How do you add an array to another array in Ruby and not end up with a multi-dimensional result?

You've got a workable idea, but the #flatten! is in the wrong place -- it flattens its receiver, so you could use it to turn [1, 2, ['foo', 'bar']] into [1,2,'foo','bar'].

I'm doubtless forgetting some approaches, but you can concatenate:

a1.concat a2
a1 + a2              # creates a new array, as does a1 += a2

or prepend/append:

a1.push(*a2)         # note the asterisk
a2.unshift(*a1)      # note the asterisk, and that a2 is the receiver

or splice:

a1[a1.length, 0] = a2
a1[a1.length..0] = a2
a1.insert(a1.length, *a2)

or append and flatten:

(a1 << a2).flatten!  # a call to #flatten instead would return a new array

You can just use the + operator!

irb(main):001:0> a = [1,2]
=> [1, 2]
irb(main):002:0> b = [3,4]
=> [3, 4]
irb(main):003:0> a + b
=> [1, 2, 3, 4]

You can read all about the array class here: http://ruby-doc.org/core/classes/Array.html

The cleanest approach is to use the Array#concat method; it will not create a new array (unlike Array#+ which will do the same thing but create a new array).

Straight from the docs (http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-concat):

concat(other_ary)

Appends the elements of other_ary to self.

So

[1,2].concat([3,4])  #=> [1,2,3,4]  

Array#concat will not flatten a multidimensional array if it is passed in as an argument. You'll need to handle that separately:

arr= [3,[4,5]]
arr= arr.flatten   #=> [3,4,5]
[1,2].concat(arr)  #=> [1,2,3,4,5]

Lastly, you can use our corelib gem (https://github.com/corlewsolutions/corelib) which adds useful helpers to the Ruby core classes. In particular we have an Array#add_all method which will automatically flatten multidimensional arrays before executing the concat.

Easy method that works with Ruby version >= 2.0 but not with older versions :

irb(main):001:0> a=[1,2]
=> [1, 2]
irb(main):003:0> b=[3,4]
=> [3, 4]
irb(main):002:0> c=[5,6]
=> [5, 6]
irb(main):004:0> [*a,*b,*c]
=> [1, 2, 3, 4, 5, 6]

a = ["some", "thing"]
b = ["another", "thing"]

To append b to a and store the result in a:

a.push(*b)

or

a += b

In either case, a becomes:

["some", "thing", "another", "thing"]

but in the former case, the elements of b are appended to the existing a array, and in the latter case the two arrays are concatenated together and the result is stored in a.