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comp.lang.ruby

Array +=

Simon Mullis

10/12/2007 10:54:00 AM

Hi All,

Question re Array method += and

>> a = []
=> []
>> a += [1, 2, 3, 4]
=> [1, 2, 3, 4]

And

>> (a ||= []) << [1, 2, 3, 4]
=> [[1, 2, 3, 4]]

But

>> ( a ||= [] ) += [ 1, 2, 3, 4 ]
SyntaxError: compile error
(irb):28: syntax error, unexpected tOP_ASGN, expecting $end
( a ||= [] ) += [ 1, 2, 3, 4 ]
^
from (irb):28

Why?

I guess it's a precedence issue. '<<' is a method in the Array class.
'+=' is not.

Is there alternate syntax I should use?

Thanks

SM
6 Answers

mortee

10/12/2007 11:35:00 AM

0

Florian Frank

10/12/2007 11:53:00 AM

0

Simon Mullis wrote:
> Is there alternate syntax I should use?
>

Did you you mean this?

(a ||= []).concat [ 1, 2, 3, 4 ]

concat modifies a, like << would. It's usually faster to concatenate
than to add and reassign.

--
Florian Frank

Robert Klemme

10/12/2007 11:56:00 AM

0

2007/10/12, mortee <mortee.lists@kavemalna.hu>:
> Simon Mullis wrote:
> >>> (a ||= []) << [1, 2, 3, 4]
> > => [[1, 2, 3, 4]]
> >
> > But
> >
> >>> ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > SyntaxError: compile error
> > (irb):28: syntax error, unexpected tOP_ASGN, expecting $end
> > ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > ^
> > from (irb):28
> >
> > Why?
> >
> > I guess it's a precedence issue. '<<' is a method in the Array class.
> > '+=' is not.

No, it's not a precedence issue. Btw, you are comparing apples and
oranges here: += and << are not equivalent (see also further below):

irb(main):001:0> a=%w{foo bar}
=> ["foo", "bar"]
irb(main):002:0> b=a.dup
=> ["foo", "bar"]
irb(main):003:0> c=a.dup
=> ["foo", "bar"]
irb(main):004:0> b << a
=> ["foo", "bar", ["foo", "bar"]]
irb(main):005:0> c += a
=> ["foo", "bar", "foo", "bar"]

<< adds the whole Array as one object while += "appends" the Array.
You rather want Array#concat.

> The difference is that << is a method, += is an assignment. (a ||= [])
> returns an object, on which you can call a method, but you can't assign
> to. You can only assign to the a variable itself, which (a ||= []) is
> not. I guess.

Exactly: the expression (a||=[]) is not an lvalue, i.e. cannot
assigned to. But you can do

( a ||= [] ).concat [ 1, 2, 3, 4 ]

Simon, please also keep in mind that += and << have different
semantics. += will create a new Array while << appends to the current
one.

Kind regards

robert

Simon Mullis

10/12/2007 1:23:00 PM

0

I love this list... I send a mail, go off to a meeting, come back and
I have explanation, discussion and solution!

Thanks to all who responded.

Cheers

SM

On 10/12/07, Robert Klemme <shortcutter@googlemail.com> wrote:
> 2007/10/12, mortee <mortee.lists@kavemalna.hu>:
> > Simon Mullis wrote:
> > >>> (a ||= []) << [1, 2, 3, 4]
> > > => [[1, 2, 3, 4]]
> > >
> > > But
> > >
> > >>> ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > > SyntaxError: compile error
> > > (irb):28: syntax error, unexpected tOP_ASGN, expecting $end
> > > ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > > ^
> > > from (irb):28
> > >
> > > Why?
> > >
> > > I guess it's a precedence issue. '<<' is a method in the Array class.
> > > '+=' is not.
>
> No, it's not a precedence issue. Btw, you are comparing apples and
> oranges here: += and << are not equivalent (see also further below):
>
> irb(main):001:0> a=%w{foo bar}
> => ["foo", "bar"]
> irb(main):002:0> b=a.dup
> => ["foo", "bar"]
> irb(main):003:0> c=a.dup
> => ["foo", "bar"]
> irb(main):004:0> b << a
> => ["foo", "bar", ["foo", "bar"]]
> irb(main):005:0> c += a
> => ["foo", "bar", "foo", "bar"]
>
> << adds the whole Array as one object while += "appends" the Array.
> You rather want Array#concat.
>
> > The difference is that << is a method, += is an assignment. (a ||= [])
> > returns an object, on which you can call a method, but you can't assign
> > to. You can only assign to the a variable itself, which (a ||= []) is
> > not. I guess.
>
> Exactly: the expression (a||=[]) is not an lvalue, i.e. cannot
> assigned to. But you can do
>
> ( a ||= [] ).concat [ 1, 2, 3, 4 ]
>
> Simon, please also keep in mind that += and << have different
> semantics. += will create a new Array while << appends to the current
> one.
>
> Kind regards
>
> robert
>
>

Simon Mullis

10/12/2007 1:25:00 PM

0

Thanks for this Robert.

The difference between the two is exactly why I wanted to use += and
not <<. I want to add many arrays to one big one and not have to use
the extra step of Array#flatten at the end.

SM



On 10/12/07, Robert Klemme <shortcutter@googlemail.com> wrote:
> 2007/10/12, mortee <mortee.lists@kavemalna.hu>:
> > Simon Mullis wrote:
> > >>> (a ||= []) << [1, 2, 3, 4]
> > > => [[1, 2, 3, 4]]
> > >
> > > But
> > >
> > >>> ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > > SyntaxError: compile error
> > > (irb):28: syntax error, unexpected tOP_ASGN, expecting $end
> > > ( a ||= [] ) += [ 1, 2, 3, 4 ]
> > > ^
> > > from (irb):28
> > >
> > > Why?
> > >
> > > I guess it's a precedence issue. '<<' is a method in the Array class.
> > > '+=' is not.
>
> No, it's not a precedence issue. Btw, you are comparing apples and
> oranges here: += and << are not equivalent (see also further below):
>
> irb(main):001:0> a=%w{foo bar}
> => ["foo", "bar"]
> irb(main):002:0> b=a.dup
> => ["foo", "bar"]
> irb(main):003:0> c=a.dup
> => ["foo", "bar"]
> irb(main):004:0> b << a
> => ["foo", "bar", ["foo", "bar"]]
> irb(main):005:0> c += a
> => ["foo", "bar", "foo", "bar"]
>
> << adds the whole Array as one object while += "appends" the Array.
> You rather want Array#concat.
>
> > The difference is that << is a method, += is an assignment. (a ||= [])
> > returns an object, on which you can call a method, but you can't assign
> > to. You can only assign to the a variable itself, which (a ||= []) is
> > not. I guess.
>
> Exactly: the expression (a||=[]) is not an lvalue, i.e. cannot
> assigned to. But you can do
>
> ( a ||= [] ).concat [ 1, 2, 3, 4 ]
>
> Simon, please also keep in mind that += and << have different
> semantics. += will create a new Array while << appends to the current
> one.
>
> Kind regards
>
> robert
>
>


--
Simon Mullis
_________________
simon@mullis.co.uk

Robert Klemme

10/12/2007 1:52:00 PM

0

2007/10/12, Simon Mullis <simon@mullis.co.uk>:
> Thanks for this Robert.
>
> The difference between the two is exactly why I wanted to use += and
> not <<. I want to add many arrays to one big one and not have to use
> the extra step of Array#flatten at the end.

That's why you should be using #concat (more efficient) unless you do
not want to modify the original array. Read the solutions properly!
;-)

Cheers

robert