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Re^4: Mini-Tutorial: Scalar vs List Assignment Operator

by ikegami (Pope)
on Aug 21, 2009 at 13:29 UTC ( #790356=note: print w/ replies, xml ) Need Help??


in reply to Re^3: Mini-Tutorial: Scalar vs List Assignment Operator
in thread Mini-Tutorial: Scalar vs List Assignment Operator

Besides flagging that wantarray could be useful here

It's not. You don't pick an assignment operator then design the function it's operand will call. You design the function first, then you write the code that will assign the return value or the part of the return value that's desired (whether it uses wantarray or not).

# f() returns three elements my $first = f(); # XXX, unless f() also allows this my ($first) = f(); # OK, uses list assignment my $first = ( f() )[0]; # OK, extracts desired scalar first my (undef, $second) = f(); # OK, uses list assignment my $second = ( f() )[1]; # OK, extracts desired scalar first # g() returns a variable number of elements my $last = g(); # XXX, unless g() also allows this my $last = ( g() )[-1]; # OK, extracts desired scalar first my $count = @all = g(); # OK, list assignment in scalar cx my $count = () = g(); # OK, list assignment in scalar cx

Like I already said, deciding what to have a sub return is a lengthy topic entirely unrelated to the behaviour of the assignment operators.


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Re^5: Mini-Tutorial: Scalar vs List Assignment Operator
by dec (Beadle) on Aug 22, 2009 at 01:23 UTC
    It's not

    Question: what is the point in writing a tutorial explaining the difference between list and scalar contexts?

    Answer: so that people who don't know the difference can gain some enlightenment.

    Question: Besides poring over man pages or searching the Internet, what other way is there to gain an understanding of when a list context is implied and when a scalar context is implied

    Answer: write a sub that calls wantarray and divine the context directly

    Have I made my point?

      huh?

      Are you saying I wrote a tutorial on contexts? I didn't, but someone else did over here.

      Are you saying you're writing a tutorial on contexts? All the power to you, but maybe should start a new thread (where people will actually see it) instead of placing it in a reply to some unrelated post.

Re^5: Mini-Tutorial: Scalar vs List Assignment Operator
by dec (Beadle) on Aug 22, 2009 at 01:35 UTC
    Further: approaching the question of "what is the difference between list and scalar context, and when are they applied?" via an experiential/experimental approach with wantarray is eminently practical. To wit, here's a simple test script I wrote when I wanted to clarify the matter for myself. This most definitely constitutes Another Way To Do It:
    #!/usr/bin/perl -w sub polymorphic { my $context=wantarray(); if ($context) { print "list context\n"; return @_; } elsif (defined($context)) { print "scalar context\n"; return join ":", @_; } else { warn "value of polymorphic() used in void context. Values were (\n +"; # so no point in returning anything, but do print print join ",", @_; print ")\n"; } } sub show_args { print "got ", scalar(@_), " args:\n"; map {print " $_\n" } @_; } show_args("a", polymorphic("b","c","d")); my $string=polymorphic(1,2,3); my @array=polymorphic(4,5,6); show_args($string,@array); # make warning 1 polymorphic("unused"); # following is tricky because we don't know context until tricky is # actually called. sub tricky { polymorphic(@_); } my $string2=tricky(qw(a b c d)); my @array2=tricky(qw(s t u v)); show_args($string2,@array2); # make warning 2 tricky(qw(fee fi fo fum)); # what happens if we use return keyword explicitly? sub tricky_2 { return polymorphic(@_); } my $string3=tricky_2(qw(a e i o u)); my @array3=tricky_2(qw(w x y z)); show_args($string3,@array3); # (turns out we can use return or not; same result--no warning) # (but this should generate one) tricky_2(5,6,7,8); sub tricky_3 { my $first=shift; my $second=shift; my @remaining=@_; print "$first, $second, ("; print join ",", @remaining; print ")\n"; };

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