2

In my C++ program, I need to know if a Lua variable is an integer number or a floating-point number. The C API provides lua_isnumber() but this function does not distinguish between int/float/double.

So far I have worked around this by using modf():

if (lua_isnumber(luaCtx, -1)) // int / unsigned int / float:
{
    luaVarName = lua_tostring(luaCtx, -2);
    double n = static_cast<double>(lua_tonumber(luaCtx, -1));

    // Figure out if int or float:
    double fractPart, intPart;
    fractPart = modf(n, &intPart);

    if (fractPart != 0.0)
    {
        luaVarType = ScriptVar::TypeTag::Float;
        luaVarData.asFloat = static_cast<float>(n);
    }
    else
    {
        luaVarType = ScriptVar::TypeTag::Integer;
        luaVarData.asInteger = static_cast<int>(n);
    }
}

Does the Lua API provide a way to infer the variable type more precisely?

glampert
  • 4,371
  • 2
  • 23
  • 49
  • 3
    Nope. You can cast it to an unsigned type using `lua_tounsigned` or `lua_checkunsigned`. Otherwise you have to cast it like you have. – Rapptz Aug 08 '14 at 04:33
  • 2
    Lua 5.3 will have both integer and floating point types. – Simple Aug 08 '14 at 05:23

1 Answers1

6
double n = lua_tonumber(L, -1);
if (n == (int)n) {
    // n is an int
} else {
    // n is a double
}

What this code does is just checking if n has any decimals or not. If n is 1.5, then casting it to int ((int)n) will floor the value to 1, so:

1.5 == 1 is false, n is a double

But if n is lets say 4:

4 == 4 is true, n is a int

This works because to lua, the only numeric number that exist is double. So when converting a number from lua to C, we can choose to use int if the number is a integer(whole number).

Community
  • 1
  • 1
guest posting no work good
  • 76
  • 1
  • 2
    This answer is up for deletion. Its hard to tell if this is a quality answer. Could you explain it a bit? – jww Aug 08 '14 at 05:20
  • OK, so no extra info about numbers, you have to check it yourself. Easy enough with a type cast / `floor()`, etc. – glampert Aug 08 '14 at 15:30