Incrementing a hex value for a function parameter in a loop instead of hardcoding it?

**Striph** · 10-04-2012

I have a constructor that takes 9 arguments, instead of harcoding each value in an array equal to each argument I was hoping I could run through a loop and set each value of the array column/row equal to the paramater name plus a counter. I named the arguments a, b, c, d, e, f, g, h, and i to try to achieve this.

Code:

Matrix::Matrix(float a, float b, float c, float d, float e, float f, float g, float h, float i)
{
    int counter = 0;

    for(int i = 0; i < MAX_SIZE; i++)
        for(int j = 0; j < MAX_SIZE; j++)
            mVals[i][j] = 0x61 + counter++;

    //Don't want to hardcore 9 of these
    //mVals[0][0] = a; 
    //mVals[0][1] = b;
}

This is what I was hoping to do, but instead its giving me the decimal value of 0x61 which is 97 + 1, +2, +3...etc.

I've tried a char() cast and I can't get anything to work. MAX_SIZE is 3, using this for a 3x3 matrix. Is this not possible since its a variable name?

**oogabooga** · 10-04-2012

Your idea would work in a scripting language, but a C++ (or C) program does not know the variable names after it is compiled. Why don't you just pass in an array?

EDIT: Actually, another idea is to use a variable argument list. See the cstdarg.h header.

EDIT2: Now that I think about it, using a variable argument list is not a very good idea as it forgoes any type checking.

Since there's only 9 values, you may as well just bite the bullet and assign them all individually.

**phantomotap** · 10-04-2012

O_o

This is C++ so I could show you a hundred different ways to emulate the look, but you'd still be doing a lot of work.

So yeah, this is one of those times when just doing the work for what you need done is for the best.

Soma

**kmdv** · 10-07-2012

If you are programming under C++11, you can take advantage of initializer lists:

Code:

class Matrix
{
public:

    Matrix(std::initializer_list<float> list) noexcept
    {
        auto it = list.begin();
        for (std::size_t row = 0; row < 3; ++row)
        {
            for (std::size_t column = 0; column < 3; ++column)
            {
                if (it != list.end())
                {
                    mElements[row][column] = *it;
                    ++it;
                }
                else
                {
                    mElements[row][column] = 0;
                }
            }
        }
    }

private:

    float mElements[3][3];
};

int main()
{
    Matrix matrix{100, 200, 300, 400, 500, 600, 700};
    ...
    return 0;
}

This also enables you to use a variable-length argument list, which fills the unspecified elements with zeros (you can make the initializer_lists nest).
With this approach you could even make your Matrix a template and get rid of the hardcoded dimensions.

The downside is that in the case of matrix math, the initializer_list solution may turn out not to be performance-wise.

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