sort vector by using lambda with &&

Question

I am trying to sort a vector elements by using lambda but I have a question. I am trying to sort it base on 2 values from a struct but lambda does not allow me to do it like that.

Here is what i am trying to do:

struct Test
{   int Current;
    int Max;
};

std::vector<Test*> VectorA

std::sort(VectorA.begin(), VectorA.end(), [](Test& test, Test& test2) {return (test.Current > test2.Current) && (test.Max > test2.Current); });

Is it possible to use it like that ?

Your vector contains Test*, not Test as your lambda implies. — wohlstad
– wohlstad, Commented Nov 6, 2022 at 16:22
Why does your lambda take non-const references as inputs? Do you intend to modify them? (Hopefully not) — Dan Mašek
– Dan Mašek, Commented Nov 6, 2022 at 16:24
Your comparaison doesn't comply with strict weak ordering neither. — Jarod42
– Jarod42, Commented Nov 6, 2022 at 16:29
thanks for telling me my mistakes i learned from it i am grateful for that i just started to learning c++ — Alex
– Alex, Commented Nov 6, 2022 at 19:17

marc_s · Accepted Answer · 2023-05-19 20:35:53Z

3

Your std::vector contains elements of type Test*, not Test.
Therefore your lambda should accept references to Test* objects, and dereference the pointers with operator->.

Since you do not need to modify these objects, it is better for your lambda to accept the arguments by a const reference.

A complete example:

#include <vector>
#include <algorithm>

struct Test
{
    int Current;
    int Max;
};

int main()
{
    std::vector<Test*> VectorA;
    std::sort(VectorA.begin(), 
              VectorA.end(),
//---------------vvvvv--------------vvvvv--------------
              [](Test* const& test, Test* const& test2)
//----------------------------vv---------------vv-----------
                { return (test->Current > test2->Current) && (test->Max > test2->Current); });
    return 0;
}

Edit: my answer above addressed only the issue of the c++ syntax itself.

As commented below by @Jarod42, there is also a semantic issue here - your comparison logic does not comply with strict weak ordering (see: Wikipedia - Weak Ordering).

edited May 19, 2023 at 20:35

marc_s

760k186 gold badges1.4k silver badges1.5k bronze badges

answered Nov 6, 2022 at 16:26

wohlstad

36k18 gold badges79 silver badges112 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jarod42 Over a year ago

Comparer is wrong too about strict weak ordering, maybe return std::tie(lhs.Current, lhs.Max) > std::tie(rhs.Current, rhs.Max);

wohlstad Over a year ago

@Jarod42 good point. I merely addressed the syntax issue. Updated my answer.

Alex Over a year ago

Thanks guys for helping me and thanks for explain and telling me my mistakes i am grateful for that

Collectives™ on Stack Overflow

sort vector by using lambda with &&

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related