Get Next Element in the std::list using iterator - c++

Question

I am trying to get difference between consecutive elements in a std::list. I attempted the following solution but as this thread says, I need to make a copy of iterator and increment it. Not sure what it mean since adding number to iterators also result in error. what am I missing here. I am using a previous version of C++ but not C++ 11
#include "stdafx.h"
#include <list>
#include <iostream>
using namespace std;
int main()
{
std::list<int> myList;
//for (int i = 10;i < 15;i++)
myList.push_back(12);
myList.push_back(15);
myList.push_back(18);
myList.push_back(19);
myList.push_back(25);
for (std::list<int>::const_iterator itr = myList.begin();itr != myList.end();++itr)
{
int x = *itr;
int y = *(itr + 1);
int diff = std::abs(x - y);
cout << diff << "\n";
}
return 0;
}

Answer 1

How about using adjacent_difference ?
std::adjacent_difference(myList.begin(), myList.end(), tempList );
See Here
If you're interested in implementation look how its implemented in Possible Implementation section in attached link. All you have to do is replace with list iterator and for output just display on screen, or store it.

Answer 2

Use increment operator to move iterator.
auto x = itr;
auto y = itr;
if(++y!=myList.end())
{
int diff = std::abs(*x - *y);
cout << diff << "\n";
}

Answer 3

You can copy the iterator and run two, but I think it's easier to get the first value outside of the loop and then use the loop to iterate through the second value, resetting the first value with the second before proceeding to the next.
Something like this:
#include <list>
#include <iostream>
int main()
{
std::list<int> myList;
myList.push_back(12);
myList.push_back(15);
myList.push_back(18);
myList.push_back(19);
myList.push_back(25);
std::list<int>::const_iterator itr = myList.begin();
if(itr != myList.end()) // is a first value
{
int last = *itr; // cache it
for (itr++; itr != myList.end(); itr++) // get next value
{
int current = *itr; //cache it
int diff = std::abs(last - current);
std::cout << diff << "\n";
last = current; // update last value
}
}
return 0;
}

Answer 4

You can use std::advance() to get to the next iterator
int x = *itr;
std::list<int>::const_iterator itr2 = itr;
std::advance(itr2,1);
if(itr2==myList.end())
break;
int y = *(itr2);
int diff = std::abs(x - y);
cout << diff << "\n";
EDIT: If c++ 11 available, see std::next

Answer 5

#include <list>
#include <iostream>
int main()
{
std::list<int> myList;
//for (int i = 10;i < 15;i++)
myList.push_back(12);
myList.push_back(15);
myList.push_back(18);
myList.push_back(19);
myList.push_back(25);
for (std::list<int>::const_iterator itr = myList.begin();
itr != std::prev(myList.end());
++itr)
{
std::list<int>::const_iterator nextIt = std::next(itr);
int diff = *itr - *nextIt;
std::cout << diff << "\n";
}
return 0;
}
Use std::next(itr) to get next iterator
and use std::prev(myList.end()) in for loop to get previous of last element in list.
Also you can change your for loop and use std::advance() to get next iterator without using std::next and std::prev
std::list<int>::const_iterator itr = myList.begin();
while (true)
{
int prevValue= *itr;
std::advance(itr, 1);
if (itr == myList.end())
{
break;
}
int diff = prevValue - *itr;
std::cout << diff << "\n";
}

Answer 6

Another way would be to simply keep track of the previous list element value, not the iterator, and to subtract that previous value from the element the iterator is currently pointing to; like so:
#include <list>
#include <cmath> // need cmath for std::abs
#include <iostream>
using namespace std;
int main()
{
std::list<int> myList;
//for (int i = 10;i < 15;i++)
myList.push_back(12);
myList.push_back(15);
myList.push_back(18);
myList.push_back(19);
myList.push_back(25);
int PreviousElement = 0;
bool Start = false;
for (std::list<int>::const_iterator itr = myList.begin(); itr != myList.end(); ++itr)
{
if (Start)
{
int diff = std::abs(*itr - PreviousElement);
cout << diff << "\n";
}
else
{
Start = true;
}
PreviousElement = *itr;
}
return 0;
}

Answer 7

I think you need two iterator to solve this question
Here's my code
#include "stdafx.h"
#include <list>
#include <iterator>
#include <iostream>
using namespace std;
int main()
{
std::list<int> myList;
//for (int i = 10;i < 15;i++)
myList.push_back(12);
myList.push_back(15);
myList.push_back(18);
myList.push_back(19);
myList.push_back(25);
int x = 0;
int y = 0;
std::list<int>::iterator itr;
std::list<int>::iterator it_next;
for (itr = myList.begin(), it_next = ++myList.begin();it_next != myList.end();itr++, it_next++)
{
x = *itr;
y = *it_next;
int diff = abs(x - y);
cout << diff << "\n";
}
system("PAUSE");
return 0;
}
I build it in VC++2005 but it should work fine in C++
Hope this will help you :)