Question

I’m struggling hard with this. Basically, I have to read a number first:

int n;
scanf("%d", &n);

Then I have to read N intervals to a vector:

vector< vector<int> > intervals;
int a, b;
for (int i = 0; i < n; i++) {
  scanf("%d %d", &a, &b);
  intervals.at(i).at(0) = a;
  intervals.at(i).at(1) = b;
}

Using GDB debugger, I get this:

3
1 4
terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check

Program received signal SIGABRT, Aborted.
0x00007ffff7260c35 in raise () from /lib/libc.so.6
(gdb) 

Any idea of what I’m doing wrong? Thank you!

Answer 1

intervals.at(i).at(0) = a;

Is illegal, since your vector is initially empty. You can either use push_back or pre-allocate the vector.

I would pre-allocate the vector, since this would require no further re-allocation on push_back:

vector< vector<int> > intervals(n);
int a, b;
for (int i = 0; i < n; i++) {
  scanf("%d %d", &a, &b);
  intervals.at(i).push_back(a);
  intervals.at(i).push_back(b);
}