You can change the way that Lua compares tables and userdata by using the "eq" metamethod (see §2.8).
The conversion rules of §2.2.1 do not apply to equality comparisons. Thus, "0"==0 evaluates to false, and t[0] and t["0"] denote different entries in a table.
The operator ~= is exactly the negation of equality (==).
The order operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, Lua tries to call the "lt" or the "le" metamethod (see §2.8). A comparison a > b is translated to b < a and a >= b is translated to b <= a.
2.5.3 - Logical Operators
The logical operators in Lua are and, or, and not. Like the control structures (see §2.4.4), all logical operators consider both false and nil as false and anything else as true.
The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation; that is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10
10 or error() --> 10
nil or "a" --> "a"
nil and 10 --> nil
false and error() --> false
false and nil --> false
false or nil --> nil
10 and 20 --> 20
(In this manual, --> indicates the result of the preceding expression.)
2.5.4 - Concatenation
The string concatenation operator in Lua is denoted by two dots ('..'). If both operands are strings or numbers, then they are converted to strings according to the rules mentioned in §2.2.1. Otherwise, the "concat" metamethod is called (see §2.8).
2.5.5 - The Length Operator
The length operator is denoted by the unary operator #. The length of a string is its number of bytes (that is, the usual meaning of string length when each character is one byte).
The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).
2.5.6 - Precedence
Operator precedence in Lua follows the table below, from lower to higher priority:
and
< > <= >= ~= ==
..
+ -
* / %
not # - (unary)
^
As usual, you can use parentheses to change the precedences of an expression. The concatenation ('..') and exponentiation ('^') operators are right associative. All other binary operators are left associative.
2. The language
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