size_t.max if the series does not contain the key and appropriate index otherwise.
auto index = [1, 2, 3, 4].sliced; auto data = [2.1, 3.4, 5.6, 7.8].sliced; auto series = index.series(data); assert(series.data[series.findIndex(3)] == 5.6); assert(series.findIndex(0) == size_t.max);
Finds an index such that series.index[index] == key.