Taking lists as input till enter is pressed - c++

Question

I am having an integer N.and next N lines contain lists that can have distinct elements from 1-100.But i am not provided length of each list.How to handle this type of input.
If say i have vector > mylist;
I need to populate this list with those lists seperated by just next line.
Say if N=3
1 2 3
4
5 6
Then mylist[0]=[1,2,3] , mylist[1]=[4] , mylist[2]=[5,6].
How to do it in c++?
Mycode : Not correct but i tried.
int main(){
int t;
cin>>t;
cin.ignore();
while(t--){
int n;
cin>>n;
cin.ignore();
lists_t lists;
std::string record;
while ( std::getline( std::cin, record ) &&
record.find_first_not_of( ' ' ) != std::string::npos && lists.size()!=n)
{
std::istringstream is( record );
lists.push_back( std::vector<int>( std::istream_iterator<int>( is ),
std::istream_iterator<int>() ) );
}
for ( const auto &l : lists )
{
for ( int x : l ) std::cout << x << ' ';
std::cout << std::endl;
}
}
}
The problem is that if i enter t=1 and n=3 then instead of following n lines it takes 4 lines and then display the data.Why ?

Answer 1

You can use standard function std::getline ans string stream std::stringstream and of course the container std::list itself.
Here is an example
#include <iostream>
#include <iterator>
#include <list>
#include <sstream>
#include <string>
int main()
{
std::list<std::list<int>> lst;
std::string record;
while ( std::getline( std::cin, record ) &&
record.find_first_not_of( ' ' ) != std::string::npos )
{
std::istringstream is( record );
lst.push_back( std::list<int>( std::istream_iterator<int>( is ),
std::istream_iterator<int>() ) );
}
for ( const auto &l : lst )
{
for ( int x : l ) std::cout << x << ' ';
std::cout << std::endl;
}
return 0;
}
The output is
1 2 3
4
5 6
if the input was the same lines.
If you need to enter a given number of lines then the code could look the following way
#include <iostream>
#include <iterator>
#include <list>
#include <sstream>
#include <string>
int main()
{
std::list<std::list<int>> lst;
std::string record;
size_t n;
std::cin >> n;
std::cin.ignore();
while ( n-- &&
std::getline( std::cin, record ) &&
record.find_first_not_of( ' ' ) != std::string::npos )
{
std::istringstream is( record );
lst.push_back( std::list<int>( std::istream_iterator<int>( is ),
std::istream_iterator<int>() ) );
}
for ( const auto &l : lst )
{
for ( int x : l ) std::cout << x << ' ';
std::cout << std::endl;
}
return 0;
}
The input could look as
3
1 2 3
4
5 6