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Asked: 2026-06-15T02:08:08+00:00

I’m having problems copying a multidimensional vector, I’ve tried many things but this is

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I’m having problems copying a multidimensional vector, I’ve tried many things but this is the last one:

vector < vector < int > > a;
vector < vector < int > > b;
a.resize(10);
b.resize(10);
a[0][0] = 123;
copy( a.begin(), a.end(), back_inserter(b) );
cout << b[0][0];

I’m trying to do a recursive loop that counts all possible routes in a grid within 10 moves. I’m trying to create a vector called current_path which would hold the current path for each recursion, when the current_path has 10 moves, It will copy the data from current_path to all_paths.

The grid goes like this:

0  1  2 3
4  5  6 7
8  9  10 11
12 13 14 15

You can only move to a square you touch so from 0 you can move to 1, 4 and 5. And from 1 to 3, 4, 5, 6 etc.

The main idea is to copy the current_path to the next function call (recursive) so it would hold the curren_path up to that point, doing that until it’s full (10 steps). After it’s copied from current_path to all_paths I suppose I have to delete the current_path?

I know how to efficiently calculate all steps but I’m having trouble copying the current_path and propably and how do I add the current_path to all_paths when I’m at 10 steps?

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1 Answer

  1. Editorial Team

    There are a few problems with your code. By the end of line 4, you have two vectors that each contain 10 empty vectors. You could visualize it like this:

    a = {{}, {}, {}, {}, {}, {}, {}, {}, {}, {}}
b = {{}, {}, {}, {}, {}, {}, {}, {}, {}, {}}

    Those inner vectors still do not have any elements so when you try and set a[0][0] to 123, you’re accessing an element that doesn’t exist, invoking undefined behaviour.

    If that had worked, your use of std::copy would simply copy each of the vectors from a and pushed it to the back of b. Since b already has 10 elements, it would now have 20 elements.

    Then you try to output b[0][0] which doesn’t exist just as much as a[0][0] didn’t either.

    The solution here is to simply use the copy assignment operator defined by std::vector:

    vector<vector<int>> a = {{1, 2, 3}, {4, 5}};
vector<vector<int>> b;
b = a;
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