CODEWITHSUNDEEP
c++iterator
user8305079
user8305079

Reputation:

Converting an integer to an iterator for std::list

I have the simple code below:

#include <iostream>
#include <list>
#include <iterator>
using namespace std;

int main()
{
    list<int> ints = { 3, 6, 1, 5, 8 };
    int index;

    index = 2;
    std::list<int>::iterator it = ints.begin() + index;
    cout << "Element at index " << index << " is " << *it << '\n';

    return 0;
}

On compiling, it says:

error: no match for ‘operator+’ (operand types are ‘std::__cxx11::list::iterator’ {aka ‘std::_List_iterator’} and ‘int’)
std::list::iterator it = ints.begin() + index;
                       ~~~~~~~~~~~~~^~~~~~~

I understand what the compiler is trying to say, but I am unsure how to resolve it. How do I convert the int to an iterator and get the element at that index? I did look for similar questions, but most of them seek the conversion in the opposite direction which can be done using std::distance, while the remaining were for different containers.

Thanks for your help!

Upvotes: 0

Views: 2847

Answers (2)

Nelfeal
Nelfeal

Reputation: 13269

Use std::next.

If you think of distance as the operator - between two iterators, then next, prev, and advance are respectively the +, -, and += between an iterator and an integer.
These functions are meant to treat all iterators the same way, even though the complexity of operations is different for different iterators. For a std::list, which uses "forward iterators", these are all linear. For a std::vector, which uses "random-access iterators", these are all constant.

Upvotes: 3

eerorika
eerorika

Reputation: 238411

Converting an integer to an iterator for std::list

Integers and iterators are quite separate concepts, so it is unclear what meaning such conversion would have. How does one convert a coconut into transitivity?

... and get the element at that index?

Now, this makes a bit more sense. List element do not have indices however. But presumably by index i, you mean the ith (zero based) successive element from beginning.

The standard library has a function for that: std::next. What this does (with non-random access iterators) is increment the iterator for given number of repetitions in a loop. Intuitively, the asymptotic complexity of such operation is linear.

Upvotes: 1

Related Questions