Lua shift element left and right in 2d array - arrays

Question

I have a 2d array : grid[x][y]
1 2 3 4 5 6 7 8
11 12 13 14 15 16 17 18
21 22 23 24 25 26 27 28
31 32 33 34 35 36 37 38
41 42 43 44 45 46 47 48
Example:
I would like to move the 3rd column down, the bottom element goes into the first row like this:
41 2 3 4 5 6 7 8
1 12 13 14 15 16 17 18
11 22 23 24 25 26 27 28
21 32 33 34 35 36 37 38
31 42 43 44 45 46 47 48
I used the following function to move the whole array left and right but in 2d array this obviously doesn't work, because you have to move the element to another array if shifting it:
function wrap( t, l )
-- change i=0 to move left and i=1 to right
for i = 1, l do
table.insert( t, 1, table.remove( t, #t ) )
end
end
I tried this function in Lua scratchpad and it doesn't work...I can't figure out the logic without losing the element.
function shift( t, direction )
for i=1,#t do
if(direction == "left") then
if(i == 1) then
tempElement = t[#t][6]
else
tempElement = t[i-1][6]
end
else
if(i == 7) then
tempElement = t[1][6]
else
tempElement = t[i+1][6]
end
end
table.insert( t[i], 6, tempElement )
table.remove( t[i], 12)
end
end
How can I shift the elements to another column but same index, so grid[5][1] goes in grid[4][1] and so on.

Answer 1

According to your comments, you are trying to shift items within a column, example:
{{11,12,13},
{21,22,23},
{31,32,33}}
to
{{31,12,13},
{11,22,23},
{21,32,33}}
The following code uses your wrap function:
g={{11,12,13},{21,22,23},{31,32,33}}
function shifItemWithinRow( array, shift )
shift = shift or 1 -- make second arg optional, defaults to 1
for i = 1, shift do
table.insert( array, 1, table.remove( array, #array ) )
end
end
function shifItemWithinColumn( grid, columnID, shiftCount )
shiftCount = shiftCount or 1 -- make second arg optional, defaults to 1
-- copy all items from g into new table, shifted:
local numRows = #grid
local newCol = {}
for i=1,numRows do --
local newI = i+shiftCount
if newI > numRows then newI = newI - numRows end
if newI < 1 then newI = numRows - newI end
newCol[newI] = grid[i][columnID]
end
-- copy all items into grid
for i=1,numRows do -- # each row
grid[i][columnID] = newCol[i]
end
end
function printGrid(g)
for i, t in ipairs(g) do
print('{' .. table.concat(t, ',') .. '}')
end
end
printGrid(g)
shifItemWithinColumn(g, 1) -- shift col 1 by +1
print()
printGrid(g)
print()
shifItemWithinColumn(g, 1, -1) -- shift col 1 by -1
printGrid(g)
This example shifts a column by +1 then by -1 (so final is same as start).