Multi-Dimensional Vectors

[Wander]

Hey. I am trying to create a program that holds coordinates for a map. I've tried multi-dimensional arrays, but there are too many coordinates for the array to hold (9 million coordinates). I was referred to multi-dimensional vectors.

I have looked up tutorials and books and everything, but I just don't get it. Can someone please explain to me how to use vectors and explain what the heck its doing. :) Please? Thank you in advance.

Email: [e-mail]

-Tyler

[Bazzy]

A 2D vector is a vector of vectors eg: vector < vector < int > >
But I think that you'd need a single vector of pairs eg: vector < pair < int, int > > ( or of your own struct/class holding the coordinates values instead of std::pair ).

[Wander]

Does this single vector of pairs work the same way as a string array? string array[int][int]

[m4ster r0shi]

If you want an array like this -> T array[x][y] (where T is a data type and x,y are ints)

You can do it using vectors like this:

vector< vector < T > > array;

array.resize(x);
for (int i=0; i<array.size(); i++)
    array[i].resize(y);

//you can now use the syntax
//array[i][j] to access the elements 

More info on vectors -> http://cplusplus.com/reference/stl/vector/

[Galik]

Do you want your array to be your 'coordinate space' (one array element for every coordinate), or do you just want to store a list of coordinates that map onto a different space?

Eg:

string place[100][100]; // 100000 individual places addressable by coordinate

Or:

struct coord
{
    int x, y;
};

coord locations[100]; // store 100 locations 

[Wander]

@m4ster r0shi: where does 'j' come from?

@galik: I want it to be like the first choice. Multi-dimensional. Two numbers for one spot.

[m4ster r0shi]

Wander wrote:

@m4ster r0shi: where does 'j' come from?

Just put indices (indexes) of your choice there. I just wanted to show that after proper initialization you can use it exactly as you would use a multi-dimensional array.

[Wander]

@m4ster r0shi: oh okay, well what exactly is that snippet doing?

[m4ster r0shi]

It reserves memory for the individual elements. vector < vector < T > > actually means a vector of vectors of T.

The outer vector is used to hold the rows. Each inner vector is a row.

This -> array.resize(x); makes your array have x rows.

This loop here:

for (int i=0; i<array.size(); i++)
    array[i].resize(y);

makes every row have y elements.

So, now that you have x rows and y elements for every row, you have a total of x*y elements, which is exactly what you want.

[Wander]

oh okay, so then i just type: array[x][y] to access a certain coordinate?

[m4ster r0shi]

Yes. After you do the initialization, yes.

[Wander]

okay thanks. lemme go try it out.

[Galik]

@Wander
Yes!

@m4ster r0shi
You can use this:
std::vector<std::vector<int> > array(x, std::vector<int>(y));

Which initialises x by y elements.

[Wander]

@m4ster roshi: I'm getting tons of weird errors with this:

int main()
{
    int x, y;

    vector< vector<string> > mapCoords[x][y];

    cout << mapCoords.size() << endl;

    mapCoords.resize(x);

    for (int i=0; i<mapCoords.size(); i++)
        mapCoords[i].resize(y);

    cout << mapCoords.size() << endl;

    return 0;
}

It doesnt seem to like my member functions.

@galik: can you try to explain to me, your method so that way I can use yours and r0shis in the future?

[m4ster r0shi]

Galik wrote:

You can use this:
std::vector<std::vector<int> > array(x, std::vector<int>(y));

But... doesn't this make x unnecessary calls to the vector copy constructor? :/

[m4ster r0shi]

Wander wrote:

I'm getting tons of weird errors with this:

This -> vector< vector<string> > mapCoords[x][y];

Should be -> vector< vector<string> > mapCoords;

Also, don't forget to #include <vector> and <string>

[Wander]

@m4ster t0shi: WOAH!! it worked and all, but when it got to the for loop it said something about requesting the runtime to terminate in an unknown way and then it crashed.

[Athar]

m4ster r0shi wrote:

But... doesn't this make x unnecessary calls to the vector copy constructor? :/

Nope. It just makes x necessary calls to the copy constructor.

[m4ster r0shi]

Athar wrote:

Nope. It just makes x necessary calls to the copy constructor.

Why? You would use something like that if you already had a vector and wanted to create a vector of copies of that vector, right? Here, you don't care what the initial values will be, so all the element by element assignments it makes are unnecessary, no?

[m4ster r0shi]

@Wander:

Post your whole code.