169 lines
3.7 KiB
Text
169 lines
3.7 KiB
Text
---
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id: custom-cpp-stl
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title: "C++ Sets with Custom Comparators"
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author: Siyong Huang, Benjamin Qi
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prerequisites:
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- sorting-custom
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- intro-ordered
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description: "?"
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frequency: 1
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---
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<resources>
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<resource source="fushar" title="Comparison Functions in C++" starred url="http://fusharblog.com/3-ways-to-define-comparison-functions-in-cpp/">Covers all of this material.</resource>
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</resources>
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<br />
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What if we want to use a C++ `set` with the `Edge` struct that was defined in "Sorting with Custom Comparators"?
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## Operator Overloading
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Works as expected, although you should make sure to include the second `const` or you'll get a compilation error. From the link above:
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> [The second const] means you cannot modify member variables of the current object.
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```cpp
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#include <bits/stdc++.h>
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using namespace std;
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struct Edge {
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int a,b,w;
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bool operator<(const Edge& y) const { return w < y.w; }
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};
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int main() {
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int M = 4;
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set<Edge> v;
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.insert({a,b,w});
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}
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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```
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## Comparator
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```cpp
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#include <bits/stdc++.h>
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using namespace std;
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struct Edge {
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int a,b,w;
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};
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bool cmp(const Edge& x, const Edge& y) { return x.w < y.w; }
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int main() {
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int M = 4;
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set<Edge,bool(*)(const Edge&,const Edge&)> v(cmp);
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.insert({a,b,w});
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}
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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```
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Lambda expressions also work:
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```cpp
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auto cmp = [](const Edge& x, const Edge& y) { return x.w < y.w; };
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int main() {
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int M = 4;
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set<Edge,bool(*)(const Edge&,const Edge&)> v(cmp);
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.insert({a,b,w});
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}
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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```
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In this case, we can shorten the declaration of `v` by writing `set<Edge,decltype(cmp)> v(cmp);` instead.
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## Functors
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Probably less confusing than the method above.
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```cpp
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#include <bits/stdc++.h>
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using namespace std;
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struct Edge {
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int a,b,w;
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};
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struct cmp {
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bool operator()(const Edge& x, const Edge& y) { return x.w < y.w; }
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};
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int main() {
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int M = 4;
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set<Edge,cmp> v;
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.insert({a,b,w});
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}
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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```
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We can also use `cmp` like a normal function by adding `()` after it.
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```cpp
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int main() {
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int M = 4;
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vector<Edge> v;
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.push_back({a,b,w});
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}
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sort(begin(v),end(v),cmp());
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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```
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## Built-In Functors
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Overloading the less than operator (`<`) automatically generates the functor `less<Edge>`. Similarly, overloading `>` automatically generates the functor [`greater<Edge>`](https://en.cppreference.com/w/cpp/utility/functional/greater). We can use this to store a set in reverse order.
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```cpp
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#include <bits/stdc++.h>
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using namespace std;
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struct Edge {
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int a,b,w;
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bool operator>(const Edge& y) const { return w > y.w; }
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};
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int main() {
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int M = 4;
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set<Edge,greater<Edge>> v;
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for (int i = 0; i < M; ++i) {
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int a,b,w; cin >> a >> b >> w;
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v.insert({a,b,w});
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}
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for (Edge e: v) cout << e.a << " " << e.b << " " << e.w << "\n";
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}
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/* Output:
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2 3 10
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1 2 9
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1 3 7
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2 4 3
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*/
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```
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## Other Containers
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The following are all valid:
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```cpp
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set<int,greater<int>> a;
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map<int,string,greater<int>> b;
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priority_queue<int,vector<int>,greater<int>> c;
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``` |