Unable to assign strings to a vector of vectors

Question

I am trying to write a code in c++ while hashing. For that I am trying to push strings in a vector of vectors. But it is showing a segmentation fault.

#include <bits/stdc++.h>

using namespace std;

int main() {
int n,ind,inr;
cin >> n;
int j=0;    
vector<string> a;
vector<vector <string>> v(n);
for(int i=0;i<n;i++)
{
    v.push_back(a);
}                   
for(int i=0;i<n;i++)
{
    int m;
    string name;
    cin>>m;
    cin>>name;
    ind=m%2039;
        v[ind][j]=name;
    j=0;
    cout<<name<<endl;
}
}

Answer 1

For starters neither of sub-vectors of the vector v has elements.

In this declaration

vector<vector <string>> v(n);

there is declared a vector with n empty sub-vectors.

Then in this loop

for(int i=0;i<n;i++)
{
    v.push_back(a);
} 

you are appending one more n empty sub-vectors to the vector v . As a result the vector v has 2 * n empty sub-vectors.

In this statement

ind=m%2039;

there is used a magic number 2039 . The result of the expression

m%2039

can be greater than or equal to 2 * n.

So in this statement

v[ind][j]=name;

using the both indices (especially for empty sub-vectors of the vector v) results in undefined behavior.

Instead of a vector of sub-vectors you could use ths standard container std::map declared for example like

std::map<unsigned int, std::vector<std::string>> m;