How safe is comparing numbers in lua with equality operator?
In my engine I have a Lua VM for scripting. In the scripts, I write things like:
stage = stage + 1 if (stage == 5) then ... end
objnum = tonumber("5") if (stage == objnum)
According to the Lua sources, Lua uses a simple equality operator when comparing doubles, the internal number type it uses.
I am aware of precision problems when dealing with floating point values, so I want to know if the comparison is safe, that is, will there be any problems with simply comparing these numbers using Lua's default '==' operation? If so, are there any countermeasures I can employ to make sure 1+2 always compares as equal to 3? Will converting the values to strings work?
I can employ to make sure 1+2 always compares as equal to 3?
You needn't worry. The number type in Lua is double, which can hold many more integers exactly than a `long int``.
You may be better off by converting to string and then comparing the results if you only care about equality in some cases. For example:
> print(21, 0.07*300, 21 == 0.07*300, tostring(21) == tostring(0.07*300)) 21 21 false true
I learned this hard way when I gave my students an assignment with these numbers (0.07 and 300) and asked them to implement a unit test that then miserably failed complaining that 21 is not equal 21 (it was comparing actual numbers, but displaying stringified values). It was a good reason for us to have a discussion about comparing floating point values.