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black_darkness

resizing a 2d vector.

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Hello could you look at my objects and tell me what is wrong with them? The problem is I am trying to resize my 30x30 vector into a 60x60 vector. But it is wrong the first 30 vectors.

 

                wid_height[0] = 60;
                wid_height[1] = 60;

                mve.resize(wid_height[0], vector<Entity>(wid_height[1]));
                
//                init_map_entity_cleanup(mve,wid_height);
                init_map_home_entity(mve);
                init_map_home(cur_map,mve);

 

Here is image one. Size is 60 as I would like.

ex1.jpg

 

Here is image 2 showing all 60.

ex2.jpg

 

Here is image 3. There is a huge problem on the first 30 vectors. There vectors only have 30 vectors in them. So the first 30 vectors only have 30 vectors inside them instead of 60 like I need.

ex3.jpg

 

Now here is an image of the last 30 vectors 30-59. They have 60 vectors each so they are working properly.

 

ex4.png

 

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The problem here is that you resize the vector and fill the new spaces with 60-element vectors, but you don't resize the first 30 vectors.

 

So that is the problem.


Could I solve this by making the vector size 0 then resize it the way I did in the beginning?

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Could I solve this by making the vector size 0 then resize it the way I did in the beginning?
That depends. Doing this will give you a vector containing 60 60-element vectors, but those vectors will not contain the data that was previously in the structure. If you want to keep the old data, you just need to loop over the vectors already present and resize them, then resize the top-level vector as before.

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It looks like (correct me if I'm wrong), that you're not using a 2D array at all.

 

Instead, you're using a vector of vectors, which is not a 2D array (it's an array of arrays). C++ doesn't have a built-in 2D vector.

Trying to massage a 'array of arrays' to behave like a 2D array is unnecessary labor. There are two far superior options:

  1. Use a 3rd party class (or make one yourself) that actually is a 2D array.
  2. Treat a 1D array (like std::vector) as a 2D array.

I usually go with the second option because it's so easy. Memory in RAM is one dimensional. 2D arrays would just line up a extra large 1D array of memory, and do some math to convert 2D coordinates to 1D coordinates. The math is very easy:

 

std::vector<Type> myArray(width * height);
 
int index = (y * width) + x;
myArray[index] = value;

 

Easy as pie. Here's the math explanation.

 

Thanks for this. I will try to read all this and implement it in my game. I have had more problems with vectors of vectors than anything else.

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It looks like (correct me if I'm wrong), that you're not using a 2D array at all.

 

Instead, you're using a vector of vectors, which is not a 2D array (it's an array of arrays). C++ doesn't have a built-in 2D vector.

Trying to massage a 'array of arrays' to behave like a 2D array is unnecessary labor. There are two far superior options:

  1. Use a 3rd party class (or make one yourself) that actually is a 2D array.
  2. Treat a 1D array (like std::vector) as a 2D array.

I usually go with the second option because it's so easy. Memory in RAM is one dimensional. 2D arrays would just line up a extra large 1D array of memory, and do some math to convert 2D coordinates to 1D coordinates. The math is very easy:

 

std::vector<Type> myArray(width * height);
 
int index = (y * width) + x;
myArray[index] = value;

 

Easy as pie. Here's the math explanation.

I implemented it like you described and it is so much faster. I guess running 10s of thousands of vectors a second is slower than running less than a 100 vectors a second. Thanks.

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