Using vector as matrix?

Question

How may I define a vector in C++11 such that its size is 4*5 so I can treat it like a matrix? (I mean using operator [] like the following)

mat[2][3];

Update: The following gives me error:

#include <memory>
class Test{
        std::vector<int> vect;
};

Error message:

implicit instantiation of undefined template 'std::__1::vector<int, std::__1::allocator<int> >'
    std::vector<int> vect;

nobody is downvoting every question of yours. If this would happen there are mechanisms in place to revert serial voting — 463035818_is_not_an_ai, Jun 24 '20 at 13:18
See https://stackoverflow.com/questions/32915825/nice-way-to-create-a-dynamic-2d-matrix-in-c-11 — auburg, Jun 24 '20 at 13:19
@idclev463035818 this happened to my last 7 questions, once I publish it within 2 seconds I get downvoting — , Jun 24 '20 at 13:20
fwiw, from your last 3 question none includes a mcve or any attempt from your side and the fourth and older question all have 0 downvotes... — 463035818_is_not_an_ai, Jun 24 '20 at 13:25

score 3 · Answer 1 · answered Jun 24 '20 at 13:18

3

You just need a nested vector, like this:

auto v = std::vector<std::vector<int>>(4, std::vector<int>(5));

and you can then index it like v[0][0].

answered Jun 24 '20 at 13:18

cigien

can't I create it as one single vector (for efficency) – Jun 24 '20 at 13:21
Yes, of course. Did you see the link that was posted in the comments? See the dupe target. – cigien Jun 24 '20 at 13:22
But I'm getting an error: implicit instantiation of undefined template 'std::__1::vector >' std::vector vect; when I type std::vector vect; – Jun 24 '20 at 13:27
1

Ok, then update the question with what you've tried, and show a minimal reproducible example. – cigien Jun 24 '20 at 13:30
Done that, take a look if possible – Jun 24 '20 at 13:41

score 0 · Answer 2 · answered Jun 27 '20 at 17:00

0

The error implicit instantiation of undefined template std::__1::vector ... means std::vector is unknown to the compiler.

You need to #include <vector>

answered Jun 27 '20 at 17:00

rustyx

2 Answers2