How to use remove-erase idiom for removing empty vectors in a vector?

Question

I have some trouble with removing a empty vector in a vector using the remove-erase idiom like Erasing elements from a vector. How can I apply this on:

vector<vector<Point> > contours; // want to remove contours.at(i).empty()
contours.erase(remove(contours.begin(), contours.end(), ??? ),contours.end());

score 10 · Accepted Answer · answered Dec 13 '11 at 15:29

10

Have you tried:

contours.erase(remove(contours.begin(), contours.end(), vector<Point>()), contours.end());

answered Dec 13 '11 at 15:29

moswald

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Indeed: it might be overkill in the sense that the compiler might emit more code than strictly necessary, but it's concise. – Steve Jessop Dec 13 '11 at 15:34
Assuming than the comparison is cheap which it should be, this is a great solution! – 111111 Dec 13 '11 at 15:35
1

The comparison should be cheap as the first thing `vector::operator==` does is to compare size() for both sides. – Blastfurnace Dec 13 '11 at 15:39
Blastfurnace thought that should be the case. – 111111 Dec 13 '11 at 15:44

score 5 · Answer 2 · answered Dec 13 '11 at 15:30

5

Use remove_if that takes a predicate.

contours.erase(
    std::remove_if(
         contours.begin(), contours.end(),
         [](const vector<Point>& v) { return v.empty(); }
         // or a functor/plain function/Boost.Lambda expression
    ), contours.end()
);

answered Dec 13 '11 at 15:30

Cat Plus Plus

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It's works! Nice suggestion, I actually looked at during it with lambda expressions, so thank you for your suggestion. – aagaard Dec 13 '11 at 15:46
Although everybody likes lambdas, I find `std::mem_fn(&std::vector::empty)` even more concise and clear. – Christian Rau Jan 14 '13 at 12:11

score 2 · Answer 3 · answered Dec 13 '11 at 15:32

2

use remove_if.

C++11

contours.erase(
    std::remove_if(contours.begin(), contours.end(), 
        [&](const Vector<Point>& vp){
            return vp.empty();
        }),
        contours.end());

C++03

struct is_empty
{
    bool operator()(const Vector<Point>& vp) constt;
    {
        return vp.empty();
    }
}


contours.erase(
         std::remove_if(contours.begin(), contours.end(), 
         is_empty,
         contours.end());

answered Dec 13 '11 at 15:32

111111

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You don't need a custom functor in C++03, a simple `std::mem_fun_ref(&std::vector::empty)` should suffice. – Christian Rau Dec 14 '12 at 09:05
The same holds for C++11, just with `std::mem_fn`, where it's also more concise and conceptually cleaner than a lambda, but Ok, that is maybe rather a matter of taste. But for C++03, not using its scarce functional facilities when they finally make perfect sense is a little bit of a crime ;) – Christian Rau Dec 14 '12 at 09:11
By the way, it should be `const` istead of `constt` and `is_empty()` instead of just `is_empty` inside the function call. – Christian Rau Dec 14 '12 at 09:12

How to use remove-erase idiom for removing empty vectors in a vector?

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