Two-dimensional array access in Perl

Question

I have an array populated with cities. I want to pass the array by reference to a sub routine and print each city to output. However, I have the following problems:

1. I can access each element before my while loop in the subroutine. But I cannot access the elements within my while loop. I get the error message:

   ... Use of uninitialized value in print at line 44, line 997 (#1) Use of uninitialized value in print at line 44, line 998 (#1) ...

The following is some code. I have commented what prints and what doesn't (I tried to cut out code that is not needed for my explanation...):

@cities;

# Assume cities is loaded successfully
&loadCities(getFileHandle('cities.txt'), $NUM_CITIES, \@cities);
&printElements(getFileHandle('names.txt'), \@cities);

sub printElements{

    my $counter = 0;
    my $arraySize = scalar $_[1];

    # Prints fine!!!
    print @{$_[1][($counter)%$arraySize];

    while ((my $line = $_[0]->getline()) && $counter < 1000){

        # Doesn't print. Generates the above error
        print @{$_[1][($counter)%$arraySize];

        $counter += 1;
    }
}

2. The Perl syntax has me super confused. I do not understand what is going on with @{$_[1]}[0]. I am trying to work it out.

1. $_[1], treat the value at this location as scalar value (memory address of the array)
2. @{...}, interpret what is stored at this memory address as an array
3. @{...} [x], access the element at index x

Am I on the right track?

Answer 1

My first tip is that you should put use strict; and use warnings; at the top of your script. This generally reveals quite a few things.

This line: print @{$_[1][($counter)%$arraySize]; doesn't have a closing }. You also don't need the parenthesis around $counter.

Like you mentioned, the best/most clear way to get the length of an array is my $arraySize = scalar @{$_[1]};.

---

You can check out the documentation here for working with references. I'll give you a quick overview.

You can declare an array as normal

my @array = (1, 2, 3);

Then you can reference it using a backslash.

my $array_ref = \@array;

If you want to use the reference, use @{...}. This is just like using a regular array.

print @{$array_ref};

You could also declare it as a reference to begin with using square braces.

my $array_ref = [1, 2, 3];
print @{$array_ref}; # prints 123

In Perl, a 2-dimensional array is actually an array of array references. Here is an example:

my @array = ( ['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i'] );
print @{$array[1]}; # prints def

Now let's try passing in an array reference to a subroutine.

my @array = ( ['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i'] );

example(\@array); # pass in an array reference

sub example {
    my @arr = @{$_[0]}; # use the array reference to assign a new array
    print @{$arr[1]};

    print @{$_[0][1]}; # using the array reference works too!
}

Now let's put it together and print the whole 2-d array.

my @array = ( ['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i'] );
example(\@array);
sub example {
    my @arr = @{$_[0]};
    for my $ref (@arr) {
        print @{$ref};
    }
} # prints abcdefghi

You could adapt this example pretty easily if you wanted to use it for your printElements subroutine.

---

Another note about printing the elements in an array. Let's take this line from the last example:

print @{$ref};

Since we are calling it every time through the loop, we may want to print a new line at the end of it.

print @{$ref} . "\n";

What does this print? Try it! Does it work?

This is where the built-in subroutine join comes in handy.

print join(" ", @{$ref}) . "\n";

For loops are generally the best way to iterate through an array. My answer here talks a little about doing it with a while loop: https://stackoverflow.com/a/21950936/2534803 You can also check out this question: Best way to iterate through a Perl array