C ++：访问向量列表中的某些元素

Question

I have list of vectors:

list< vector<int> > myList;

The structure of this list:

({1,2,3}, {4,5,6}, ...)

I want to get a certain element by his positions. For example, getFromList(myList, 0, 2) will return 3 . I tried this, but it does not work:

int getFromList(list< vector<int> > myList, int i, int j) 
{
    int ki = 0, kj = 0, num;
    for (list<vector<int>>::iterator it1 = myList.begin(); it1 != myList.end(); ++it1) {
        vector<int>::iterator it2;
        ki++;
        for (it2 = (*it1).begin(); it2 != (*it1).end(); ++it2) {
            kj++;
            if (ki == i && kj == j) {
                num = (*it2);
            }
        }
    }

    return num;
}

Answer 1

The solution that Cássio provided in the comments won't work as you cannot randomly access elements of a list

Instead you can use std::next defined in the header <iterator> to do this like so:

return std::next(myList.begin(), i)->at(j);

Note that this method doesn't do any bounds checking on the size of the list that you pass in. Before returning this you should check that i is a valid index.

Answer 2

Here is a demonstrative program

#include <iostream>
#include <list>
#include <vector>
#include <iterator>
#include <stdexcept>


int getFromList( const std::list<std::vector<int>> &myList, size_t i, size_t j )
{
    if ( !( i < myList.size() ) ) throw std::out_of_range( "The frst index is out of the range" );

    auto it = std::next( myList.begin(), i );

    if ( !( j < it->size() ) ) throw std::out_of_range( "The second index is out of the range" );

    return it->operator []( j );
}    

int main()
{
    std::list<std::vector<int>> myList = { { 1, 2, 3 }, { 4, 5, 6, 7 }, { 8, 9, 10, 11, 12 } };

    std::cout << "myList[" << 2 << "][" << 3 << "] = " << getFromList( myList, 2, 3 ) << std::endl;

    return 0;
}

Its output is

myList[2][3] = 11

Pay attention that the first parameter of the function is const reference.

As for your function then it has undefined behaviour when one of the indices is out of the valid range because the function returns uninitialized value of variable num .

Answer 3

I found some decision on my own:

int getFromList(list< vector<int> > myList, int i, int j) 
{
    list<vector<int>>::iterator iter = myList.begin();
    advance(iter, i);
    vector<int> x = (*iter);

    return x[j];
}