Splitting Array in C++

Question

I'm making a game and I have an array of floats that make a 3D models that I've placed in with opengl. What I want to do is separate the x,y&z coords and make them into a multi-dimensional array. I want to do this so I can adjust the y position in relation to the terrain. The for loop that I have placed in my init function is as follows:

    for (int x = 0; x < sizeof(desert_scene_plainVerts); x++) {
    if (((x + 3)%3) == 0) {
        //x coord
        terrainxPos[x/3] = desert_scene_plainVerts[x];
    }
    else if (((x + 1)%3) == 0) {
        //z coord
        terrainzPos[(x-2)/3] = desert_scene_plainVerts[x];
    }
    else{
        //y coord
        terrainzPos[(x-1)/3] = desert_scene_plainVerts[x];
    }
}

I am getting an error on this line:

terrainzPos[(x-1)/3] = desert_scene_plainVerts[x];

The error goes as follows:

Thread 1: EXC_BAD_ACCESS (code=2, address= 0x10 etc.)

Does anybody know what I am doing wrong.

I think perhaps (x-1)/3 is giving a floating point number instead of an integer. Try using ceil((x-1)/3) or (x-1)/3 >> 0 — kumarharsh
– kumarharsh, Commented Dec 21, 2013 at 20:03
@KumarHarsh No, it doesn't do floating point arithmetic: x is an int, and the C dialects won't convert any integers to floats unless one of the operands is a float. — cmaster - reinstate monica
– cmaster - reinstate monica, Commented Dec 21, 2013 at 20:35
@cmaster oh dear lord... yes ofcourse, you're right! too much JS has killed my C :( — kumarharsh
– kumarharsh, Commented Dec 22, 2013 at 6:04

Joe Z · Accepted Answer · 2013-12-21 20:24:30Z

1

Your logic looks ok, assuming you've declared terrainxPos[], terrainyPos[] and terrainzPos[] correctly. If those are vectors, then make sure you resize() them properly.

However, your loop may be clearer and easier to reason about written like this:

for (int x = 0, v = 0; x < sizeof(desert_scene_plainVerts); x += 3, v++) {
    terrainxPos[v] = desert_scene_plainVerts[x + 0];
    terrainyPos[v] = desert_scene_plainVerts[x + 1];
    terrainzPos[v] = desert_scene_plainVerts[x + 2];
}

edited Dec 21, 2013 at 20:24

answered Dec 21, 2013 at 20:07

Joe Z

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3 Comments

David Graovac Over a year ago

what do you mean by resize() them 'properly'?

Joe Z Over a year ago

@DavidGraovac: Resize them to be large enough hold all of the plainVerts. BTW, I just noticed what's likely the real problem: sizeof. I think you likely mean desert_scene_plainVerts.size() if it's a vector, or sizeof(desert_scene_plainVerts) / sizeof(desert_scene_plainVerts[0]) if it's an array.

Joe Z Over a year ago

@DavidGraovac: You need to resize it before the loop. ie. terrainxPos.resize( desert_scene_plainverts.size() / 3 );

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