Get all positions of elements in STL vector that are greater than a value

Question

I would like to know how can I find the index positions of elements that verify a certain condition (for example greater than). For example if I have a vector of int values<

Answer 1

I think I'd use std::copy_if:

std::vector x{3, 2, 5, 8, 2, 1, 10, 4, 7};
std::vector y(x.size());

std::iota(y.begin(), y.end(), 0);
std::copy_if(y.begin(), y.end(), 
             std::ostream_iterator(std::cout, " "), 
             [&](size_t i) { return x[i] > 5; });

For me, this gives 3 6 8, the indices of 8, 10 and 7 in x -- exactly what we want.

If you're stuck with a C++98/03 compiler/library, you'll use std::remove_copy_if instead (and reverse the sense of the comparison). In this case, you obviously won't be able to use a lambda for the comparison either.