Creating 2-D array from 1-D array

Ruby
1

Hello, I am relatively new to Ruby. I have an existing array:
a = [1 , 2 , 3 , 4 , 5 , 6]
and I want to create this 2-D array:
b = [[1 , 2][2 , 4][5 , 6]]
I am using the following code, which works, but seems somewhat clumsy:
b = Array.new
i = 0
k = 0
for item in a
if i == 0
a1 = item
i = 1
elsif i == 1
a2 = item
i = 0
b[k] = [a1, a2]
puts “b…#{k}, #{b[k]}”
k = k + 1
end
end
I would appreciate any advice to make this code look more elegant or
ruby-like. In particular, I suspect there is a way to avoid the
indices.
Thanks for the help.

2

Here are some suggestions, which should help reduce the use of indices
and local variables.

When constructing the array b, you don’t need to reference a position.
You can start with an empty array, and push the new pairs onto the end:

instead of b = Array.new, write b = []
instead of b[k] = [a1, a2], write b << [a1, a2]

That gets rid of k.

To get the pairs, I suggest you investigate ‘slice’.

e.g. a.slice(0, 2) returns a two-element array from the 0th position of
a: [1, 2]

You will also need to change your looping mechanism if you use slice,
but you should be able to get rid of a1 and a2.

3

Thanks a lot for the suggestions.
I also found a good tip at:

Now my code looks like:
a = [1 , 2 , 3 , 4 , 5 , 6]
b = []
(0…a.length-1).step(2) do |i|
b << [a[i], a[i+1]]
end
print b
There might also be a simple solution using slice, need to work on it.
Much better! Thanks again

4

You can also use Hash constructor:

a = [1 , 2 , 3 , 4 , 5 , 6]
siliced = Hash[*a].to_a

Sandro

5

On Sun, Nov 27, 2011 at 7:06 AM, J. Marshal [email protected] wrote:

   a1 = item

ruby-like. In particular, I suspect there is a way to avoid the
indices.
Thanks for the help.

–
Posted via http://www.ruby-forum.com/.

There are some very helpful methods in Enumerable

(http://rubydoc.info/stdlib/core/1.9.3/Enumerable).

In this case, check out each_slice.

ary = [1 , 2 , 3 , 4 , 5 , 6]
sliced = ary.each_slice 2

sliced # => #<Enumerator: [1, 2, 3, 4, 5, 6]:each_slice(2)>

You can think of it like the array you showed,

for instance you can iterate over its elements

or map them to new values or whatever.

sliced.each do |element|
element # => [1, 2], [3, 4], [5, 6]
end

sliced.map(&:reverse) # => [[2, 1], [4, 3], [6, 5]]

But if you really need an array, you can get it with to_a

sliced.to_a # => [[1, 2], [3, 4], [5, 6]]

6

Many thanks for the tips. You guys are great!

7

On Sun, Nov 27, 2011 at 4:50 PM, Sandro P.
[email protected] wrote:

You can also use Hash constructor:

a = [1 , 2 , 3 , 4 , 5 , 6]
siliced = Hash[*a].to_a

Beware that this will only work if the odd elements are unique.

Hash[*[1,2,1,3,1,4]].to_a
=> [[1, 4]]