#inserting a set in a set of sets using stl

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haughty knot
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I am learning about c++ stl, and while studying sets, I am stuck at this problem of inserting a set in a set of sets. Here is what my approach looks like. Kindly point out the flaws and redirect me to the correct approach. Thanks
set<set<int> > nestedSet; nestedSet.insert(set<int>({1,3,2}));

smoky ingotBOT
#

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dark ingot
haughty knot
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yes, i mean a nested set. I understand there would possibly be a problem around the definition of the < operator for comparing two sets. But im sure there's some way around it?

dark ingot
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but why do you want to nest sets?

unborn cove
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Maths does deal with sets of sets.

dark ingot
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fair enough yeah just wanted to check it was that and not that they actually just wanted std::vectors

haughty knot
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@dark ingot any discrete mathematical representation has to be expressible in code as well, so I think it should be possible to implement

dark ingot
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but anyway set takes an optional type argument which allows you to specify a comparison operator

By default that comparison uses std::less which uses operator<. The problem is you don't really want to be implementing custom operators for a set since 1. its a standard type and you can't necessarily do that in some cases and 2. you might not necessarily want all sets to behave this way since its quite a generic type

instead there are a couple of options

  1. wrap your set inside a class e.g. MathSet and then you know that every MathSet is a set with the specific properties you want. And then you can do all the operators correctly for that class
  2. provide your own comparison type instead of std::less that knows how to compare sets in the way you want it to (e.g. std::set<std::set<int>, SetCompare>)
  3. you can probably specialize std::less but I can't remember if its allowed to do that with standard types
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I would likely go with the first option. Just because it means you can define custom operations on sets in a nicer way. And change out your implementation inside at a later date if you want to. And do all the nice invariant stuff. But it leads to more boilerplate

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but if you just want an "it works" solution you probably want a custom comparitor

haughty knot
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@dark ingot i still dont get the result. there seems to be an issue with the constructor accepting the initializer list as an argument.

rapid glade
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That is probably a side-effect of set<set<int> > nestedSet; not being valid.

dark ingot
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I think set is one of those containers that only compile errors once you try to use a specific aspect of it which it doesn't support

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in that case its the constructor being invalid

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because it can't insert the elements

dark ingot
haughty knot
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nestedSet.insert(set<int>({1,3,2})); alongwith defining a custom comparator and initialising the nestedSet as set<set<int>,Cmp> nestedSet;

dark ingot
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whats the error?

rapid glade
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Just copy the text, don't send pictures.

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Even better, send a link to godbolt.org that reproduces the error so you can get a link back with the fixed code.

haughty knot
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@rapid glade @dark ingot heres the code link https://godbolt.org/z/nh1M98o5K

Compiler Explorer - C++ (x86-64 gcc 13.2)

using namespace std;
using Cmp = std::integral_constant<decltype(bcmp), bcmp>;

bool cmp(set<int> s, set<int> t){
int smin = INT_MAX;
for(auto i : s){
smin = min(smin, i);
}
int tmin = INT_MAX;
for(auto i : t){
tmin = min(tmin, i);
}
return smin < tmin;
}

int main(){
set<int> s;
s.insert(1);
...

rapid glade
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I have no clue what your Cmp with that std::integral_constant was supposed to be.

haughty knot
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@rapid glade yeah it works. how did you figure out to put the comparator within a struct?

rapid glade
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Because the second template parameter for std::set is a type, so you can't just put in a value.

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You'd think the comparator would be a value and not a type, but apparently not.

haughty knot
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yeah, pretty cool. thanks.

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!solved

smoky ingotBOT
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