2D Vectors

I have a usual vector that I have declared like this:

size_t size = 100000;
std::vector<int> Test(size);


Now I want to have a 2D vector instead of this.
Like an ordinary array it would look like for example:
Test[5][5];


But now if I want to create a 2D for the vector with the same size(100000).
How would the syntax look like for this ?

Code:

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std::vector< std::vector<int> > v;

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Also, you don't need to specify a size for vector. It's dynamic, so it will resize itself if needed.
But also need to do:

Code:

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v.push_back( std::vector() );

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To create the first dimension so you can access the second dimension:

Code:

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v[0].push_back(0);
int n = v[0][0]

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A 2D vector would be a vector of vectors (of int).

Code:

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std::vector<std::vector<int> > vec2d(100, std::vector<int>(1000));

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Typedefs can reduce some typing:

Code:

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typedef std::vector<int> IntVector
typedef std::vector<IntVector> IntVector2D
IntVector2D vec2d(100, IntVector(1000));

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Now vec2d holds 100 vectors each holding 1000 integers.

I think I understand. To be sure, if I take the example for:

std::vector<std::vector<int> > vec2d(100, std::vector<int>(1000));

Will I access the 2D vector like this:

vec2d[100][1000];

Yes, but again, with a vector, you do no have to specify a size! Unless you use it as a buffer, you never need to specify size.

more like vec2d[99][999];

Quote:

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Originally Posted by robwhit

vec2d[99][999];

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How do you figure that?
Sure, the highest index will be 99 and 999, but the declaration or definition should be vec2d[100][1000].

Yes I will change my code to not specify size.
If I dont specify size, I am not really sure how it should be written.

Should it be written like this if you dont specify size:
std::vector<std::vector<int> > vec2d((), std::vector<int>());

I already showed you:
http://cboard.cprogramming.com/showp...68&postcount=2

Yes ofcourse, now I understand and ofcourse when you access the 2D-array with size 100 and 1000 it should be written like this instead:
vec2d[99][999];
As the first position starts at 0.

This:

Code:

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std::vector<std::vector<int> > vec2d(100, std::vector<int>(1000));

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is equivalent to this:

Code:

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int arr2d[100][1000];

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You should specify the size if you know what it is before hand. If you don't know, then you can use push_back to add elements.

To access existing elements in a 2-D array or vector, you use:

Code:

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std::cout << vec2d[0][0];
std::cout << vec2d[99][999];
std::cout << arr2d[0][0];
std::cout << arr2d[99][999];

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But remember that you can only access the elements in the vector(s) if they exist. So if you don't specify a size you can't access those elements until you've push_back'd enough items.

Quote:

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Originally Posted by Daved

You should specify the size if you know what it is before hand. If you don't know, then you can use push_back to add elements.

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Kind of depends on the performance of your application. In typical programs, it won't matter, since it's lightning fast anyway,
You would get what some call a memory hog ;)

It also depends on what it is meant for. The dimensions can have a known fixed size and you may want to be able to start accessing things right away.

>>vec2d[99][999];
>How do you figure that?
the op asked how to access it, not how to declare/define it.

I see, so you were basing it on access and not declaration/definition.