Accessing return values of Lua-functions in C++ and vice versa

Question

I have some functions in Lua and some functions in C++ and I have managed to get the correct results from my addition, power and division. The only Problem I have is with the factorial, because I am not able to pass the correct number to it, which should be the result of the division.

in Lua:

function powLua(a, n)
   b = 1
   for i=1,n do b = b * a end
   return b
end

function divisionLua(a, b)
   c = b/a
   return c
end

aLua = additionLua(2, 3)
bLua = additionLua(1, 3)

print("Result of additionLua is a = "..aLua.."\nResult of additionLua is b = "..bLua.."\n")

fac = factorialLua(divLua) <---- this is the part that doesn't work

print("factorial of divLua is "..fac.."\n")

in C++:

int addition(lua_State* L)
{
    int x = lua_tonumber(L, 1);
    int y = lua_tonumber(L, 2);

    lua_pushnumber(L, x + y);

    return 1;
}

int fac_calc(int x)
{
    if (x <= 1)
        return 1;

    return x * fac_calc(x-1);
}

int factorial(lua_State* L)
{   
    int fac = lua_tonumber(L, 1);

    lua_pushnumber(L, fac_calc(fac));

    return 1;
}

int power(lua_State* L, int a, int n)
{
    int b;

    lua_getglobal(L, "powLua");
    lua_pushnumber(L, a);
    lua_pushnumber(L, n);

    if (lua_pcall(L, 2, 1, 0) != 0)
        printf("error running function 'powLua': %s", lua_tostring(L, -1));

    if (!lua_isnumber(L, -1))
        printf("function `powLua' must return a number\n");
    b = lua_tonumber(L, -1);
    lua_pop(L, 1);  /* pop returned value */
    return b;
}

int division(lua_State* L, int a, int b)
{
    int c;

    lua_getglobal(L, "divisionLua");
    lua_pushnumber(L, a);
    lua_pushnumber(L, b);

    lua_pcall(L, 2, 1, 0);

    c = lua_tonumber(L, -1);
    lua_pop(L, -1);

    return c;
}

int main(int argc, char* argv[])
{
    lua_State* L = luaL_newstate();

    luaL_openlibs(L); 

    lua_pushcfunction(L, addition); 
    lua_setglobal(L, "additionLua"); 

    lua_pushcfunction(L, factorial);
    lua_setglobal(L, "factorialLua");

    luaL_dofile(L, "test01.lua");

    lua_getglobal(L, "aLua");
    int a = lua_tonumber(L, -1);
    lua_pop(L, -1);

    lua_getglobal(L, "bLua");
    int b = lua_tonumber(L, -1);
    lua_pop(L, -1);

    int pwr = power(L, a, b);

    int div = division(L, a, pwr);
    lua_pushnumber(L, div);
    lua_setglobal(L, "divLua");

    cout << "Result of a^b in powLua is pwr: " << a << "^" << b << " = " << pwr << endl;

    cout << "Result of pwr/a in divisionLua is: " << pwr << "/" << a << " = " << div << endl;

    lua_close(L);
}

my Output is this:

Result of additionLua is a = 5

Result of additionLua is b = 4

factorial of divLua is 1

Result of a^b in powLua is pwr: 5^4 = 625

Result of pwr/a in divisionLua is: 625/5 = 125

I guess I have to push the result of the division onto the stack before I open the "test01.lua" file, but it didn't work either.

I have also succesfully testet the factorial function by giving it a direct value. For example factorialLua(5) gives me an output of 120.

Does anybody know where my mistake is?

Edit:

Setting "divLua" before running the script with a testvalue like this:

int div = 5;
lua_pushnumber(L, div);
lua_setglobal(L, "divLua");
...
...
luaL_dofile(L, "test01.lua");

leads to the correct result of factorialLua(divLua), however when I set "divLua" as a value of division(), like this:

int div = division(L, 5, 30); //div should be 6 now
lua_pushnumber(L, div);
lua_setglobal(L, "divLua");
...
...
luaL_dofile(L, "test01.lua");

the result of factorialLua(divLua) is 1 and not 720 how it should be.

The reason must be that division() has to be called after the luaL_dofile, but lua_setglobal(L, "divLua") has to be before of it. So I need to let "div" know the result of a calculation which will be done later.

Any suggestions?

Answer 1

There are a couple of issues above:

• You're trying to compute factorialLua(divLua) in the script before divLua is set from C++.
• You're calling lua_pop with a negative index but it expects a positive integer.
• Assuming the first two points are addressed, you're trying to compute 125! which is going to be a really large number.

Note, the reason you're currently getting 1 for factorialLua is because lua_tonumber returns 0 if the given stack index isn't actually a number. Since divLua is still nil when this was called, you are effectively computing factorialLua(0).

The two obvious ways to go about fixing this:

1. Do a lua_setglobal(L, "divLua") before you actually execute your script.
2. Move your factorial computation into C++ land to a point where divLua is actually defined.