Sorting objects vector with lambda, not sorting in the end

Question

I'm trying to write a sort function for vectors using lambda, and I've seen this suggested at so many places, but I've always had problems doing this.

std::sort(eventvec.begin(), eventvec.end(), [](const event& lhs, const event& rhs) {
    return lhs.getim < rhs.getim;});

I had multiple errors while writing it and then I kind of stacked it on a function within a function as it needed a static function.

The functions declared in the class are:

double event::getut() { return unift; }

static double getim(event &a) { return a.getut(); }

In the end the sorting is not in order at all. There are negative values as well in the attribute. Any suggestions?

What kind of errors are there? I think you should be using return lhs.getim() < rhs.getim(); instead of what you wrote. — Tanveer Badar
– Tanveer Badar, Commented Oct 23, 2020 at 8:39
You're not calling the function. return lhs.getut() < rhs.getut() but that means you need to make getut() const. Like this: double event::getut() const { return unift; } — Retired Ninja
– Retired Ninja, Commented Oct 23, 2020 at 8:39
@TanveerBadar that would ask for arguments, and what do I enter as an argument because lhs.getim(lhs) doesn't work. — wakanada
– wakanada, Commented Oct 23, 2020 at 8:46
@RetiredNinja error: passing 'const event' as 'this' argument discards qualifiers [-fpermissive] AND --------------------------------------------------------------------------------------- note: in call to 'double event::getut()' double event::getut() ^~~~~ — wakanada
– wakanada, Commented Oct 23, 2020 at 8:47
You didn't read all of what I wrote. "that means you need to make getut() const. Like this: double event::getut() const { return unift; }" — Retired Ninja
– Retired Ninja, Commented Oct 23, 2020 at 9:01

Caleth · Accepted Answer · 2020-10-23 08:51:51Z

3

You are sorting your events by the address of event::getim, which is the same for every event.

I think you mean to call a member function of event, which needs to be callable from a const event

double event::getut() const { return unift; }

std::sort(eventvec.begin(), eventvec.end(), [](const event& lhs, const event& rhs) {
return lhs.getut() < rhs.getut(); });

answered Oct 23, 2020 at 8:51

Caleth

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2 Comments

Caleth Over a year ago

@wakanada you need to edit event::getut as shown. It needs to be callable from const event

wakanada Over a year ago

yeah, got it. Thanks

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