How can I shift elements inside STL container

Question

I want to shift elements inside container on any positions to the left or right. The shifting elements are not contiguous.

e.g I have a vector {1,2,3,4,5,6,7,8} and I want to shift {4,5,7} to the left on 2 positions, the expected result will be {1,4,5,2,7,3,6,8}

Is there an elegant way to solve it ?

Answer 1

You can write your own shifting function. Here's a simple one:

#include <iterator>
#include <algorithm>

template <typename Container, typename ValueType, typename Distance>
void shift(Container &c, const ValueType &value, Distance shifting)
{
    typedef typename Container::iterator Iter;

    // Here I assumed that you shift elements denoted by their values;
    // if you have their indexes, you can use advance
    Iter it = find(c.begin(), c.end(), value);
    Iter tmp = it;

    advance(it, shifting);

    c.erase(tmp);

    c.insert(it, 1, value);
}

You can then use it like that:

vector<int> v;
// fill vector to, say, {1,2,3,4,5}
shift(v, 4, -2); // v = {1,4,2,3,5}
shift(v, 3, 1); // v = {1,4,2,5,3}

This is a naive implementation, because when shifting multiple elements, find will iterate many times on the beginning of the container. Moreover, it assumes that every element is unique, which might not be the case. However, I hope it gave you some hints on how to implement what you need.

Answer 2

Can't you do a simple insert and then erase?

Don't forget that you'll invalidate any iterators that refer to elements above the removal or insertion point, whichever is lowest.

HTH

cheers,

Rob