Many collection objects, e .g ., stack, can be implemented with tabular representation (an array) . Such an implementation provides fast index and saves space on linked structures . However, its space is bounded at creation time . This note presents a hybrid data structure, called pool, which combines tabular and indexed data structures. A pool is a number of segments of contiguous memory space. It provides unbounded space . Its first segment is allocated initially . When more space is needed, a new segment is allocated in double size of previous segment . All segments allocated can be accessed via an index table. The size of index table is log n, where n is the number of elements . A pool can provide index operation in 0(log n) and iterations almost as fast as tabular implementation .