Map with index.

Please add

map(Func2<? super T, int index, ? extends R> func)

Where the second parameter of Func2 is the index of the element.

A similar overload could be useful on operators like scan, reduce, filter etc.

[zsxwing]

By the way, mapWithIndex can be implemented by zipWith(Observable.range(0, Integer.MAX_VALUE)).

[artem-zinnatullin]

Why not use rxjava-extras? 😃

RxJava already has ~4k methods, and it has compose() and some other ways to be extendable.

I’d say, that existing operators are enough to do what you want, though, it’s a little bit less effective than native operator because it creates new Pair each time:

Observable
  .just("a", "b", "c")
  .map(new Func1<String, Pair<Integer, String>>() {
    private int counter; // Or AtomicInteger if needed.

    @Override
    public Pair<Integer, String> call(String s) {
      return new Pair<Integer, String>(counter++, s);
    }
  })

// I have nothing personal against indexed operators, but at the same time I’d vote for keeping API “small”.