pairwise_test

[Dr.Ruud]

Inspired by List::MoreUtils, I came up with the following.
Please comment on the name of the sub, and on any problems
you see, and on what not.

#!/usr/bin/perl
use strict;
use warnings;

# pairwise_firstidx BLOCK ARRAY1 ARRAY2

• 
  # returns the first index for which the BLOCK returns a defined value,
  # optionally only for a list of index values
  
  sub pairwise_do (&\@\@;@)
  {
  use vars qw/$a $b/;
  my $op = shift; # the BLOCK
  
  my ($caller_a, $caller_b) = do
  { my $pkg = caller();
  no strict 'refs';
  \*{$pkg.'::a'}, \*{$pkg.'::b'};
  };
  
  my $retval;
  local (*$caller_a, *$caller_b);
  
  for ( $_[2]
  ?
  @_[2..$#_]
  :
  $[ .. $#{$_[0]} >= $#{$_[1]}
  ?
  $#{$_[0]}
  :
  $#{$_[1]}
  ) {
  (*$caller_a, *$caller_b) = \($_[0][$_], $_[1][$_]);
  defined ($retval = $op->()) and return $retval;
  }
  return;
  }
  
  my @x = (0..6, 70..99);
  my @y = (0..6, 80..99);
  
  # find the index of the first non-equal value for
  # two arrays, and within a specific index range:
  print pairwise_do { $a != $b ? $_ : undef } @x, @y, 2..9;
  
  
  __END__

 

[Brian McCauley]

Inspired by List::MoreUtils, I came up with the following.
Please comment on the name of the sub, and on any problems
you see, and on what not.

# pairwise_firstidx BLOCK ARRAY1 ARRAY2

• 
  # returns the first index for which the BLOCK returns a defined
  # value,

• 
  
  Well, there's your first problem. It doesn't return the index. It
  returns the return value of the block.

 

[Dr.Ruud]

Brian McCauley schreef:

Dr.Ruud:

Inspired by List::MoreUtils, I came up with the following.
Please comment on the name of the sub, and on any problems
you see, and on what not.

# pairwise_firstidx BLOCK ARRAY1 ARRAY2

• 
  # returns the first index for which the BLOCK returns a defined
  # value,

• 
  
  Well, there's your first problem. It doesn't return the index. It
  returns the return value of the block.
  

  defined ($retval = $op->()) and return $retval;

• 
  
  Ah yes, the documentation is outdated. And with that the name of the
  function is wrong. (Or I made it too versatile.)
  
  The BLOCK can return the index value through $_. I see that as the
  normal usage, but other usage is not discouraged.
  
  Maybe 'pairwise_do' is a better function name?
  
  What is the next problem?

 

[Michele Dondi]

# find the index of the first non-equal value for
# two arrays, and within a specific index range:
print pairwise_do { $a != $b ? $_ : undef } @x, @y, 2..9;

Maybe useful for other purposes, but in this case, what's the *big*
advantage over List::Util's

print first { $x[$_] != $y[$_] } 2..9; # ?

If the range were not explicit... then yes, somewhat more clumsy:

print first { $x[$_] != $y[$_] } 1..max($#x,$#y);

However what one feels more missing in Perl 5 is probably a way to do
things in a parallel manner on two or more lists. That's what Perl 6
zip operator and new pointy subs are really great for:

for @x Z @y -> $a, $b { say "<$a> <$b>" if $a !$ $b}

(Don't know 'bout map() & C. - I'm really a baby-Perl-6'er)


Michele

 

[Dr.Ruud]

Michele Dondi schreef:

Dr.Ruud:

# find the index of the first non-equal value for
# two arrays, and within a specific index range:
print pairwise_do { $a != $b ? $_ : undef } @x, @y, 2..9;

Maybe useful for other purposes, but in this case, what's the
*big* advantage over List::Util's

print first { $x[$_] != $y[$_] } 2..9; # ?

Thanks, that is a good alternative for this case. I'll check out the
implementation of first() too.

I want mine to be shortcutting, as first() most probably is too.
And to work on defined() in stead of true/false, so maybe I should call
it first_defined(). Or change to true/false.

A variant of List::MoreUtils::firstidx() could look like this:

$i = firstidx { $a != $b } @x, @y;
$i = firstidx { $a != $b } @x, @y, 2..9;

I'll try to come up with an example that uses it more to the full, I
need to write tests anyway.

If the range were not explicit... then yes, somewhat more clumsy:

print first { $x[$_] != $y[$_] } 1..max($#x,$#y);

ITYM 0 .. max($#x,$#y).

 

[Michele Dondi]

print first { $x[$_] != $y[$_] } 1..max($#x,$#y);

ITYM 0 .. max($#x,$#y).

No, I meant... yes, I did!


Michele