How to sort with a lambda? - c++

Question

sort(mMyClassVector.begin(), mMyClassVector.end(),
[](const MyClass & a, const MyClass & b)
{
return a.mProperty > b.mProperty;
});
I'd like to use a lambda function to sort custom classes in place of binding an instance method. However, the code above yields the error:
error C2564: 'const char *' : a function-style conversion to a built-in type can only take one argument
It works fine with boost::bind(&MyApp::myMethod, this, _1, _2).

Answer 1

Got it.
sort(mMyClassVector.begin(), mMyClassVector.end(),
[](const MyClass & a, const MyClass & b) -> bool
{
return a.mProperty > b.mProperty;
});
I assumed it'd figure out that the > operator returned a bool (per documentation). But apparently it is not so.

Answer 2

You can use it like this:
#include<array>
#include<functional>
using namespace std;
int main()
{
array<int, 10> arr = { 1,2,3,4,5,6,7,8,9 };
sort(begin(arr),
end(arr),
[](int a, int b) {return a > b; });
for (auto item : arr)
cout << item << " ";
return 0;
}

Answer 3

Can the problem be with the "a.mProperty > b.mProperty" line? I've gotten the following code to work:
#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>
#include <sstream>
struct Foo
{
Foo() : _i(0) {};
int _i;
friend std::ostream& operator<<(std::ostream& os, const Foo& f)
{
os << f._i;
return os;
};
};
typedef std::vector<Foo> VectorT;
std::string toString(const VectorT& v)
{
std::stringstream ss;
std::copy(v.begin(), v.end(), std::ostream_iterator<Foo>(ss, ", "));
return ss.str();
};
int main()
{
VectorT v(10);
std::for_each(v.begin(), v.end(),
[](Foo& f)
{
f._i = rand() % 100;
});
std::cout << "before sort: " << toString(v) << "\n";
sort(v.begin(), v.end(),
[](const Foo& a, const Foo& b)
{
return a._i > b._i;
});
std::cout << "after sort: " << toString(v) << "\n";
return 1;
};
The output is:
before sort: 83, 86, 77, 15, 93, 35, 86, 92, 49, 21,
after sort: 93, 92, 86, 86, 83, 77, 49, 35, 21, 15,

Answer 4

You can sort an array like this:
#include <bits/stdc++.h>
using namespace std;
int main() {
int q[] = {1, 3, 5, 7, 9, 2, 4, 6, 8 ,10};
sort(q, q + 10, [&](int A, int B) { return A < B; });
for (int i = 0; i < 10; i++)
cout << q[i] << ' ';
return 0;
}
before sort: 1 3 5 7 9 2 4 6 8 10
after sort: 1 2 3 4 5 6 7 8 9 10
I'd always like to use lambda to sort a array of struct in acm contests like this:
struct item {
int a, b;
};
vector<item> q;
sort(q.begin(), q.end(), [&](item t1, item t2) {
return t1.a < t2.a;
});