Sort multidimensional map based on pair value - c++

Question

I have a map defined as follows:
std::map<std::string, std::vector<std::pair<std::string, std::string>>> groupList;
GOAL:
I have a list of groups of commands. I want to sequence through each group in order. In order to do this, each group has a "sequence" value. I want to sort the overall list based on this value from smallest to largest. Essentially, I have a list of elements, which each have an accompanying list of other values within them specific to each parent. I want to sort the list of parents by a specific pair or value within the child lists.
For me to visualize this, I created an array in PHP with a similar structure. I'm not sure exactly how to visualize a C++ map so this is just me making assumptions. 1, 2, 3 are the keys of the map.
Array
(
[1] => Array
(
[groupID] => 1
[sequence] => 0
[command] => DefaultState
)
[2] => Array
(
[groupID] => 2
[sequence] => 2
[command] => Restart
)
[3] => Array
(
[groupID] => 3
[sequence] => 1
[command] => Beep
)
)
I'd like to sort this map based on the value of a particular pair inside, in this case "sequence". Element "2" should be below element "3" when sorting using the "sequence" value. The end result would look like this:
Array
(
[1] => Array
(
[groupID] => 1
[sequence] => 0
[command] => DefaultState
)
[3] => Array
(
[groupID] => 3
[sequence] => 1
[command] => Beep
)
[2] => Array
(
[groupID] => 2
[sequence] => 2
[command] => Restart
)
)
I'm sorry to mix languages here but there's no simple way for me to dump a map that depicts its structure (that I know of).
At first, my map was set up like so:
std::map<std::string, std::map<std::string, std::string>> groupList;
This was easier for me to add elements and then access them later, but I figured a vector pair would be easier to use for sorting. I would prefer to use the latter definition for ease. I was looking at using std::sort and boost but I've had no luck implementing for this specific case.
Any comments/help is welcome. Thanks!

Answer 1

Whenever I see a datastructure, I imagine to have a definition for it handy:
struct Item {
int groupID;
int sequence;
std::string command;
}
Now you can trivially define the array:
Item arr[] = {
{ 1, 0, "DefaultState" },
{ 2, 2, "Restart" },
{ 3, 1, "Beep" },
};
Since it's a simple aggregate with value semantics, you can just define an order and sort it:
struct Item {
int groupID;
int sequence;
std::string command;
bool operator<(Item const& other) const {
return sequence < other.sequence;
}
};
Demo
Adding a streaming operator<< and we get a full working demo:
Live On Coliru
#include <iostream>
struct Item {
int groupID;
int sequence;
std::string command;
bool operator<(Item const& other) const {
return sequence < other.sequence;
}
friend std::ostream& operator<<(std::ostream& os, Item const& i) {
return os << "Item { group:" << i.groupID << ", sequence:" << i.sequence << ", command:'" << i.command << "' }";
}
};
#include <algorithm>
int main() {
Item arr[] = {
{ 1, 0, "DefaultState" },
{ 2, 2, "Restart" },
{ 3, 1, "Beep" },
};
std::sort(std::begin(arr), std::end(arr));
for (auto& item : arr)
std::cout << item << '\n';
}
Prints
Item { group:1, sequence:0, command:'DefaultState' }
Item { group:3, sequence:1, command:'Beep' }
Item { group:2, sequence:2, command:'Restart' }

Answer 2

Expanding on my earlier answer, and in case you're really looking for something more advanced, here's what comes to mind using Boost MultiIndex containers:
Live On Coliru
#include <fstream>
#include <iostream>
struct Item {
int groupID;
int sequence;
std::string command;
friend std::ostream& operator<<(std::ostream& os, Item const& i) {
return os << "Item { group:" << i.groupID << ", sequence:" << i.sequence << ", command:'" << i.command << "' }";
}
};
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/member.hpp>
namespace bmi = boost::multi_index;
using Table = bmi::multi_index_container<
Item,
bmi::indexed_by<
bmi::ordered_unique<
bmi::tag<struct by_group>,
bmi::member<Item, int, &Item::groupID>
>,
bmi::ordered_unique<
bmi::tag<struct by_sequence>,
bmi::member<Item, int, &Item::sequence>
>
>
>;
#include <algorithm>
#include <map>
int main() {
Table arr = {
{ 1, 0, "DefaultState" },
{ 2, 2, "Restart" },
{ 3, 1, "Beep" },
};
for (auto& item : arr.get<by_group>())
std::cout << item << '\n';
std::cout << "\nsorted by sequence:\n";
for (auto& item : arr.get<by_sequence>())
std::cout << item << '\n';
}
Prints
Item { group:1, sequence:0, command:'DefaultState' }
Item { group:2, sequence:2, command:'Restart' }
Item { group:3, sequence:1, command:'Beep' }
sorted by sequence:
Item { group:1, sequence:0, command:'DefaultState' }
Item { group:3, sequence:1, command:'Beep' }
Item { group:2, sequence:2, command:'Restart' }