how to store vector in a vector c++

Question

I want to store some int vectors into a vector, so firstly I initialize a vector type vector, then iterate for begin to end, and store int numbers into each subvectoer from 0 to 4 like below code:

std::vector<std::vector<int>  > v(12);

void fun_1(std::vector<int> a )
{
    for (int i = 0; i < 5; ++i)
    {
        a.push_back(i);
    }
}
int main()
{

    for (int i = 0; i < 12; ++i)
    {
        fun_1(v[i]);
    }

    cout<<v[0].at(2);
}

So after I done that, there is always a segmentation fault which I think is because subvector is still empty, they are not assigned, so I am wondering how can I modify this to reach my goal? Thank for any idea.

Just like you did. Put the `cout` inside of the function. – juanchopanza Jan 05 '17 at 22:00 — juanchopanza, Jan 05 '17 at 22:00
You need to pass the vector to the function by reference. – Jan 05 '17 at 22:00 — , Jan 05 '17 at 22:00
@latedeveloper thank you! problem solved – MMzztx Jan 05 '17 at 22:04 — MMzztx, Jan 05 '17 at 22:04

score 0 · Accepted Answer · answered Jan 05 '17 at 22:04

0

Function fun_1 accepts its input parameter by copy so it only modifies the local copy of the vector, not the original one passed in. So the original one remains empty. Once you attempt to access one of its (non-existent) elements, it throws an exception.

If you change fun_1 to accept its argument by reference:

void fun_1(std::vector<int>& a )

This should work just fine.

answered Jan 05 '17 at 22:04

Argenet

379
2
9

thank you so much! problem solved – MMzztx Jan 05 '17 at 22:06

score 0 · Answer 2 · answered Jan 05 '17 at 22:06

0

You send parameter to your function by copy , you should send this by reference:

void fun_1(std::vector<int> &a )
{
    for (int i = 0; i < 5; ++i)
    {
        a.push_back(i);
    }
}

answered Jan 05 '17 at 22:06

Morteza Jalambadani

2,190
6
21
35

score 0 · Answer 3 · answered Jan 05 '17 at 22:07

0

You are currently passing the vector by value in fun_1, so once you return, all of the changes you made to a are discarded. fun_1 should look something like

void fun_1(std::vector<int> &a )
{
   for (int i = 0; i < 5; ++i)
   {
      a.push_back(i);
   }
}

This will cause the call to fun_1 in main to actually manipulate v, instead of manipulating a copy of v.

answered Jan 05 '17 at 22:07

Trevor

366
2
11

Looks like @Morteza beat me to it. – Trevor Jan 05 '17 at 22:08

how to store vector in a vector c++

3 Answers3