gistfile1.cpp

#include <iostream>
#include <vector>

// Returns a vector of indices in the range [st, end) which correspond to
// values for which pred is true.
template <typename Iterator, typename Func>
std::vector<std::size_t> find_indices(Iterator st, Iterator end, Func pred) {
  std::vector<std::size_t> indices;
  for (auto i=st; i<end; i++) {
    if (pred(*i)) indices.push_back(std::distance(st, i));
  }
  return indices;
}

// Returns true if val >= 2, else false.
bool example_pred(int val) {
  return val >= 2;
}

int main()
{
  // We use a vector as input, but find_indices()
  // should work for anything you can iterate over.
  std::vector<int> data_array { 0, 1, 2, 3, 0, 3, 2, 1, 0 };

  // This particular predicate checks for all values >= 2,
  // but you could use whatever arbitrary logic you like.
  //auto indices = find_indices(begin(data_array), end(data_array), [](int i){return i>=2;});

  // If you like ugly things, you could use a function
  // pointer instead of a lambda.
  auto indices = find_indices(begin(data_array), end(data_array), example_pred);

  for (auto &i: indices) {
    std::cout << "Index: " << i << "   Value: " << data_array[i] << std::endl;
  }

  return 0;
}