sorting a list containing circles c++

Question

I'd like to run this simple code:

#include <cstdlib> //std :: rand ()
#include <vector> //std::vector<>
#include <list> //std::list<>
#include <iostream> //std::ostream_iterator
#include <iterator> //std:: cout std::ostream_iterator
#include <algorithm> //std::reverse, std::generate
#include <map>
#include "ColorRGB.hpp"
#include "point2d.hpp"
#include "circle.hpp"
#include <cmath>

int main(){

std::list<Circle>lk; 

    Point2d a(7.5,3.2);
    Point2d b(6.5,2.2);
    Point2d c(5.5,1.2);

    ColorRGB d(0, 0, 0);
    ColorRGB e(0, 1, 1);
    ColorRGB f(1, 1, 0);

    Circle c1(a, 2, d);
    Circle c2(b, 1, e);
    Circle c3(c, 0.4, f);

    lk.push_back(c1);
    lk.push_back(c2);
    lk.push_back(c3);

sort(lk.begin(), lk.end(), [] (Circle const& lhs,Circle const& rhs)
    ->bool{return(lhs.getrad() <= rhs.getrad())};
    return 0;
    }

but get the compile message:

Juliuss-MacBook-Pro-2:uebung4 JONG$ g++ -o 4_1 4_1.cpp
4_1.cpp:42:30: error: expected expression
    sort(lk.begin(), lk.end(), [] (Circle const& lhs,Circle const& r...
                               ^
1 error generated.

To be fair, it's highly probable the list isn't going to be any better. — chris, Jun 01 '14 at 20:19
Add `-std=c++11` to the compiler options, otherwise gcc does not know what a lambda is. — nwp, Jun 01 '14 at 20:31
Related: Among the other things mentioned, your comparator is also wrong. The comparator for a standard library sort (list, free-function, or otherwise), must enforce a *strict weak order*, and yours does not. As is a `<=` given two values A and B with identical radius will answer "true" for the sort-comparator's `cmp(A,B)` and `cmp(B,A)`. This tends to wreak havoc on the partitioning algorithms. If the values are "equal", `cmp(A,B)` and `cmp(B,A)` should both answer **false**; not true. Change the comparison logic to strictly `<`. — WhozCraig, Jun 01 '14 at 21:11

score 1 · Accepted Answer · edited May 23 '17 at 12:11

You can not sort a list using std::sort because std::sort requires a random access iterator and list has a bidirectional iterator only.

Use a vector instead. Alternatively, if you insist on using list, there is a member function called list.sort(). It can accept a custom comparator as well.

Related question / answers here

Also, use an SSCCE in the question. That means getting rid of/providing the code of custom classes as much as possible.

You also had errors in the braces. Here is a code that is self-contained and also compiles. It show the usage of both std::sort with vector and the list::sort function:

#include <list>
#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>

class Circle
{
    int r;
public:
    Circle(int r) : r(r) {}
    int getrad() const { return r; }
};

int main(){
    std::vector<Circle> vk;
    std::list<Circle> lk;

    Circle c1(2);
    Circle c2(1);
    Circle c3(3);

    vk.push_back(c1);
    vk.push_back(c2);
    vk.push_back(c3);

    lk.push_back(c1);
    lk.push_back(c2);
    lk.push_back(c3);

    std::sort(vk.begin(), vk.end(),
        [] (const Circle& lhs, const Circle& rhs) -> bool {
            return lhs.getrad() < rhs.getrad();
        });

    lk.sort(
        [] (const Circle& lhs, const Circle& rhs) -> bool {
            return lhs.getrad() < rhs.getrad();
        });

    return 0;
}

Still, I recommend vector.

After the OPs update:

As pointed out by @nwp, use a C++11 compiler if you use lambas:

g++ -std=c++11 -Wall -Wextra filename.cpp

As pointed out by @WhozCraig , the comparator also needed a fix.

i get the same compile error if i place lk.sort instead of sort — user3697516, Jun 01 '14 at 20:20
Then how about answering the comments first? We don´t know your error because you didn´t write it. — deviantfan, Jun 01 '14 at 20:23

sorting a list containing circles c++

1 Answers1