Finding indices of original source array in a bin

[loopless]

I must admit to being completely lost in performing a simple operation with boost::histogram

• assuming I have a histogram with 3 bins into which I insert an unsorted std::vector of "double"
• I want to iterate over all the entries in, say, bin 2, but I need to iterate over the indices from the original vector, not their values
  For example, with a vector of size 10, I want to know that entries 4,7,9 are in bin 2

Obviously, I can iterate over my original vector and call axis(0).index(value) to find what bin each entry is in, it just seems somewhat a round-about way of doing it.