Newbie at ruby - syntactic sugar for Range / Array

[Neil Laurance]

Hi there,

Was hoping someone could point me in the right direction. I'm currently
learning a little ruby, and going through some of the puzzles from the
Ruby Quiz book.

One of the examples uses a very strange looking construct:

a,b,c,d = *0 .. 3

As far as I understand this, *0 .. 3 equates to (0..3).to_a ? I've been
searching the online Ruby documentation, but couldn't find anwhere that
describes this syntax. Could anyone point me to a description for this?

Many thanks, toolkit

 

[dblack]

Hi --

Hi there,

Was hoping someone could point me in the right direction. I'm currently
learning a little ruby, and going through some of the puzzles from the
Ruby Quiz book.

One of the examples uses a very strange looking construct:

a,b,c,d = *0 .. 3

As far as I understand this, *0 .. 3 equates to (0..3).to_a ? I've been
searching the online Ruby documentation, but couldn't find anwhere that
describes this syntax. Could anyone point me to a description for this?

The * is the "unar[r]ay" (unary un-array) operator, or at least
so-called by me The construct you're talking about is indeed a
little odd. It's one of the times when a range pretends to be an
array. In fact, you don't need to un-array an array to get it to do
parallel assignment:

a,b,c,d = [0,1,2,3]

So *0..3 is almost like saying: you're a range, but pretend that
you're a thing that can be un-arrayed, namely an array. That seems to
be the purpose of the * in this case -- to "trick" the range into
thinking it's an array.

If you look at ranges as array-like lists of values in the first place
(which I don't), then it might make sense in a less convoluted way


David

--
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[Rick DeNatale]

So *0..3 is almost like saying: you're a range, but pretend that
you're a thing that can be un-arrayed, namely an array. That seems to
be the purpose of the * in this case -- to "trick" the range into
thinking it's an array.

If you look at ranges as array-like lists of values in the first place
(which I don't), then it might make sense in a less convoluted way

Or you can think of the * before the last rvalue in an assignment as a
signal to replace the rvalue with a series of rvalues obtained from
the elements of the array resulting from sending to_ary to the
original rvalue.

The original rvalue doesn't need to be an array, or a range, just
anything which responds to to_ary

And actually in Ruby 1.8, it looks like it can be any object, since it
seems to actually use to_a instead of to_ary which is defined in
Object to return an array containing the receiver. But this is
supposed to change in Ruby 1.9

--
Rick DeNatale

IPMS/USA Region 12 Coordinator
http://ipmsr12.denhaven2.com/

Visit the Project Mercury Wiki Site
http://www.mercuryspacecraft.com/

 

[dblack]

Hi --

Or you can think of the * before the last rvalue in an assignment as a
signal to replace the rvalue with a series of rvalues obtained from
the elements of the array resulting from sending to_ary to the
original rvalue.

The original rvalue doesn't need to be an array, or a range, just
anything which responds to to_ary

And actually in Ruby 1.8, it looks like it can be any object, since it
seems to actually use to_a instead of to_ary which is defined in
Object to return an array containing the receiver. But this is
supposed to change in Ruby 1.9

Yes, that's a more cogent explanation, and it accounts for:

a, b = "abc\ndef" => a == "abc\n", b = "def"

So the only weird thing is that ranges can turn themselves into arrays



David

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[Rick DeNatale]

So the only weird thing is that ranges can turn themselves into arrays

Nothing at all wierd about that, any enumerable can, which makes
perfect sense when you think about it.

--
Rick DeNatale

IPMS/USA Region 12 Coordinator
http://ipmsr12.denhaven2.com/

Visit the Project Mercury Wiki Site
http://www.mercuryspacecraft.com/

 

[Matt Todd]

_Why The Lucky Stiff calls it a splat. I like this because it's a very
vivid description of a bug-swatter splatting the entrails of the array
(or array-able) object out into separate, individual pieces. From
something compact to its little pieces strewn out in order.

M.T.

 

[Rick DeNatale]

Nothing at all wierd about that, any enumerable can, which makes
perfect sense when you think about it.

And an afterthought.

They don't get turned into anything. The message to_a returns an an
array which represents the receiver, unless the receiver happens to be
an array, in which case it simply returns itself.

--
Rick DeNatale

IPMS/USA Region 12 Coordinator
http://ipmsr12.denhaven2.com/

Visit the Project Mercury Wiki Site
http://www.mercuryspacecraft.com/

 

[dblack]

Hi --

_Why The Lucky Stiff calls it a splat.

I think everyone does, except me That's certainly what it was
always called by Perl people 10 or 12 years ago when I first
encountered it in Perl.


David

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[dblack]

Hi --

And an afterthought.

They don't get turned into anything. The message to_a returns an an
array which represents the receiver, unless the receiver happens to be
an array, in which case it simply returns itself.

Let me rephrase my point: the only weird thing is that ranges are
enumerable


David

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[Logan Capaldo]

Yes, that's a more cogent explanation, and it accounts for:

a, b = "abc\ndef" => a == "abc\n", b = "def"

irb(main):047:0> a, b = "abc\ndef"
=> ["abc\ndef"]
irb(main):048:0> p a
"abc\ndef"
=> nil
irb(main):049:0> p b
nil
=> nil
irb(main):050:0> RUBY_VERSION
=> "1.8.4"

Oh I see, it works with *"abc\ndef".

Weird seeing it there.

 

[dblack]

Hi --

Yes, that's a more cogent explanation, and it accounts for:

a, b = "abc\ndef" => a == "abc\n", b = "def"

irb(main):047:0> a, b = "abc\ndef"
=> ["abc\ndef"]
irb(main):048:0> p a
"abc\ndef"
=> nil
irb(main):049:0> p b
nil
=> nil
irb(main):050:0> RUBY_VERSION
=> "1.8.4"

Oh I see, it works with *"abc\ndef".

Yes -- I forgot the *, which sort of made my point pointless


David

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