stevieb has asked for the wisdom of the Perl Monks concerning the following question:

Hello friends and esteemed Monks,

This isn't a problem per-se, just something I've observed and am wondering if there's a way around it.

I am currently building several wildlife cameras that will be attached to my house and some of my cabins, each camera will be attached to a Raspberry Pi, and will stream back results to a central preview server that I will display on a television screen (up to eight inside of a single browser window).

To enhance what I've already got working, I decided that I want pan/tilt capabilities to my cameras, so for the tilt, I'm using a servo (with the servo() capability of RPi::WiringPi), and for the pan, I'm going to use a stepper motor with a ULN2003 integrated circuit, for which I'm now writing a new distribution, RPi::StepperMotor for. Everything is working great thus far, but I have a question specifically related to constant.

Normally, with a list (ie. array in this case), we can get the count of elements and assign it elsewhere:

my $array = [qw(1 2 3)];
my $array_len = @$array;

# $array_len == 3

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However, in the module I'm writing, I'd prefer to use a constant:

use constant STEPPER_SEQUENCE => [
    [qw(1 0 0 1)],
    [qw(1 0 0 0)],
    [qw(1 1 0 0)],
    [qw(0 1 0 0)],
    [qw(0 1 1 0)],
    [qw(0 0 1 0)],
    [qw(0 0 1 1)],
    [qw(0 0 0 1)],
];

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All is still well and great. However, what I can't seem to figure out, is how to extract the count of elements in the constant list. Here is an example. It prints only aref ok. I've tried a few things I've found in the documentation and elsewhere, but I've either overlooked something, or it's not possible to extract the info I want here:

use warnings;
use strict;
use feature 'say';

use constant STEPPER_SEQUENCE => [
    [qw(1 0 0 1)],
    [qw(1 0 0 0)],
    [qw(1 1 0 0)],
    [qw(0 1 0 0)],
    [qw(0 1 1 0)],
    [qw(0 0 1 0)],
    [qw(0 0 1 1)],
    [qw(0 0 0 1)],
];

# the below line houses the normal "ARRAY(0x...)"

use constant STEP_COUNT => STEPPER_SEQUENCE;

# ... and because STEPPER_SEQUENCE returns the "ARRAY(0x...)",
# this fails:

say "const ok" if STEPPER_SEQUENCE == 3;

my $aref = [qw(1 2 3)];

say "aref ok" if @$aref == 3;

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I have tried:

@{ STEPPER_SEQUENCE }
@( STEPPER_SEQUENCE )
(STEPPER_SEQUENCE)
@STEPPER_SEQUENCE
etc, etc, etdc

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Is it that I'm oblivious to something, or is what I'm attempting not possible?

Thanks!

-stevieb

Replies are listed 'Best First'.
Re: Get element count of CONSTANT list (aref))
by haukex (Archbishop) on Mar 12, 2018 at 21:37 UTC

    constants are basically just sub FOO () { ... } (Constant Functions). So I first tried your @{ STEPPER_SEQUENCE }, but Perl reminded me: Ambiguous use of @{STEPPER_SEQUENCE} resolved to @STEPPER_SEQUENCE and Global symbol "@STEPPER_SEQUENCE" requires explicit package name. However, once you disambiguate:

    use constant STEPPER_SEQUENCE => [
    [qw(1 0 0 1)], [qw(1 0 0 0)], [qw(1 1 0 0)], [qw(0 1 0 0)],
    [qw(0 1 1 0)], [qw(0 0 1 0)], [qw(0 0 1 1)], [qw(0 0 0 1)],
];
print 0+@{STEPPER_SEQUENCE()}, "\n";
print 0+@{+STEPPER_SEQUENCE}, "\n";
print 0+@{&STEPPER_SEQUENCE}, "\n";
__END__
8
8
8

    [download]

    Update: The thread constant vector might be relevant here too. Also added the third example, although note that calls to constant functions with & are not subject to inlining.

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      Don't know how I didn't follow that path, probably just because I was focused for so long on it.

      use warnings;
use strict;
use feature 'say';

use constant STEPPER_SEQUENCE => [
        [qw(1 0 0 1)],
        [qw(1 0 0 0)],
        [qw(1 1 0 0)],
        [qw(0 1 0 0)],
        [qw(0 1 1 0)],
        [qw(0 0 1 0)],
        [qw(0 0 1 1)],
        [qw(0 0 0 1)],
    ];

use constant STEP_COUNT => 0+@{ STEPPER_SEQUENCE() };

say STEP_COUNT;

      [download]

      Output:

      8

      [download]

      Works perfectly! Thanks once again haukex!

      -stevieb

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Re: Get element count of CONSTANT list (aref))
by tybalt89 (Monsignor) on Mar 12, 2018 at 22:17 UTC 
    #!/usr/bin/perl

# http://perlmonks.org/?node_id=1210767

use strict;
use warnings;

use constant STEPPER_SEQUENCE => [
    [qw(1 0 0 1)],
    [qw(1 0 0 0)],
    [qw(1 1 0 0)],
    [qw(0 1 0 0)],
    [qw(0 1 1 0)],
    [qw(0 0 1 0)],
    [qw(0 0 1 1)],
    [qw(0 0 0 1)],
];

print STEPPER_SEQUENCE->@* . "\n";

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      ++ tybalt89, I had thought about and actually tested that as well, but I can't use that. If I used postfix-deref, then that would force the encompassing distribution (or other consumers) to require 5.20 at minimum, which I refuse to go above 5.10.

      Besides, it results in the same type of behaviour:

      use warnings;
use strict;
use feature 'say';

use constant STEPPER_SEQUENCE => [
        [qw(1 0 0 1)],
        [qw(1 0 0 0)],
        [qw(1 1 0 0)],
        [qw(0 1 0 0)],
        [qw(0 1 1 0)],
        [qw(0 0 1 0)],
        [qw(0 0 1 1)],
        [qw(0 0 0 1)],
    ];

use constant STEP_COUNT => STEPPER_SEQUENCE->@*;

say STEP_COUNT;

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      Output:

      ARRAY(0x17985b8)ARRAY(0x17981f8)ARRAY(0x17c8e68)ARRAY(0x17edc90)ARRAY(
+0x17edd80)ARRAY(0x17a7708)ARRAY(0x17ee0c8)ARRAY(0x1805590)

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        Besides, it results in the same type of behaviour:

        In list context, yes, but like @{...} you can force it to scalar context: 0+STEPPER_SEQUENCE->@* is 8. And BTW, there is also STEPPER_SEQUENCE->$#* which will returns 7. tybalt89's code forced the scalar context with the dot . operator.

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Re: Get element count of CONSTANT list (aref))
by stevieb (Canon) on Mar 12, 2018 at 22:07 UTC

    For anyone who may be interested, here's the mock-up prototype code I've got so far to turn the stepper motor clockwise (cw()) or counter-clockwise (ccw()). There's a lot of repetitiveness I will be removing, and adding in many more checks etc, but this works quite well thus far:

    package Step;

use 5.010;
use strict;
use warnings;

use Carp qw(croak);
use Data::Dumper;
use RPi::Const qw(:all);
use WiringPi::API qw(:perl);

our $VERSION = '2.3601';

use constant FULL => 2;
use constant HALF => 1;
use constant NUM_PINS => 0..3;

use constant STEPPER_SEQUENCE => [
    [qw(1 0 0 1)],
    [qw(1 0 0 0)],
    [qw(1 1 0 0)],
    [qw(0 1 0 0)],
    [qw(0 1 1 0)],
    [qw(0 0 1 0)],
    [qw(0 0 1 1)],
    [qw(0 0 0 1)],
];

use constant STEP_COUNT => 0+@{ STEPPER_SEQUENCE() };

sub new {
    my ($class, %args) = @_;

    my $self = bless {}, $class;

    if (! exists $args{pins}){
        croak "'pins' parameter is required to use this module\n";
    }

    setup_gpio();

    $self->_pins($args{pins});

    my $delay = $args{delay} // 0.01;
    $self->delay($delay);

    my $speed = $args{speed} // 'half';
    $self->speed($speed);

    return $self;
}
sub cw {
    my ($self, $degrees) = @_;

    my $step_counter = 0;
    my $pins = $self->_pins;

    for (1..$self->_turns($degrees)){
        for my $gpio_pin (NUM_PINS){
            if (STEPPER_SEQUENCE->[$step_counter][$gpio_pin]){
                write_pin($pins->[$gpio_pin], HIGH);
            }
            else {
                write_pin($pins->[$gpio_pin], LOW);
            }
        }

        $step_counter += $self->_phases;

        if ($step_counter >= STEP_COUNT){
            $step_counter = 0;
        }

        $self->_wait;
    }

    for (@$pins){
        write_pin($_, LOW);
        pin_mode($_, INPUT);
    }
}
sub ccw {
    my ($self, $degrees) = @_;

    my $step_counter = 0;
    my $pins = $self->_pins;

    for (1..$self->_turns($degrees)){
        for my $gpio_pin (NUM_PINS){
            if (STEPPER_SEQUENCE->[$step_counter][$gpio_pin]){
                write_pin($pins->[$gpio_pin], HIGH);
            }
            else {
                write_pin($pins->[$gpio_pin], LOW);
            }
        }

        $step_counter += $self->_phases * -1;

        if ($step_counter < 0){
            $step_counter = STEP_COUNT + $self->_phases * -1;
        }

        $self->_wait;
    }

    for (@$pins){
        write_pin($_, LOW);
        pin_mode($_, INPUT);
    }
}
sub speed {
    my ($self, $speed) = @_;
    if (defined $speed){
        if (! grep {$speed ne $_} qw(full half)){
            croak "'speed' parameter must be either 'full' or 'half'\n
+";
        }
        $self->{speed} = $speed;
    }
    return $self->{speed};
}
sub delay {
    my ($self, $delay) = @_;
    $self->{delay} = $delay if defined $delay;
    return $self->{delay};
}
sub _wait {
    my ($self) = @_;
    select(undef, undef, undef, $self->delay);
}
sub _pins {
    my ($self, $pins) = @_;

    if (defined $pins){
        if (@$pins != 4){
            croak "the 'pins' parameter must include an aref with four
+ elements\n";
        }

        for (@$pins){
            pin_mode($_, OUTPUT);
            write_pin($_, LOW);
        }
        $self->{pins} = $pins;
    }

    return $self->{pins};
}
sub _phases {
    return $_[0]->speed eq 'full' ? FULL : HALF;
}
sub _turns {
    my ($self, $degrees) = @_;
    return $self->_phases == 1
        ? int($degrees / 5.625 + 0.5) * 64
        : int($degrees / 11.25 + 0.5) * 64;
}

1;

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    ...and the calling script (the param to ccw() is the degrees in which to turn:

    use warnings;
use strict;

use lib '.';

use Step;

my $s = Step->new(
    pins => [12, 16, 20, 21],
    speed => 'full'
);


#$s->cw(90);
$s->ccw(90);

    [download]

    Feedback on any part would be appreciated, particularly with the cw() and ccw() method names. I thought about left() and right(), but perhaps those could be ambiguous in certain scenarios. I do totally like them better. Perhaps they could be aliases with the documentation specifying this?

    Update: After some consideration, I think I might alias right(); up() to cw() and left(), down() to ccw(). Thoughts much welcome.

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Re: [SOLVED] Get element count of CONSTANT list (aref))
by stevieb (Canon) on Mar 13, 2018 at 21:36 UTC
[reply]

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