Why = works when inserting string in vector<string> and then pushing in 2d string vector, but-CodePudding

I was creating a vector of strings of size 4x4 with all characters as dots i.e. I was creating:

....
....
....
....

And then I had to push this vector of strings in a vector of vector of strings like in the code below:

int main()
{
    vector<vector<string>> ans;
    int n=4;
    vector<string> matrix(n);
    for(int i=0;i<n;i  )
    {
        for(int j=0;j<n;j  )
        {
            matrix[i][j]='.';    //inserting '.' as characters
        }
    }
    
    ans.push_back(matrix);
    
     for(int i=0;i<n;i  )   //printing the just inserted matrix
    {
        for(int j=0;j<n;j  )
        {
           cout<<ans[0][i][j]<<" ";
        }
        cout<<endl;
    }
    
}

This was when I am printing it back, it gives garbage/nothing. But, I change the insertion matrix[i][j]='.'; to matrix[i] ='.';, it is working fine.

int main()
{
    vector<vector<string>> ans;
    int n=4;
    vector<string> matrix(n);
    for(int i=0;i<n;i  )
    {
        for(int j=0;j<n;j  )
        {
            matrix[i] ='.';    //inserting '.' by  =
        }
    }
    
    ans.push_back(matrix);
    
     for(int i=0;i<n;i  )   //printing the just inserted matrix
    {
        for(int j=0;j<n;j  )
        {
           cout<<ans[0][i][j]<<" ";   //works fine
        }
        cout<<endl;
    }
    
}

What is the cause of this behaviour?

CodePudding user response：

I don't see how this doesn't crash outright:

vector<string> matrix(n);  // THIS IS A VECTOR OF EMPTY STRINGS

for(int i=0;i<n;i  )
{
    for(int j=0;j<n;j  )
    {
        matrix[i][j]='.';    //inserting '.' as characters
    }
}

When you reference matrix[i], you get back an empty string because you haven't assigned anything to that string. Attempting to access any element in that string and assigning a char to it is going to ~~throw an exception~~ be undefined behavior.

There's an easy fix:

for(int i=0;i<n;i  )
{
    matrix[i] = string(n ,' ');
    for(int j=0;j<n;j  )
    {
        matrix[i][j]='.';    //inserting '.' as characters
    }
}

Or simply:

for(int i=0;i<n;i  )
{
    matrix[i] = string(n ,'.');
}

CodePudding user response：

This code:

int n=4;
vector<string> matrix(n);

Will create a vector filled with n default-constructed strings; i.e. a vector with 4 empty strings. So, in the following loop, you're accessing each string with an out-of-bound index (which is undefined behavior):

for (int i = 0; i < n; i  ) {
    for (int j = 0; j < n; j  ) {
        matrix[i][j] = '.';
               // ^ Out of bounds: matrix[i].size() == 0
    }
}

In your second case:

for (int i = 0; i < n; i  ) {
    for (int j = 0; j < n; j  ) {
        matrix[i]  = '.';
               // ^ You're appending '.' to the string.
               //   That's fine
    }
}

you're appending to the strings, which is fine.

If you want to have a vector of n strings, each containing n times character '.', you may just skip the loop and do:

int n = 4;
std::vector<std::string> matrix(n, std::string(n, '.'));

As a side note, you're not showing it but most probably you are:

using namespace std;

You shouldn't do it.