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Re^2: Getting for() to accept a tied array in one statement

by perlancar (Hermit)
on Apr 16, 2019 at 13:05 UTC ( [id://1232648]=note: print w/replies, xml ) Need Help??


in reply to Re: Getting for() to accept a tied array in one statement
in thread Getting for() to accept a tied array in one statement

Your code which wraps tie in @{ ... } does not work because all elements are FETCH-ed first before the loop block code is executed. I.e. in the above case, FETCH(0) and FETCH(1) are called before "first" and "second" are print-ed by print(). The @{ ... } basically turns the tied array into list of values. I need to iterate over the tied array so the code in the loop block (in this case, print()) is executed along with FETCH/FETCHSIZE.
  • Comment on Re^2: Getting for() to accept a tied array in one statement

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Re^3: Getting for() to accept a tied array in one statement
by hdb (Monsignor) on Apr 16, 2019 at 13:37 UTC

    I disagree. Here is the full code and FETCH is only called once for each element within the body of the loop:

    use strict; use warnings; package MyClass; use Tie::Array; our @ISA = ('Tie::Array'); our @data; # mandatory methods sub TIEARRAY { my $class = shift; bless \@data, $class; @data = @_ +; return \@data } sub FETCH { print "FETCH: "; my ($self, $index ) = @_; return $dat +a[$index] } sub FETCHSIZE { print "<FETCHSIZE> "; return scalar @data } package main; for( @{ tie my @x, "MyClass", "first", "second"; \@x } ) { print "In loop = "; print "$_\n"; }

    Output:

    In loop = FETCH: first In loop = FETCH: second

      Ah sorry, I stand corrected. I was reading your code as: for( do { tie my @x, "MyClass", "first", "second"; @x } ) {

      So indeed: for( @{ tie my @x, "MyClass", "first", "second"; \@x } ) {

      will make for() iterate over a tied array. In fact,

      use strict; use warnings; package MyClass; use Tie::Array; our @ISA = ('Tie::Array'); our @data; # mandatory methods sub TIEARRAY { my $class = shift; bless \@data, $class; @data = @_ +; return \@data } sub FETCH { print "FETCH: "; my ($self, $index ) = @_; return $dat +a[$index] } sub FETCHSIZE { return scalar @data } package main; sub wrapper { tie my @x, "MyClass", @_; \@x; } for( @{wrapper("first", "second")} ) { print "In loop = "; print "$_\n"; }

      also works. So it's quite close to what I want, yay. Any improvement is welcome :-)

        Try this:

        sub wrapper : lvalue { tie my @x, "MyClass", @_; @x; } for( wrapper("first", "second") ) { print "In loop = "; print "$_\n"; }

        As I said initially I am not expert in this, so there might be undesired side effects...

        And FETCHSIZE is called once only...

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