Is there an equivalent to the “for … else” Python loop in C++?

Question

Python has an interesting for statement which lets you specify an else clause.

In a construct like this one:

for i in foo:
  if         


        

Answer 1

There probably isn't a single solution that fits best all problems. In my case a flag variable and a range-based for loop with an auto specifier worked best. Here's an equivalent of the code in question:

bool none = true;
for (auto i : foo) {
  if (bar(i)) {
    none = false;
    break;
  }
}
if (none) baz();

It is less typing than using iterators. Especially, if you use the for loop to initialize a variable, you may use that instead of the boolean flag.

Thanks to auto typing it is better than std::none_of, if you want to inline the condition rather than call bar() (and if you are not using C++14).

I had a situation where both conditions occurred, the code looked something like this:

for (auto l1 : leaves) {
  for (auto x : vertices) {
    int l2 = -1, y;
    for (auto e : support_edges[x]) {
      if (e.first != l1 && e.second != l1 && e.second != x) {
        std::tie(l2, y) = e;
        break;
      }
    }
    if (l2 == -1) continue;

    // Do stuff using vertices l1, l2, x and y
  }
}

No need for iterators here, because v indicates whether break occurred.

Using std::none_of would require specifying the type of support_edges[x] elements explicitly in arguments of a lambda expression.