Get Lua table size in C

Question

How can I get a size of a Lua table in C?

static int lstage_build_polling_table (lua_State * L) {
    lua_settop(L, 1);
    luaL_checktype(L, 1, LUA_TTABLE);
    lua_objlen(L,1);
    int len = lua_tointeger(L,1);
    printf("%d\n",len);
    ...
}

My Lua Code:

local stages = {}
stages[1] = stage1
stages[2] = stage2
stages[3] = stage3

lstage.buildpollingtable(stages)

It´s printing 0 always. What am I doing wrong?

Dunaway

"Size of a table" is a strange term to use. You are getting the length of the sequence in a table (possibly 0), if the table has a sequence as defined in the manual, otherwise indeterminate. – Girovard

Answer 1

lua_objlen returns the length of the object, it doesn't push anything on the stack.

Even if it did push something on the stack your lua_tointeger call is using the index of the table and not whatever lua_objlen would have pushed on the stack (if it pushed anything in the first place, which it doesn't).

You want size_t len = lua_objlen(L,1); for lua 5.1.

Or size_t len = lua_rawlen(L,1); for lua 5.2.

Linea

Actually, the type is size_t. Also, since Lua 5.2 the name is lua_rawlen: lua.org/manual/5.2/manual.html#lua_rawlen – Concert

Note: these methods will only work for "lists", i.e. lua tables with [1..] consecutive indices. Details discussed on #2706293 – Swadeshi

Answer 2

In the code you gave, just replace lua_objlen(L,1) with lua_len(L,1).

lua_objlen and lua_rawlen return the length and do not leave it on the stack.

lua_len returns nothing and leaves the length on the stack; it also respect metamethods.

Claro

Note: these methods will only work for "lists", i.e. lua tables with [1..] consecutive indices. Details discussed on #2706293 – Swadeshi