c++ class constructor for array

Question

I am writing a Matrix2D class. At the beginning I was using constructor as folows,

My code:

Matrix2D(float a,float b, float c,float d)
{
    a_=a;
    ....
}

However, I have just realized that it would be a lot better if I could use multi dimensional array [2][2]. That's where the problem lies, How do I write constructor for array ?

class Matrix
{
    float matrix[2][2];
    public:
    Matrix2D(float a,float b,float c, float d)
    {
        matrix[2][2]={a,b,c,d} // not valid
    }
}

Just to let you know, I don't ask for a complete code. I just need someone to put me on a right track.

BTW, the class name and constructor name need to match.

Ben Voigt
– Ben Voigt

2012-10-01 19:40:11 +00:00
Commented Oct 1, 2012 at 19:40 — Ben Voigt
– Ben Voigt, Commented Oct 1, 2012 at 19:40
thx, its just typo, they match in a source code

Tomas
– Tomas

2012-10-02 07:12:50 +00:00
Commented Oct 2, 2012 at 7:12 — Tomas
– Tomas, Commented Oct 2, 2012 at 7:12

Luchian Grigore · Accepted Answer · 2012-10-01 19:27:38Z

4

For C++11 you can do:

Matrix(float a,float b,float c, float d) :
   matrix{{a,b},{c,d}}
{
}

There's no clean alternative for C++03.

edited Oct 1, 2012 at 19:27

answered Oct 1, 2012 at 19:21

Luchian Grigore

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3 Comments

Jesse Good Over a year ago

You need to take off the parentheses, i.e. matrix{{a,b},{c,d}} because it's an array.

Jesse Good Over a year ago

LWS uses gcc 4.7.2

Tomas Over a year ago

ok, I have tried it and apparently vs 2010 doesnt support this feature, so I will have to go with " matrix[0][0]=a; " and so on.. Thx for support

Pete Becker · Accepted Answer · 2012-10-01 19:11:02Z

2

matrix[0][0] = a; // initialize one element

and so on.

answered Oct 1, 2012 at 19:11

Pete Becker

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1 Comment

Tomas Over a year ago

Thx for the answer. That's the way I'll do it

iruel · Accepted Answer · 2012-10-01 19:29:07Z

1

matrix[0][0] = value you want to matrix [n][n] = value you want but count up in a loop so the matrix can be dynamic in size or you can reuse your code later.

for(int ii(0); ii < first dimension size; ++ii)
{
   for(int ll(0); ii < second dimension size; ++ll)
   {
     matrix[ii][ll] = value you want;
   }
}

this will make your code more extensible and more useful outside of this application and maybe it's not useful or maybe it is.

answered Oct 1, 2012 at 19:29

iruel

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Comments

Bartlomiej Lewandowski · Accepted Answer · 2012-10-01 19:15:18Z

0

If it will be a matrix 2X2, then you can pass a float array and then loop through it.

for example

for(int x = 0;x<4;x++)
{
    matrix[0][x] = myarray[x];
}

answered Oct 1, 2012 at 19:15

Bartlomiej Lewandowski

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7 Comments

Bartlomiej Lewandowski Over a year ago

@LuchianGrigore how is that invalid?

Code-Apprentice Over a year ago

@coolbartek If matrix is declared as float matrix[2][2] then matrix[0][3] is obviously invalid.

Bartlomiej Lewandowski Over a year ago

it looks fine for me, an two-dimensional array is actually a normal array, where you can access the elements by [0][x]. An array[x][y] is the same as an array[0][x*y]

Ben Voigt Over a year ago

@coolbartek: Don't you mean that array[y][x] is the same as array[0][x+y*width]?

Ben Voigt Over a year ago

@Luchian, are you forgetting that a[i] is the same as *(a + i) for built-in types?

|

Ben Voigt · Accepted Answer · 2012-10-01 19:34:18Z

0

Luchian's version is best if you have a C++11 compiler. Here's one that works for all versions of C++:

struct matrix_holder { float matrix[2][2]; };

class Matrix : matrix_holder
{
    static matrix_holder pack(float a,float b,float c, float d)
    {
        matrix_holder h = { {{a, b}, {c, d}} };
        return h;
    }

public:
    Matrix(float a,float b,float c, float d) : matrix_holder(pack(a,b,c,d))
    {
    }
};

The optimizer will inline away the helper.

edited Oct 1, 2012 at 19:34

answered Oct 1, 2012 at 19:27

Ben Voigt

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