How to sort with a lambda?

c++ sorting lambda char constants

225,048

Solution 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.

Solution 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;
}

Solution 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,

225,048

BTR

Updated on March 16, 2022

Comments

BTR about 2 years
```
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).
- BTR about 13 years
  
  The vector is of a struct which contains an integer and two strings. The property here would be an integer.
- GManNickG about 13 years
  
  Show us a small compilable example.
sellibitze about 13 years

What you have written so far makes little sense. If mProperty is supposed to be an int a.mProperty>b.mProperty will definitely yield a bool.
BTR about 13 years

Then you understand my confusion. I think it might be something weird with my VC10 Express (no service pack). I moved the project onto a machine with Visual Studio 2010 Team and it worked without the "-> bool".
BTR about 13 years

Yeah, something screwy with the setup I was on. Compiling on my laptop without it just fine on the Team edition of Visual Studio 2010. What clued me in what that I'd switched back to bind and the error wouldn't go away. I was on VC10 Express. Bug?
BTR about 13 years

And yes, the property is int32_t.
BTR about 13 years

Hey GMan, I noticed that you have found Lambda bugs with VS per your site: gmannickg.com/?p=38. Possibly related issue in the Express edition?
Warpspace over 10 years

Shouldn't it be operator<, not operator>?
pancake almost 10 years

Yes it should be <, for standard ascending order. I edited the answer to make it clear it was a descending sort but apparently my edit was unhelpful and got wiped!
David Grayson almost 8 years

I just tried this in GCC 5.3.0 and the -> bool part is not needed.
Conrad Jones about 5 years

it was highly likely explicitly specifying the -> boot return type was required due to a VC bug.
Admin over 3 years

make it longer, thats why op wants to use a lambda