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get a digit of an enum

Cplusc (457)
hello forum
I have a enum data type which is 

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enum Type {

	D1Q3,
	D2Q9,
	D3Q19,
	D3Q27

};


The first number is the dimension of my problem and the second number is another factor which I need to get for future calculation. I don't know how the get any arbitrary digit out of an enum datatype. I tried to convert enum to string and then get the second digit of string using stringify. by the way I want to get any digit of enum without converting to string. can anyone help me with this.
thanks in advance.
Last edited on
Peter87 (11235)
Perhaps something like:

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int get_dimension(Type t)
{
	switch (t)
	{
	case D1Q3:
		return 1;
	case D2Q9:
		return 2;
	case D3Q19:
	case D3Q27:
		return 3;
	}
	return -1;
}
Peter87 (11235)
Alternatively you could encode these two numbers into the underlying value of each enumerator so that you can easily extract them.

Something like:

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const int max_type_factor = 1000;

#define TYPE_ENUMERATOR(d, q) D ## d ## Q ## q = (d) * max_type_factor + (q),

enum Type
{
	TYPE_ENUMERATOR(1, 3)
	TYPE_ENUMERATOR(2, 9)
	TYPE_ENUMERATOR(3, 19)
	TYPE_ENUMERATOR(3, 27)
};

int get_dimension(Type t)
{
	return t / max_type_factor;
}

int get_factor(Type t)
{
	return t % max_type_factor;
}
Last edited on
deleted account xyzzy (5768)
Unscoped enums make it easy to get the underlying numeric value of the enumerated constant, they implicitly convert to integral values:
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#include <iostream>

enum Type { D1Q3, D2Q9, D3Q19, D3Q27 };

int main()
{
   std::cout << D1Q3 << ' ' << D3Q27 << '\n';
}
0 3

https://www.learncpp.com/cpp-tutorial/unscoped-enumeration-input-and-output/

Scoped enums take a bit of a nudge to suss out the underlying value:
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#include <iostream>

enum class Type { D1Q3, D2Q9, D3Q19, D3Q27 };

int main()
{
   std::cout << static_cast<int>(Type::D1Q3) << ' '
             << static_cast<int>(Type::D3Q27) << '\n';
}
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https://www.learncpp.com/cpp-tutorial/scoped-enumerations-enum-classes/
Cplusc (457)
Thanks for your answer. I'm looking for something just for example take D2Q9 and return the dimension=2 and number node=9. I thought about first option before but you need to repeat it twice to get dimension and node. maybe I can use pair for the fist option.
Last edited on
Peter87 (11235)
Cplusc wrote:
maybe I can use pair for the fist option

Or define your own struct type for more descriptive names than first and second.
Cplusc (457)
thanks for the help.
jonnin (11437)
basically you are asking for a variable name via code.
you don't normally do this:

int ab12hoopajoo{42};
cout << //what now, I want to peel the number 12 from the variable name here, how?

you can do this via preprocessor macros, but its a horrible solution and usually an indication of a bad idea.
instead, consider a map or similar structure that ties the name as a string and the value together.
keskiverto (10402)
Yes. Enum is named values, similar to constant variables.
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enum Type { D1Q3, D2Q9, D3Q19, D3Q27 };
// or
constexpr int D1Q3  = 0;
constexpr int D2Q9  = 1;
constexpr int D3Q19 = 2;
constexpr int D3Q27 = 3;

In both cases you can refer to literal constant 2 with name D3Q19 in your code.

The names in the code are only for your convenience; they do not exist in running executable.
kbw (9488)
This is exactly the sort of thing that wise_enum helps with:
https://github.com/quicknir/wise_enum
helios (17574)
I really do not understand why the language is still missing simple enum-string conversion. It's so much simpler to solve this in the compiler than in a library.
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