's often easiest to use simple assignment: +dealing with just matrices, it's often easiest to use simple assignment: for $x (1 .. 10) { for $y (1 .. 10) { @@ -165,19 +165,19 @@ just dealing with 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. -If you just wanted to append to a row, you'd have +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 I<couldn't> just say: +Notice that I I<couldn't> say just: push $LoL[0], "wilma", "betty"; # WRONG! @@ -186,22 +186,22 @@ 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 -are you going to do that? Well, if you only want one +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: print $LoL[0][0]; If you want to print the whole thing, though, you can't -just say +say print @LoL; # WRONG -because you'll just get references listed, and perl will never -automatically dereference things for you. Instead, you have to +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() 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 ) { @@ -231,7 +231,7 @@ sometimes is easier to take a temporary on your way through: } } -Hm... that's still a bit ugly. How about this: +Hmm... that's still a bit ugly. How about this: for $i ( 0 .. $#LoL ) { $aref = $LoL[$i]; @@ -243,7 +243,7 @@ Hm... that's still a bit ugly. How about this: =head1 Slices -If you want to get at a slide (part of a row) in a multidimensional +If 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. @@ -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][$y]; - } + } That same loop could be replaced with a slice operation: @@ -266,16 +266,16 @@ That same loop could be replaced with a slice operation: but as you might well imagine, this is pretty rough on the reader. Ah, but what if you wanted a I<two-dimensional slice>, such as having -$x run from 4..8 and $y run from 7 to 12? Hm... here's the simple way: +$x run from 4..8 and $y run from 7 to 12? Hmm... here's the simple way: @newLoL = (); for ($startx = $x = 4; $x <= 8; $x++) { for ($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 @@ -308,6 +308,6 @@ perldata(1), perlref(1), perldsc(1) =head1 AUTHOR -Tom Christiansen <tchrist@perl.com> +Tom Christiansen <F<tchrist@perl.com>> Last udpate: Sat Oct 7 19:35:26 MDT 1995 |