index negative number in array in lua - arrays

Question

problem: I have an array called X. X[1:20]. I initialized to 0 to start with.
I want to pass a number to each element of array after adding 1 to it. So I read number and store in local variable z. sometimes "number" can be positive and negative.
However, array is only for positive (i.e. x [1....20] )
When "number" is negative lua gives error message that "attempt to perform arithmetic on field '?' (a nil value).
What should I do?
local x= {}
local number
local z
for i = 1, 20 do
x [i] = 0; -- array initialization
end
for y = 1, 5 do
z = number -- I am reading a "number" from hardware & it is a negative integer number
x[z] = x[z]+1 ;
end

Answer 1

It's because you are reading x[z], not just assigning it.
A simple fix would be to give it a default value.
Example:
x[z] = (x[z] or 0) + 1
This will make the code assume x[z] is equal to zero by default.

Answer 2

Your initialization code is only setting the positive values of the array X to 0. Simply modify the loop to initialize all the possible values of Z.
for i = -20, 20 do
x [i] = 0; -- array initialization
end
This assumes Z ranges from -20 to 20.
Negative index values work, but can sometimes lead to unexpected behavior when using the ipairs() function. The ipairs() function will only iterate through the values of the table starting at index 1 until the first uninitialized (nil) entry.
t = {}
for i = -2, 2 do t[i] = i end
print("all values")
for i = -2, 2 do
print(i, t[i])
end
print("positive integer values")
for k, v in ipairs(t) do
print(k, v)
end
Gives the following results:
all values
-2 -2
-1 -1
0 0
1 1
2 2
positive integer values
1 1
2 2

Answer 3

You can't index tables using negative numbers in Lua. You have got few possibilities:
Type cast the index to string and index with this string. It will make your table associative (you can index with any string):
z = tostring(number)
This is good if you do not have any boundaries for your indices
If you have limited interval of numbers (-20 to 20), you can do a transformation to the positive indices when accessing the values:
MIN_VALUE = -20
z = number - MIN_VALUE
This approach should give better performance, since the array is addressed directly.

Answer 4

You never set "number" to anything, therefore it gives you the error
attempt to perform arithmetic on field '?' (a nil value)
Notice the words "a nil value", because this is why this error occurred. So next time set "number" to something, or add an "or ___" when trying to do something like calculations, or setting variables.