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In an array of arrays, how do I print the whole array or bits of it?

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Contributed by Anonymous Monk on Aug 30, 2000 at 14:40 UTC
Q&A  > Data Structures


Answer: In an array of arrays, how do I print the whole array or bits of it?
contributed by jreades

Because an array is ordered, it's very easy to print out entire array of arrays in *some* kind of order (assuming that you've been keeping it in some kind of order).

Take, for example, the following array of arrays.

my @level1 = ( [1, 2, 3, 4, 5], [6, 7, 8, 9, 10] );

To print the entire array, you need to do the following:

for (my $i = 0; $i < @level1; $i++) { for (my $j = 0; $j < @{$level1[$i]}; $j++) { print STDOUT "\$i: " . $i . " \$j: " . $j . " value: " . $le +vel1[$i][$j] . "\n"; } }

There are several ways of making this code faster, but only at the cost of readability.

Of course, life gets more difficult if you don't know in advance what kind of array-of-array (or List of Lists: LOL) you're dealing with.

That's where the ref function comes in handy.

my @level1 = ( [ 1, 2, 3, [4, 5] ], 6, 7, [ 8, [9, [10] ] ], ); &read_array(\@level1); exit 0; sub read_array { my $array_reference = shift; foreach (@$array_reference) { if (ref($_) eq 'ARRAY') { &read_array($_); } else { print $_ . "\n"; } } }

Here we shifted to basic recursion -- something that calls itself -- to handle the fact that we don't know how deep the LOL runs nor how many elements any given list will contain.

Using subroutines to print an LOL is slightly slower (there's overhead for subroutine calls), but in this case the flexibility gain makes it worthwhile.

As for printing a section of an LOL, you'll have to define the problem a little more because there's any number of ways of tackling it.

Answer: In an array of arrays, how do I print the whole array or bits of it?
contributed by indigo

Basically, you write a program to do what you want. :) See the previous respondant for examples. Perl has number of builtins to facilitate array operations, such as foreach, map, and grep. For example, if you have a blended array of values and references, you can do this:

my @lol = ([1, 2, 3], 'x', [qw(A B C)], 'foo'); print join "\n", map {ref $_ ? join ', ', @$_ : $_} @lol;
Additionally, you may wish to look at the Data::Dumper module, which can display complex data structures in a human readable format.

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