looping over set inside map in cpp - c++

Question

I am getting error in for loop ,compiler saying that 's' must have a pointer type (in s->second.begin()).I just want to iterate over set below.
vector<vector<int>> verticalTraversal(TreeNode* root) {
map<int, map<int, set<int>>> mep;
solve(root, 0, 0, mep);
vector<vector<int>> result;
for (auto p : mep) {
vector<int> temp;
map<int,set<int>> s = p.second;
for (auto ity = s->second.begin(); ity != s->second.end(); ity++) {
//getting error here in s->second.begin()
}
//reverse(temp.begin(), temp.end());
result.push_back(temp);
}
return result;
}

Answer 1

So here's some code that iterates through each set (and prints out all the integers).
vector<vector<int>> verticalTraversal() {
map<int, map<int, set<int>>> mep;
solve(root, 0, 0, mep);
vector<vector<int>> result;
for (map<int, map<int, set<int>>>::iterator i = mep.begin(); i != mep.end(); ++i) {
for (map<int, set<int>>::iterator j = i->second.begin(); j != i->second.end(); ++j) {
for (set<int>::iterator k = j->second.begin(); k != j->second.end(); ++k) {
cout << *k << '\n';
}
}
}
return result;
}
First thing you should notice is that there are three loops not two, since you have a set inside a map inside a map you have three levels to iterate though (not two as in your code).
Second thing is I removed all auto and all range based loops and been completely explicit about the types involved. Hopefully that makes the code easier to understand.
Once you've understood it you can put the auto back in if you like, or even replace the loops with range based loops.

Answer 2

The best thing to do is to write a small program, and then get an idea/experiment with the smaller program to understand the basic concepts.
Here is an example:
#include <map>
#include <set>
#include <iostream>
int main()
{
std::map<int, std::map<int, std::set<int>>> mep;
mep.insert({0, {{0, {0,1,2,3}}}}); // populate the map
mep.insert({1, {{1, {1,2,3,4,5}}}}); // populate the map
// traverse each set in the mep map
for (auto p : mep)
{
std::map<int, std::set<int>> s = p.second;
for (auto ity = s.begin(); ity != s.end(); ity++)
{
std::set<int>& theSet = ity->second;
for (auto& setV : theSet )
std::cout << setV << " ";
std::cout << "\n";
}
}
}
Output:
0 1 2 3
1 2 3 4 5
Also note that you really should be using references to the data within the map instead of copying them by value. Things like this:
for (auto p : mep)
and
std::map<int, std::set<int>> s = p.second;
incurs a copy. Rather, you should do this:
for (auto& p : mep)
...
std::map<int, std::set<int>>& s = p.second;