Lua PANIC error - c++

Question

I am creating a Sudoku solver in C++ while implementing Lua scripting for the actual solving of the puzzle. I have created the following Lua code, but get a
PANIC: unprotected error in call to Lua API (attempt to call a nil value)
error whenever my C++ code reaches the first instance of lua_call.
When compiling the code in SciTE, I get the following error:
lua: SudokuSolver.lua:99: 'end' expected (to close 'for' at line 61)
near ''
Adding three 'end's to the end of the function that has the for loop at line 61 clears that error, but causes errors in the C++ program. Can someone please look at my Lua and see if there's any syntax errors or other issues which may be causing this? Thank you
CODE
-- Table Declaration
SudokuGrid = {}
function RecieveGrid ( _Pos, _Value )
-- Recives the cell value at _Pos position from C++
SudokuGrid[_Pos] = _Value
end
function SolveSudoku ( _Pos )
-- Recursive function which solves the sudoku puzzle
local iNewValue = 1
-- If Position is 82+, all cells are solved
if( _Pos >= 82 ) then
return true
end
-- If Position already has a value
if( SudokuGrid[_Pos] ~= 0) then
return SolveSudoku( _Pos + 1 )
else
while(true) do
SudokuGrid[_Pos] = iNewValue
iNewValue = iNewValue + 1
-- If the new value of the cell is higher than 9 its not valid
if( SudokuGrid[_Pos] > 9 ) then
--Reset value
SudokuGrid[_Pos] = 0
return false
end
if( IsValid( _Pos ) and SolveSudoku( _Pos + 1 ) ) then
return true
end
end
end
end
function IsValid ( _Pos )
-- Calculate Column and Row in Grid
x = _Pos % 9
if( x == 0 ) then
x = 9
end
y = math.ceil(_Pos / 9)
-- Check Rows
for i=1, 9 do
CheckVal = ((y - 1) * 9) + i
if( CheckVal == _Pos ) then
-- Do nothing
else if ( SudokuGrid[_Pos] == SudokuGrid[CheckVal]and SudokuGrid[_Pos] ~= 0 ) then
return false
else
-- Do nothing
end
end
-- Check Columns
for i=1, 9 do
CheckVal = ((i - 1) * 9) + x
if( CheckVal == _Pos ) then
-- Do nothing
else if ( SudokuGrid[_Pos] == SudokuGrid[CheckVal] and SudokuGrid[_Pos] ~= 0 ) then
return false
else
-- Do nothing
end
end
-- Check 3X3 Grid
SquareCol = math.ceil(x/3)
SquareRow = math.ceil(y/3)
StartVal = (SquareCol - 1) * 27 + (SquareRow * 3) -2
for j=0, 2 do
for i=0, 2 do
CheckVal = StartVal + i
if( CheckVal == _Pos ) then
-- Do nothing
else if ( SudokuGrid[_Pos] == SudokuGrid[CheckVal] and SudokuGrid[_Pos] ~= 0 ) then
return false
else
-- Do nothing
end
end
StartVal = StartVal + 9
end
return true
end
function SendGrid ( _Pos )
-- Sends the value at _Pos to C++
return SudokuGrid[_Pos]
end

Answer 1

The syntax error is in all lines containing else if:
else if ( SudokuGrid[_Pos] == SudokuGrid[CheckVal]and SudokuGrid[_Pos] ~= 0 ) then
In Lua, use elseif instead. Using else if would need more closing end.
elseif SudokuGrid[_Pos] == SudokuGrid[CheckVal] and SudokuGrid[_Pos] ~= 0 then