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

I tried:

somearray = ["some", "thing"]
anotherarray = ["another", "thing"]
somearray.push(anotherarray.flatten!)

I expected

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

but got

["some", "thing", nil]

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.

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.

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]

Here are two ways, notice in this case that the first way assigns a new array ( translates to somearray = somearray + anotherarray )

somearray = ["some", "thing"]

anotherarray = ["another", "thing"]

somearray += anotherarray # => ["some", "thing", "another", "thing"]

somearray = ["some", "thing"]
somearray.concat anotherarray # => ["some", "thing", "another", "thing"]

Try this, it will combine your arrays removing duplicates

array1 = ["foo", "bar"]
array2 = ["foo1", "bar1"]

array3 = array1|array2

http://www.ruby-doc.org/core/classes/Array.html

Further documentation look at "Set Union"

(array1 + array2).uniq

This way you get array1 elements first. You will get no duplicates.

Elaborating on @Pilcrow's answer the only suitable answer for huge arrays is concat (+) since is fast and does not allocate a new object to be garbage-collected when operating inside a loop.

Here's the benchmark:

require 'benchmark'

huge_ary_1 = Array.new(1_000_000) { rand(5_000_000..30_000_00) }

huge_ary_2 = Array.new(1_000_000) { rand(35_000_000..55_000_00) }

Benchmark.bm do |bm|
  p '-------------------CONCAT ----------------'
  bm.report { huge_ary_1.concat(huge_ary_2) }

  p '------------------- PUSH ----------------'
  bm.report { huge_ary_1.push(*huge_ary_2)  }
end

Results:

       user     system      total        real
"-------------------CONCAT ----------------"
  0.000000   0.000000   0.000000 (  0.009388)
"------------------- PUSH ----------------"
  example/array_concat_vs_push.rb:13:in `block (2 levels) in <main>': stack level too deep (SystemStackError)

As you can see using push throws an ERROR: stack level too deep (SystemStackError) when the arrays are big enough.

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

Just another way of doing it.

[somearray, anotherarray].flatten
=> ["some", "thing", "another", "thing"]

The question, essentially, is "how to concatenate arrays in Ruby". Naturally the answer is to use concat or + as mentioned in nearly every answer.

A natural extension to the question would be "how to perform row-wise concatenation of 2D arrays in Ruby". When I googled "ruby concatenate matrices", this SO question was the top result so I thought I would leave my answer to that (unasked but related) question here for posterity.

---

In some applications you might want to "concatenate" two 2D arrays row-wise. Something like,

[[a, b], | [[x],    [[a, b, x],
 [c, d]] |  [y]] =>  [c, d, y]]

This is something like "augmenting" a matrix. For example, I used this technique to create a single adjacency matrix to represent a graph out of a bunch of smaller matrices. Without this technique I would have had to iterate over the components in a way that could have been error prone or frustrating to think about. I might have had to do an each_with_index, for example. Instead I combined zip and flatten as follows,

# given two multi-dimensional arrays that you want to concatenate row-wise
m1 = [[:a, :b], [:c, :d]]
m2 = [[:x], [:y]]

m1m2 = m1.zip(m2).map(&:flatten)
# => [[:a, :b, :x], [:c, :d, :y]]

If the new data could be an array or a scalar, and you want to prevent the new data to be nested if it was an array, the splat operator is awesome! It returns a scalar for a scalar, and an unpacked list of arguments for an array.

1.9.3-p551 :020 > a = [1, 2]
 => [1, 2] 
1.9.3-p551 :021 > b = [3, 4]
 => [3, 4] 
1.9.3-p551 :022 > c = 5
 => 5 
1.9.3-p551 :023 > a.object_id
 => 6617020 
1.9.3-p551 :024 > a.push *b
 => [1, 2, 3, 4] 
1.9.3-p551 :025 > a.object_id
 => 6617020 
1.9.3-p551 :026 > a.push *c
 => [1, 2, 3, 4, 5] 
1.9.3-p551 :027 > a.object_id
 => 6617020 

I'm surprised nobody has mentioned reduce, which works well when you have an array of arrays:

lists = [["a", "b"], ["c", "d"]]
flatlist = lists.reduce(:+)  # ["a", "b", "c", "d"]

a = ['a', 'b']
b = ['c', 'd']
arr = [a, b].flatten

This won't remove dups, but

a|b

removes dups.

somearray = ["some", "thing"]

anotherarray = ["another", "thing"]

somearray + anotherarray

I find it easier to push or append arrays and then flatten them in place, like so:

somearray = ["some", "thing"]
anotherarray = ["another", "thing"]
somearray.push anotherarray # => ["some", "thing", ["another", "thing"]]
#or
somearray << anotherarray # => ["some", "thing", ["another", "thing"]]
somearray.flatten!  # => ["some", "thing", "another", "thing"]
somearray # => ["some", "thing", "another", "thing"]

somearray = ["some", "thing"]
anotherarray = ["another", "thing"]
somearray + anotherarray # => ["some", "thing", "another", "thing"]
somearray.concat anotherarray # => ["some", "thing", "another", "thing"]
somearray.push(anotherarray).flatten # => ["some", "thing", "another", "thing"]
somearray.push *anotherarray # => ["another", "thing", "another", "thing"]