Question

Is there a common idiom for doing something twice, as in the following situation?

    for ( int i = 0; i < num_pairs; i++ ) {
        cards.push_back( Card(i) );
        cards.push_back( Card(i) );
    }

I have a feeling that there’s a clearer way than introducing a new loop variable counting from 0 to 1, especially since it isn’t used except for counting.

    for ( int i = 0; i < num_pairs; i++ )
        for ( int j = 0; j < 2; j++ )
            cards.push_back( Card(i) );

(Card is just some class I made up and not relevant to the question.)

Answer 1

I’ve got a few suggestions. See the last for my recommendation:

1. In my opinion insert(it, N, value) beats std::fill_n:

   for ( int i = 0; i < num_pairs; i++ ) {
       cards.insert(cards.end(), 2, Card(i) );
   }
   

2. If order isn’t important, you could just generate the cards once, and duplicate after the fact

   std::copy(cards.begin(), cards.end(), std::back_inserter(cards));
   

3. Using a dirty lambda and integer division trick. Warning this has the hallmark of a premature optimization: reduces readability for no good reason.

   std::vector<Card> cards(num_pairs * 2);
   int i = 0;
   std::generate_n(cards.begin(), num_pairs, [&i] () { return Card(i++/2); });
   

   (assumes Card is default-constructible. If not, use cards.back_inserter())

4. My recommendation

   The following wins in both performance and expression of intent:

   std::vector<Card> cards;
   cards.reserve(num_pairs*2);
   for (int i = 0; i < num_pairs; ++i)
   {
       cards.emplace_back(i);
       cards.emplace_back(i);
   }