I have an array that contains a number of items, for example: 

@arr = qw ( up down in out left right );

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Say I want to select specific items from this list, say, "down", "out" and "left". This might be represented by another array: 

@sel = ( 1, 3, 4 );

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I'm looking to build a function that will take both of these arrays as an input and to produce an output that is an array containing the selected items. Thus the usage might be: 

@my_items = Magic(@arr, @sel);

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Then I thought it might be better to simply build a bit map of the items I want to select, thereby eliminating the machinations with manipulating arrays with every call, viz: 

$bitmask = MoreMagic(@arr, @sel);

[...other work...]

@my_items = BetterMagic(@arr, $bitmask);

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This led to my postings on Taking arbitrary elements from an array. From that study, I now have the BetterMagic function... but how to build the MoreMagic function?

I've tried looking at vec() and map() in the perlfunc docs... or to try and use rotates, etc but I'm not having any joy. An example: 

@arr = qw ( up down in out left right );  # 6 elements
@sel = ( 1, 3, 4 );

$mask = 0b000000; # Need to set this to have as 
                  # many bits as the size of @arr
                  # Maybe better to pass it in 
                  # by reference?


printf("      org \$mask: %0b\n", $mask);
@revsel = reverse(@sel);
foreach $item (@revsel) {
   vec($mask, $item, 1) = 1;
   printf("\$item: $item  \$mask: %0b\n", $mask);
}
printf("    final \$mask: %0b\n", $mask);

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It seems like something pretty simple to do but I'm going around in circles trying to figure it out.

Any suggestions??

Thanks.

In reply to How to Use a Bitfield to Select Elements of an Array by ozboomer

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