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;
}
}

Ask yourself, what is the size of the nested vectors in `v` (or another way: what is the size of `a`)? — NathanOliver, Mar 17 '20 at 13:50
BTW: https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h — Jabberwocky, Mar 17 '20 at 13:55
Not only that, but you'll be surprised to learn that the size of `v` turns out to be not `n` values, but twice as much. As far as segfaults go, there are so many different reasons for them, here, it's difficult to decide where to start. — Sam Varshavchik, Mar 17 '20 at 13:55

score 1 · Answer 1 · answered Mar 17 '20 at 13:56

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;

Unable to assign strings to a vector of vectors

1 Answers1