Re: metamethods... are __gt and _

On 22 September 2013 13:10, Jayanth Acharya <jayachar88@gmail.com> wrote:

On Sun, Sep 22, 2013 at 5:29 PM, liam mail <liam.list@googlemail.com> wrote:

On 22 September 2013 12:47, Jayanth Acharya <jayachar88@gmail.com> wrote:

In the list of published metamethods available in every document that I read, I seem to be unable to find __gt and __ge anywhere. Are those really no present, or am I missing something (like checking the right place, doc) ?

If those are really absent, then does it mean we cannot use metatables to support the > and >= operators for say table comparison ?

See [1] and the notes after each of the "le" and "lt" operators, the operators you mention come for free when you define other operators.

http://www.lua.org/manual/5.1/manual.html#2.8

Thanks Liam. Probably 5 minutes after posting that question, I found a mail written by Peter Hill way back 2004, asking this question, and also the answer, but I had completely missed the note in the manual.

So, if I understand this correctly, I cannot use the < and <= operators in Lua code while comparing 2 tables say 'a' and 'b', where the intent was to check (a < b). However I could rewrite the Lua code as (b > a). Right ?

> "the operators you mention come for free when you define other operators."

This does not mean you can not use the operators you have defined that would be bizarre.

Maybe a simple piece of code will help in understand what is happening.

$ cat ./ops.lua

local a, b = 1, 2

local c, d = a > b, a < b

$ luac-5.2.2 -l -l ./ops.lua

main <./ops.lua:0,0> (11 instructions at 0x8ad4b50)

0+ params, 4 slots, 1 upvalue, 4 locals, 2 constants, 0 functions

1 [1] LOADK 0 -1 ; 1

2 [1] LOADK 1 -2 ; 2

3 [2] LT 1 1 0

4 [2] JMP 0 1 ; to 6

5 [2] LOADBOOL 2 0 1

6 [2] LOADBOOL 2 1 0

7 [2] LT 1 0 1

8 [2] JMP 0 1 ; to 10

9 [2] LOADBOOL 3 0 1

10 [2] LOADBOOL 3 1 0

11 [2] RETURN 0 1

constants (2) for 0x8ad4b50:

1 1

2 2

locals (4) for 0x8ad4b50:

0 a 3 12

1 b 3 12

2 c 11 12

3 d 11 12

upvalues (1) for 0x8ad4b50:

0 _ENV 1 0

You can see that the third instruction[1] is LT and the seventh is also, Lua just swapped the stack slot indices to provide both operators.

[1] If you take the first instruction to be number 1