Checking errors for adding tables with __add metamethod?
So, I'm trying to learn metamethods. I'm trying to make a script that can add arrays of numbers together, but will error if there's non-number elements in either table. Here's what I have:
01 | t1 = {1, 2, 3} |
02 | t2 = {4, 5, 6,"swag noob"} |
03 |
04 | mt = { |
05 | __add = function(t1, t2) |
06 | for _, v in pairs(t1) do --Check table 1 for non-number values |
07 | if not type(v) == "number" then |
08 | error("Numbers. You need to use numbers!") |
09 | return |
10 | end |
11 | end |
12 |
13 | for _, v in pairs(t2) do --Same for table 2 |
14 | if not type(v) == "number" then |
15 | error("Numbers. You need to use numbers!") |
Although "swag noob" is not a number value, the output won't error. It will add 1 to 4, 2 to 5, and 3 to 6. That's not what I want. When I take 6 out of the table and leave "swag noob", I get...
Workspace.Script:24: attempt to perform arithmetic on field '?' (a string value)
The errors are supposed to be caught in the beginning of the metamethod, but they aren't.
1 answer
This is an order-of-operations mistake.
If you check the Lua operator precedence article, you'll see that not is higher precedence than ==. What does that mean?
That means that not a == b is interpretted as (not a) == b -- which is NOT a ~= b.
You should use ~= when you want to see if things are different!
I would suggest a few code-cleanup-things. Repeating code is bad -- you should have a function that checks for non-number values. You can also take advantage of assert to eliminate the if entirely:
1 | function onlyContainsNumber(tab) |
2 | for _, el in pairs(tab) do |
3 | assert(type(el) == "number", "table must contain only numbers") |
4 | end |
5 | end |
6 |
7 | ..... |
8 | onlyContainsNumbers(t1) |
9 | onlyContainsNumbers(t2) |
I would argue that you usually don't want to add tables of different lengths -- because you'll lose some information, that is probably a mistake when it happens.
Even so, you can simplify your usage of add using math.min, which gives you the smaller of a few numbers (so you can get the shorter of the two lengths)
01 | local sum = {} |
02 | for i = 1, math.min(#t1, #t2) do |
03 | sum[i] = t1[i] + t2[i] |
04 | -- Usually `table.insert` describes that you are appending |
05 | -- so it is what you want. |
06 | -- In this case, using `[i] = ` better illustrates the relationship |
07 | -- between sum[i] and t1[i] and t2[i]. |
08 | -- (It's also much faster) |
09 | end |
10 | return sum |
You probably want to set the metatable of sum to be mt, too!