SPUG: re: slice

[ced at carios2.ca.boeing.com]

Replying to Rob G.'s question.  Sorry to lose the thread... 
It's Monday :). 


> ...but I was surprised:

> $DATA{ $key }[0] = $val;  # same as $DATA{ $key }->[0] = $val;

> is proper syntax to assign a value to a anon array ref stored in a hash,
> correct?

Right. Either works.

> So I was expecting array slices to behave in the same manner by replacing a
> single subscript with a range of subscripts.  But this didn't seem to work:

> $DATA{ $key }[1,2,3,4] = @prices[0,1,2,3];

> Thinking I understand and really understanding are !=.  Is the difference
> between the 2 lines that the slice turns the array operation into a list
> operation?  
> And, therefore, the [1,2,3,4] no longer is telling perl that
> $DATA{ $key } is an array reference?  So perl thinks that I'm trying to do
> what?

I believe the $ in $DATA{}[] will propagate a scalar context for 
both the index [1,2,3,4] and the slice @prices[] on the RHS. In 
both cases, the list will reduce to the final element.

So, the above becomes:

   $DATA[ $key }[4] = $prices[3];

A hash slice here would have to written:

   @{ $DATA{ $key } }[1,2,3,4] = @prices[0,1,2,3];

or, more virtuously, since you'd save keystrokes: 

   @{ $DATA{ $key } }[1..4] = @prices[0..3];

A bit tricky because $DATA{ $key } is an array reference 
and the binding precedence won't allow you to get away with: 

  @$DATA{ $key }[1,2,3,4] 

The tokener thinks $DATA alone contains the array ref. so
you have to give it some help with the {}. 

Hope this isn't too far off the mark. Again, Perl Data Structure
Cookbook probably has far better explanations. 


Rgds,
--
Charles DeRykus

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