Casting

Lua does few implicit conversions and many explicit ones.

From / to	How
any to string	tostring(x) or string.format("%s", x).
string to number	tonumber(s); returns nil on failure.
string in base N	tonumber(s, 16) for hex (or any base 2..36).
number to integer (truncate)	math.floor(x) or x // 1.
number to integer (ceil)	math.ceil(x).
to clean boolean	x and true or false.

Arithmetic between a number and a string that looks like a number works in 5.1 / 5.2 / 5.3 but raises in 5.4. The operator does not rely on this; use tonumber first.

print("3" + 1)               -- 4 in 5.3, error in 5.4
print(tonumber("3") + 1)     -- 4, portable

References

• number for the integer / float distinction.

• string for string.format formatting codes.