List of vectors

Question

I am trying to figure out how to properly use a list of vectors.

I understand how to declare

list< vector<int> > variableName;

but I do not know how to actually set anything to it or pull any information out of it.

More specifically, I am trying to make a list of vectors of objects and I would like to be able to set and pull information from this.

list< vector<ClassObject> > listOfVectorsOfClass;

Can anyone help me out?

Solution 1

You can access information with iterators:

 list< vector<ClassObject> >::iterator list_it;
 vector<ClassObject>::iterator vec_it;
 for (list_it = listOfVectorOfClass.begin(); list_it != listOfVectorOfClass.end(); 
         ++ list_it)
 {

     for (vec_it = list_it->begin(); vec_it != list_it->end(); ++ vec_it)
     {
          //do something with vec_it
          //for example call member function of Class
          (*vec_it).print();
     }
}//can use const_iterator depends on what you will do on class objects

It is the same thing as you access list of vectors of int.

Solution 2

Do you want a sample ?!

list< vector<int> > variableName;

variableName.push_back({1, 2, 3});
variableName.push_back({4, 2, 6});

for (auto &v : variableName)
{
    for (auto &x : v)
        cout << x << " ";
    cout << endl;
}

Solution 3

#include <cstdio>
#include <list>
#include <vector>
#include <iterator>

int main(){
    std::list< std::vector<int> > li;
    li.push_back(std::vector<int>());
    li.push_back(std::vector<int>());
    std::vector<int> v3(3);
    v3.push_back(1);
    v3.push_back(2);
    v3.push_back(3);
    li.push_back(v3);
     for (std::list<std::vector<int> >::iterator it1 = li.begin(); it1 != li.end(); ++it1){
        std::vector<int>::iterator it2;
        for (it2 = (*it1).begin(); it2 != (*it1).end(); ++ it2){
            std::printf("element: %d\n",(*it2));
        }
     }
return OK;
}

if you are familiar with new C++11 syntax then it could be even more simple:

std::list< std::vector<int> > li;

li.push_back({1, 2, 3});//push_back vector
li.push_back({4, 5, 6});//again, vector implicitly

for (auto &v : li)
{
    //even better auto const since we need read only access to elements
    for (auto const &i : v)
        std::printf("element: %d\n",i);
}

Comments

[Mahesh]

+1 But you could directly use list_it->begin() in the inner loop.

[taocp]

@Mahesh yeah, agree. Will update it now

[Christian Rau]

By the way, why on earth declare the iterators outside of their for loops?

[Christian Rau]

C++11 and printf, ouch!

[4pie0]

I know it is little mixed, but not forbidden, right?

[Christian Rau]

Of course it's not forbidden (that's why it's not worth a downvote anyway, though unfortunately not an upvote either). But if youre using the C++ version from <cstdio>` and not using namespace std, you should at least qualify it appropriately (std::printf).

[4pie0]

@ChristianRau well you can always find some post to downvote this is not difficult

[Admin]

This is perfect! A list of vectors is really confusing but you really laid out the foundation for me. After showing me it like this, it makes much more sense. Still a little confusing so I have to move slowly but I think I can manage it from here. Thank you guys so much!

[Christian Rau]

@tacp And that is because...?

[taocp]

@ChristianRau I updated the post. Probably because of my bad coding style. I agree it should be put outside the first for loop.

[Christian Rau]

@tacp Wow, that's even worse.