Ruby splat and << operators

Question

I want to do this:

a << *b

but this happens in irb:

1.9.3p327 :020 > a
 => [1, 2, 3, 4] 
1.9.3p327 :021 > b
 => [5, 6, 7] 
1.9.3p327 :022 > a << *b
SyntaxError: (irb):22: syntax error, unexpected tSTAR
a << *b
      ^

Am I missing something?

Answer 1

Look the reason here :

 a = [1, 2, 3, 4] 
 b = [5, 6, 7] 
 p a.<<(*b)
 # ~> -:3:in `<<': wrong number of arguments (3 for 1) (ArgumentError)
 # ~>  from -:3:in `<main>'

<< method expects only one argument.So now as below splat(*) is an operator,which will create 5,6,7 ,which << method do not expect,rather it expects only one object. Thus design of Ruby don't allowing * before b.

 a = [1, 2, 3, 4] 
 b = [5, 6, 7] 
 p a << *
 # ~> -:3: syntax error, unexpected *

 a = [1, 2, 3, 4] 
 b = [5, 6, 7] 
 p a << *b
 # ~> -:3: syntax error, unexpected *
 # ~> p a << *b
 # ~>         ^

That is why the 2 legitimate errors :

• wrong number of arguments (3 for 1) (ArgumentError)

• syntax error, unexpected *

Probably you can use -

 a = [1, 2, 3, 4] 
 b = [5, 6, 7] 
 p a.push(*b)
 # >> [1, 2, 3, 4, 5, 6, 7]

Answer 2

i have commented it eariler and now adding it to answer ;-)

a += b will do your job

Answer 3

Try this:

 a.concat(b)

it will output:

=> [1, 2, 3, 4, 5, 6, 7] 

Answer 4

Check the type signature: ary << obj → ary. It expects exactly one parameter. Try: a.push(*b) (push(obj, ... ) → ary)

Answer 5

Given

a = [1, 2, 3, 4]
b = [5, 6, 7]

The solution you want is

a.concat b
# => [1, 2, 3, 4, 5, 6, 7]

However, you might find this a little interesting

b.each &a.method(:<<)
a
# => [1, 2, 3, 4, 5, 6, 7]

This says "call method a#<< on each element in b"

Equivalent to

b.each &a.method(:push)
a
# => [1, 2, 3, 4, 5, 6, 7]