take input from user in 2d vector

Question

I am trying to take values in a 2d vector called alloc from the user but the alloc zero index is the only changed index and the values of each index override it i don't know why (i.e: only the first index of the 2d vector is affected)

Code:

cout<<"Please Enter n"<<endl;
cin>>n;
cout<<"Please Enter m"<<endl;
cin>>m;


std::vector<vector<int>> alloc ;
int val;
cout<<"Please Enter values of allocation matrix"<<endl;
for(int i=0;i<n;i++){
vector <int> temp;
for(int j=0;j<m;j++){

cin>>val;
temp.push_back(val);
}
alloc.push_back(temp);
temp.clear();
}

Answer 1

for(int i=0;i<n;i++) {
  vector <int> temp;
  for(int j=0;j<m;j++) {
    cin>>val;
    temp.push_back(val);
  }
  alloc.push_back(temp);
  temp.clear();
}

alloc.push_back(temp); doesn't make a copy of temp, it merely adds a pointer to temp to the end of alloc. temp [and its contents] ceases to exist after the for(i) loop exits.

You need to dynamically allocate the "inside" vector instances instead of using a stack object [i.e. temp in this case].