Comparing 2 std::lists of std::pairs - c++

Question

I am hoping you can help me out here. I have searched for other answers, but I havent found something that matches my specific situation (but if you do find one, please let me know the URL!). I have seen a lot of suggestions about using std::map instead of list and I dont mind switching the container if need be.
Currently, I have two Lists of pairs i.e.
std:list <std::pair<string,string>> outputList1;
std:list <std::pair<string,string>> outputList2;
I have populated each list with User Settings that I have retrieved from an SQL database (I omit the SQL retrieval code here).
Example list:
outputList1 (first, second)
CanSeeAll, True
CanSubmit, False
CanControl, False
OutputList2:
CanSeeAll, False
CanSubmit, True
CanControl, False
I want to iterate through both lists and find the mismatches. For example, find the first string of the first pair of the first list to find the matching first string in the second list, then compare the second string to determine whether they match, then print out the non matching pairs to a new string (eventually to file), and so on.
In this example, the final string would have CanSeeAll and CanSubmit as the final output since those are the two that mismatch.
Here is what I've tried so far, but I get a blank string:
std::list <std::pair<std::string,std::string>>::iterator it1 = outputList1.begin();
std::list <std::pair<std::string,std::string>>::iterator it2 = outputList2.begin();
string token;
while (it1 != outputList1.end()){
if((*it1).first == ((*it2).first))
{
if((*it1).second != ((*it2).second))
{
token.append((*it1).first);
token.append(",");
token.append((*it1).second);
token.append("\r\n");
}
it1++;
it2 = outputList2.begin();
}
it2++;
if (it2 == outputList2.end())
it1++;
}
I know this logic is flawed as it will skip the first pair on the second list after the first iteration, but this is the best I can come up with at the moment, and I am banging my head on the keyboard a the moment.
Thanks everyone!

Answer 1

As I understand the problem,
you want to compare every element of one list, to every other element of another list.
You could use a pair of nested range based for loops.
#include <list>
#include <string>
int main(){
std::list<std::pair<std::string,std::string>> l1;
std::list<std::pair<std::string,std::string>> l2;
for (auto x: l1){
for (auto y: l2){
//compare x to y
}
}
}

Answer 2

The answer uses an auxiliary map but, have in mind you will get better result if you use two maps (or hash tables) instead of two list.
// create a map for elements in l2
std::map<std::string, std::string> l2map;
// move elements from l2 to the map so we get O(N*log(N)) instead of O(n²)
for (std::list<std::pair<std::string,std::string> >::iterator it = l2.begin();
it != l2.end();
++it)
{
l2map.insert(*it);
}
// walk l1 and look in l2map
for (std::list<std::pair<std::string,std::string> >::iterator l1it = l1.begin();
l1it != l1.end();
++l1it)
{
// look for the element with the same key in l2
// l1it->first is the key form l1
std::map<std::string, std::string>::iterator l2it = l2map.find(l1it->first);
if (l2it != l2map.end()) {
// found, then compare
if (l1it->second != l2it->second) { // l1it->second is the value from l1
// mismatch
}
} else {
// not in l2
}
}

Answer 3

You could use std::mismatch with the pre-condition: all settings occur in the same order in both lists (you could do a sort if this is not the case)
auto iterPair = std::mismatch(l1.begin(), l1.end(), l2.begin());
while (iterPair.first != l1.end()) {
// TODO: Handle the mismatching iterators
iterPair = std::mismatch(iterPair.first + 1, l1.end(), iterPair.second + 1);
}

Answer 4

If the keys in your lists come in the same order, as in your example, you can traverse the lists linearly:
std::ostringstream s;
std:list<std::pair<string, string>>::const_iterator i2(outputList2.cbegin());
for(auto const &pair: outputList1) {
if(pair.second != i2->second) {
s << pair.first << ": " << pair.second << " != " << i2->second << endl;
}
++i2;
}
Alternatively, use STL algorithms:
#include <algorithm>
typedef std::list<std::pair<std::string, std::string>> List;
std::ostringstream s;
for(
auto itrs(
std::mismatch(
outputList1.cbegin(), outputList1.cend(), outputList2.cbegin()
, [](auto const &l, auto const &r){ return l.second == r.second; }))
; itrs.first != outputList1.cend()
; itrs = std::mismatch(itrs.first, outputList1.cend(), itrs.second
, [](auto const &l, auto const &r){ return l.second == r.second; }))
{
s << itrs.first->first << ": "
<< itrs.first->second << " != " << itrs.second->second
<< std::endl;
}