Rotating keys in a table

[megalukes]

Is there a function to rotate keys in Lua? Something like this:

Code: Select all

-- table.rotate(t,value) value accepting positive and negative numbers

table = {1, 2 ,3 ,4 ,5}
table.rotate(table,1) -- {2, 3, 4, 5, 1}

table2 = {1, 2 ,3 ,4 ,5}
table.rotate(table2,-1) -- {5, 1, 2, 3, 4}

I was so used with it in Ruby, but now I think it's awkward there isn't anything similar in Lua (I didn't find it anywhere, somebody must've done, somewhere).

[raidho36]

No, there isn't one. Lua tables are much more than simple arrays so it makes no sense to implement something with that narrow use case into core language, which also prides itself on being minimalistic. Depending on your use case, you can simply shift lookup index left or right and wrap it around, or implement such function yourself which is trivial. Note that shifting lookup key is preferable because actually rearranging values inside the table is computationally expensive by comparison.

[ivan]

Shifting by 1 in either direction is easy by using table.insert/remove, for example:

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-- left right by 1
t = {1, 2 ,3 ,4 ,5}
local last = table.remove(t)
table.insert(t, 1, last) -- {5, 1, 2, 3, 4 }

Note that your code overwrites the 'table' global so we have to be careful with that.
Unfortunately, shifting by more than 1 element becomes a pain and it's not super efficient.
Like Raidho said, the important question is: is there any good reason to shift all that data around?
An easier way would be write a special function for accessing shifted tables:

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t = {1, 2 ,3 ,4 ,5}

function get_value_by_shift(t, index, shift)
  index = (index + shift - 1)%#t + 1
  return t[index]
end

[raidho36]

Not particularly efficient but that will do the business.

Code: Select all

function table.rotate ( t, num )
	for i = 1, math.abs ( num ) do
		if num < 0 then
			table.insert ( t, 1, table.remove ( t ) )
		else
			table.insert ( t, table.remove ( t, 1 ) )
		end
	end
end

Paste this anywhere (actually atop) in your code and you'll be able to use it just as in your mock code in OP.

[airstruck]

Not much trouble if you don't need to do it in place.

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local function rotate (t, n)
    local len, out = #t, {}
    for i = 1, len do out[i] = t[1 + (i - 1 + n) % len] end
    return out
end

Writing a special shifted accessing function does sound like a good solution, though.

[megalukes]

Thank you everyone for the tips and snippets.