ruby multi-dimensional array, move element

Question

Assuming I have an array or arrays:

arr = [["Foo1", "Bar1", "1", "W"],
       ["Foo2", "Bar2", "2", "X"],
       ["Foo3", "Bar3", "3", "Y"],
       ["Foo4", "Bar4", "4", "Z"],
       ["Foo5", "Bar5", "5", "A"]]

Is there a way I can move an individual array in the 2d array based on some criteria?

For example, if element 3 of the inner array is "4", then move it to the top?

So the above arr would then look like:

  [["Foo4", "Bar4", "4", "Z"],
   ["Foo1", "Bar1", "1", "W"],
   ["Foo2", "Bar2", "2", "X"],
   ["Foo3", "Bar3", "3", "Y"],
   ["Foo5", "Bar5", "5", "A"]]

I have tried things like to try and select the inner array:

arr.map {|row| row.select {|i| i[3] == "4"} }

tombeynon · Accepted Answer · 2018-03-06 13:58:59Z

2

You certainly can. Easiest way is to find the match first, and if it exists move it to the desired position.

index = arr.index{|i| i[2] == '4' }
arr.insert(0, arr.delete_at(index)) if index

Remember the indices in an array are 0 based, so you're looking at index 2 if you want to check the third element in the array.

answered Mar 6, 2018 at 13:58

tombeynon

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4 Comments

user3437721 Over a year ago

Thanks for this. I may have been overthinking it

tombeynon Over a year ago

We've all been there!

Stefan Over a year ago

You can use arr.unshift(element) instead of arr.insert(0, element)

tombeynon Over a year ago

You can and that would be better if moving it to the start of the array is required. The OP asked about moving it in general though so I thought I'd show that as it was more flexible.

Aleksei Matiushkin · Accepted Answer · 2018-03-06 14:21:13Z

0

Most generic way would be probably using Array#sort:

▶ criteria = ->(arr) { arr[2] == "4" }
#⇒ #<Proc:0x005570424c2568@(pry):8 (lambda)>
▶ arr.sort do |e1, e2|
▷   criteria.(e1) ? -1 : criteria.(e2) ? 1 : e1 <=> e2
▷ end
#⇒ [["Foo4", "Bar4", "4", "Z"],
#   ["Foo1", "Bar1", "1", "W"],
#   ["Foo2", "Bar2", "2", "X"],
#   ["Foo3", "Bar3", "3", "Y"],
#   ["Foo5", "Bar5", "5", "A"]]

That way you might update criteria as you want and everything will still work.

Or, more explicitly preserving the original order (credits to @StefanPochmann):

arr.sort_by.with_index { |e, i| [criteria[e] ? 0 : 1, i] }

edited Mar 6, 2018 at 14:21

answered Mar 6, 2018 at 14:06

Aleksei Matiushkin

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2 Comments

Stefan Pochmann Over a year ago

The second way has the indexes still in the result. And how about sort_by? That even allows adding indexes as an afterthought that doesn't appear in the result: arr.sort_by.with_index { |e, i| [criteria[e] ? 0 : 1, i] }

Aleksei Matiushkin Over a year ago

@StefanPochmann indeed, thanks, updated, the original version was indeed a complete crap.

Stefan Pochmann · Accepted Answer · 2018-03-06 14:25:38Z

0

Just another way, also works if there's more than one inner array to be moved:

arr.partition { |e| e[2] == '4' }.sum([])

answered Mar 6, 2018 at 14:25

Stefan Pochmann

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