Array +=

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

Simon M. wrote:

    from (irb):28

Why?

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

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.

mortee

2007/10/12, mortee [email protected]:

           ^
    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

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

Simon M. 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.

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

2007/10/12, Simon M. [email protected]:

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!
:wink:

Cheers

robert

Yep… concat it is!

Cheers!

SM

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