C++ setting array values in a class

Question

I am having trouble with what seems like a very simple concept. I have a class like such:

class Projectile
{
public:
    int count;
    int projectiles[1][3];

    Projectile();
    void newProjectile();

};

Projectile::Projectile()
{
    count = 0;
}

void Projectile::newProjectile()
{
    projectiles[0][0] = { 1, 2, 3 };
}

I am trying to set the values inside the projectiles array, and I must be doing this incorrectly. How can I go about dynamically adding a set of values into this property?

If there is just 1 row, what is the point of having a 2 dimensional array ? — Mahesh
– Mahesh, Commented Sep 15, 2011 at 5:51
The idea will be to add additional rows later. I just need to set a size initially.. i think. — grep
– grep, Commented Sep 15, 2011 at 5:56
Then vector<vector<int>> is what you want, as I mentioned. — Mahesh
– Mahesh, Commented Sep 15, 2011 at 5:58

Gilad Naor · Accepted Answer · 2011-09-15 05:42:26Z

3

projectiles[0][0] refers to a specific location in a two-dimensional array, its type is int

If you want to dynamically add items, then you can use std::vector<int> (see here)

answered Sep 15, 2011 at 5:42

Gilad Naor

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3 Comments

Gilad Naor Over a year ago

Please look at at link I gave. If it still doesn't help, I'll see what I can do... :-)

grep Over a year ago

Ah I see. Never worked with these. It is possible to store a vector as a value in a vector? Like a 2 dim vector? What i am trying to achieve is a 2 dimensional array like structure which holds an x and y coordinate for each member of the array. Is this possible with vectors?

Gilad Naor Over a year ago

Yes (Mahesh pointed this out in his answer). Although there might be better alternatives, such as std::vector< std::pair<int, int> > if you want to store x, y coordinates.

Mahesh · Accepted Answer · 2011-09-15 06:16:16Z

2

 projectiles[0][0] = { 1, 2, 3 };

This isn't correct. Initializer lists can only given at the point of declaration. You have to assign values independently to each location of the array elements. std::vector<std::vector> twoDimArray; is what you want.

struct foo{

    std::vector<std::vector<int> > twoDimArray;
    void create(int size){

        std::vector<int> oneDimArray(size);
        // vector as of now can just accommodate size number of elements. They aren't
        // assigned any values yet.


        twoDimArray.push_back(oneDimArray); // Copy it to the twoDimArray

        // Now if you wish to increase the size of each row, just push_back element to 
        //  that row. 
        twoDimArray[0].push_back(8);
    }
};

edited Sep 15, 2011 at 6:16

answered Sep 15, 2011 at 5:42

Mahesh

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11 Comments

grep Over a year ago

Could you provide an example of how to set this up :) ? it would be much appreciated.

Mahesh Over a year ago

@Headspin - See the update. But if you are new to std::vector, first play with a vector that can hold primitive types like int. Then it is much easier to understand of vector holding another vector.

grep Over a year ago

This is a great example. Just one last question. How would I store this as a property in my class?

Mahesh Over a year ago

Could you explain what exactly do you meant by "store this as a property" ?

grep Over a year ago

As in, how would I declare it as a public variable accessible to the rest of the class?

|

user466534 · Accepted Answer · 2011-09-15 05:45:58Z

1

try this

void Projectile::newProjectile()
{
    projectiles[0][0] = 1;
    projectiles[0][1]=2;
    projectiles[0][2]=3;

}

answered Sep 15, 2011 at 5:45

user466534

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